RFC 2178
This document is obsolete. Please refer to RFC 2328.






Network Working Group                                             J. Moy
Request for Comments: 2178                  Cascade Communications Corp.
Obsoletes: 1583                                                July 1997
Category: Standards Track


                             OSPF Version 2

Status of this Memo



   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract



   This memo documents version 2 of the OSPF protocol. OSPF is a link-
   state routing protocol.  It is designed to be run internal to a
   single Autonomous System.  Each OSPF router maintains an identical
   database describing the Autonomous System's topology.  From this
   database, a routing table is calculated by constructing a shortest-
   path tree.

   OSPF recalculates routes quickly in the face of topological changes,
   utilizing a minimum of routing protocol traffic.  OSPF provides
   support for equal-cost multipath.  An area routing capability is
   provided, enabling an additional level of routing protection and a
   reduction in routing protocol traffic.  In addition, all OSPF routing
   protocol exchanges are authenticated.

   The differences between this memo and RFC 1583 are explained in
   Appendix G. All differences are backward-compatible in nature.
   Implementations of this memo and of RFC 1583 will interoperate.

   Please send comments to ospf@gated.cornell.edu.

Table of Contents



    1        Introduction ........................................... 5
    1.1      Protocol Overview ...................................... 5
    1.2      Definitions of commonly used terms ..................... 6
    1.3      Brief history of link-state routing technology ........  9
    1.4      Organization of this document ......................... 10
    1.5      Acknowledgments ....................................... 11
    2        The link-state database: organization and calculations  11
    2.1      Representation of routers and networks ................ 11



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    2.1.1    Representation of non-broadcast networks .............. 13
    2.1.2    An example link-state database ........................ 14
    2.2      The shortest-path tree ................................ 18
    2.3      Use of external routing information ................... 20
    2.4      Equal-cost multipath .................................. 22
    3        Splitting the AS into Areas ........................... 22
    3.1      The backbone of the Autonomous System ................. 23
    3.2      Inter-area routing .................................... 23
    3.3      Classification of routers ............................. 24
    3.4      A sample area configuration ........................... 25
    3.5      IP subnetting support ................................. 31
    3.6      Supporting stub areas ................................. 32
    3.7      Partitions of areas ................................... 33
    4        Functional Summary .................................... 34
    4.1      Inter-area routing .................................... 35
    4.2      AS external routes .................................... 35
    4.3      Routing protocol packets .............................. 35
    4.4      Basic implementation requirements ..................... 38
    4.5      Optional OSPF capabilities ............................ 39
    5        Protocol data structures .............................. 40
    6        The Area Data Structure ............................... 42
    7        Bringing Up Adjacencies ............................... 44
    7.1      The Hello Protocol .................................... 44
    7.2      The Synchronization of Databases ...................... 45
    7.3      The Designated Router ................................. 46
    7.4      The Backup Designated Router .......................... 47
    7.5      The graph of adjacencies .............................. 48
    8        Protocol Packet Processing ............................ 49
    8.1      Sending protocol packets .............................. 49
    8.2      Receiving protocol packets ............................ 51
    9        The Interface Data Structure .......................... 54
    9.1      Interface states ...................................... 57
    9.2      Events causing interface state changes ................ 59
    9.3      The Interface state machine ........................... 61
    9.4      Electing the Designated Router ........................ 64
    9.5      Sending Hello packets ................................. 66
    9.5.1    Sending Hello packets on NBMA networks ................ 67
    10       The Neighbor Data Structure ........................... 68
    10.1     Neighbor states ....................................... 70
    10.2     Events causing neighbor state changes ................. 75
    10.3     The Neighbor state machine ............................ 76
    10.4     Whether tocome adjacent    ............................ 82
    10.5     Receiving Hello Packets ............................... 83
    10.6     Receiving Database Description Packets ................ 85
    10.7     Receiving Link State Request Packets .................. 88
    10.8     Sending Database Description Packets .................. 89
    10.9     Sending Link State Request Packets .................... 90
    10.10    An Example ............................................ 91



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    11       The Routing Table Structure ........................... 93
    11.1     Routing table lookup .................................. 96
    11.2     Sample routing table, without areas ................... 97
    11.3     Sample routing table, with areas ...................... 97
    12       Link State Advertisements (LSAs) ......................100
    12.1     The LSA Header ........................................100
    12.1.1   LS age ............................................... 101
    12.1.2   Options .............................................. 101
    12.1.3   LS type .............................................. 102
    12.1.4   Link State ID ........................................ 102
    12.1.5   Advertising Router ................................... 104
    12.1.6   LS sequence number ................................... 104
    12.1.7   LS checksum .......................................... 105
    12.2     The link state database .............................. 105
    12.3     Representation of TOS ................................ 106
    12.4     Originating LSAs ..................................... 107
    12.4.1   Router-LSAs .......................................... 110
    12.4.1.1 Describing point-to-point interfaces ................. 112
    12.4.1.2 Describing broadcast and NBMA interfaces ............. 113
    12.4.1.3 Describing virtual links ............................. 113
    12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 114
    12.4.1.5 Examples of router-LSAs .............................. 114
    12.4.2   Network-LSAs ......................................... 116
    12.4.2.1 Examples of network-LSAs ............................. 116
    12.4.3   Summary-LSAs ......................................... 117
    12.4.3.1 Originating summary-LSAs into stub areas ............. 119
    12.4.3.2 Examples of summary-LSAs ............................. 119
    12.4.4   AS-external-LSAs ..................................... 120
    12.4.4.1 Examples of AS-external-LSAs ......................... 121
    13       The Flooding Procedure ............................... 122
    13.1     Determining which LSA is newer ....................... 126
    13.2     Installing LSAs in the database ...................... 127
    13.3     Next step in the flooding procedure .................. 128
    13.4     Receiving self-originated LSAs ....................... 130
    13.5     Sending Link State Acknowledgment packets ............ 131
    13.6     Retransmitting LSAs .................................. 133
    13.7     Receiving link state acknowledgments ................. 134
    14       Aging The Link State Database ........................ 134
    14.1     Premature aging of LSAs .............................. 135
    15       Virtual Links ........................................ 135
    16       Calculation of the routing table ..................... 137
    16.1     Calculating the shortest-path tree for an area ....... 138
    16.1.1   The next hop calculation ............................. 144
    16.2     Calculating the inter-area routes .................... 145
    16.3     Examining transit areas' summary-LSAs ................ 146
    16.4     Calculating AS external routes ....................... 149
    16.4.1   External path preferences ............................ 151
    16.5     Incremental updates -- summary-LSAs .................. 151



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    16.6     Incremental updates -- AS-external-LSAs .............. 152
    16.7     Events generated as a result of routing table changes  153
    16.8     Equal-cost multipath ................................. 154
             Footnotes ............................................ 155
             References ........................................... 158
    A        OSPF data formats .................................... 160
    A.1      Encapsulation of OSPF packets ........................ 160
    A.2      The Options field .................................... 162
    A.3      OSPF Packet Formats .................................. 163
    A.3.1    The OSPF packet header ............................... 164
    A.3.2    The Hello packet ..................................... 166
    A.3.3    The Database Description packet ...................... 168
    A.3.4    The Link State Request packet ........................ 170
    A.3.5    The Link State Update packet ......................... 171
    A.3.6    The Link State Acknowledgment packet ................. 172
    A.4      LSA formats .......................................... 173
    A.4.1    The LSA header ....................................... 174
    A.4.2    Router-LSAs .......................................... 176
    A.4.3    Network-LSAs ......................................... 179
    A.4.4    Summary-LSAs ......................................... 180
    A.4.5    AS-external-LSAs ..................................... 182
    B        Architectural Constants .............................. 184
    C        Configurable Constants ............................... 186
    C.1      Global parameters .................................... 186
    C.2      Area parameters ...................................... 187
    C.3      Router interface parameters .......................... 188
    C.4      Virtual link parameters .............................. 190
    C.5      NBMA network parameters .............................. 191
    C.6      Point-to-MultiPoint network parameters ............... 191
    C.7      Host route parameters ................................ 192
    D        Authentication ....................................... 193
    D.1      Null authentication .................................. 193
    D.2      Simple password authentication ....................... 193
    D.3      Cryptographic authentication ......................... 194
    D.4      Message generation ................................... 196
    D.4.1    Generating Null authentication ....................... 196
    D.4.2    Generating Simple password authentication ............ 197
    D.4.3    Generating Cryptographic authentication .............. 197
    D.5      Message verification ................................. 198
    D.5.1    Verifying Null authentication ........................ 199
    D.5.2    Verifying Simple password authentication ............. 199
    D.5.3    Verifying Cryptographic authentication ............... 199
    E        An algorithm for assigning Link State IDs ............ 201
    F        Multiple interfaces to the same network/subnet ....... 203
    G        Differences from RFC 1583 ............................ 204
    G.1      Enhancements to OSPF authentication .................. 204
    G.2      Addition of Point-to-MultiPoint interface ............ 204
    G.3      Support for overlapping area ranges .................. 205



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    G.4      A modification to the flooding algorithm ............. 206
    G.5      Introduction of the MinLSArrival constant ............ 206
    G.6      Optionally advertising point-to-point links as subnets 207
    G.7      Advertising same external route from multiple areas .. 207
    G.8      Retransmission of initial Database Description packets 209
    G.9      Detecting interface MTU mismatches ................... 209
    G.10     Deleting the TOS routing option ...................... 209
             Security Considerations .............................. 210
             Author's Address ..................................... 211

1.  Introduction



   This document is a specification of the Open Shortest Path First
   (OSPF) TCP/IP internet routing protocol.  OSPF is classified as an
   Interior Gateway Protocol (IGP).  This means that it distributes
   routing information between routers belonging to a single Autonomous
   System.  The OSPF protocol is based on link-state or SPF technology.
   This is a departure from the Bellman-Ford base used by traditional
   TCP/IP internet routing protocols.

   The OSPF protocol was developed by the OSPF working group of the
   Internet Engineering Task Force.  It has been designed expressly for
   the TCP/IP internet environment, including explicit support for CIDR
   and the tagging of externally-derived routing information. OSPF also
   provides for the authentication of routing updates, and utilizes IP
   multicast when sending/receiving the updates.  In addition, much work
   has been done to produce a protocol that responds quickly to topology
   changes, yet involves small amounts of routing protocol traffic.

1.1.  Protocol overview



   OSPF routes IP packets based solely on the destination IP address
   found in the IP packet header. IP packets are routed "as is" -- they
   are not encapsulated in any further protocol headers as they transit
   the Autonomous System. OSPF is a dynamic routing protocol.  It
   quickly detects topological changes in the AS (such as router
   interface failures) and calculates new loop-free routes after a
   period of convergence.  This period of convergence is short and
   involves a minimum of routing traffic.

   In a link-state routing protocol, each router maintains a database
   describing the Autonomous System's topology.  This database is
   referred to as the link-state database. Each participating router has
   an identical database.  Each individual piece of this database is a
   particular router's local state (e.g., the router's usable interfaces
   and reachable neighbors).  The router distributes its local state
   throughout the Autonomous System by flooding.




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   All routers run the exact same algorithm, in parallel. From the
   link-state database, each router constructs a tree of shortest paths
   with itself as root.  This shortest-path tree gives the route to each
   destination in the Autonomous System.  Externally derived routing
   information appears on the tree as leaves.

   When several equal-cost routes to a destination exist, traffic is
   distributed equally among them.  The cost of a route is described by
   a single dimensionless metric.

   OSPF allows sets of networks to be grouped together.  Such a grouping
   is called an area.  The topology of an area is hidden from the rest
   of the Autonomous System.  This information hiding enables a
   significant reduction in routing traffic.  Also, routing within the
   area is determined only by the area's own topology, lending the area
   protection from bad routing data.  An area is a generalization of an
   IP subnetted network.

   OSPF enables the flexible configuration of IP subnets.  Each route
   distributed by OSPF has a destination and mask.  Two different
   subnets of the same IP network number may have different sizes (i.e.,
   different masks).  This is commonly referred to as variable length
   subnetting.  A packet is routed to the best (i.e., longest or most
   specific) match.  Host routes are considered to be subnets whose
   masks are "all ones" (0xffffffff).

   All OSPF protocol exchanges are authenticated.  This means that only
   trusted routers can participate in the Autonomous System's routing.
   A variety of authentication schemes can be used; in fact, separate
   authentication schemes can be configured for each IP subnet.

   Externally derived routing data (e.g., routes learned from an
   Exterior Gateway Protocol such as BGP; see [Ref23]) is advertised
   throughout the Autonomous System.  This externally derived data is
   kept separate from the OSPF protocol's link state data.  Each
   external route can also be tagged by the advertising router, enabling
   the passing of additional information between routers on the boundary
   of the Autonomous System.

1.2.  Definitions of commonly used terms



   This section provides definitions for terms that have a specific
   meaning to the OSPF protocol and that are used throughout the text.
   The reader unfamiliar with the Internet Protocol Suite is referred to
   [Ref13] for an introduction to IP.






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   Router
      A level three Internet Protocol packet switch.  Formerly called a
      gateway in much of the IP literature.

   Autonomous System
      A group of routers exchanging routing information via a common
      routing protocol.  Abbreviated as AS.

   Interior Gateway Protocol
      The routing protocol spoken by the routers belonging to an
      Autonomous system. Abbreviated as IGP.  Each Autonomous System has
      a single IGP.  Separate Autonomous Systems may be running
      different IGPs.

   Router ID
      A 32-bit number assigned to each router running the OSPF protocol.
      This number uniquely identifies the router within an Autonomous
      System.

   Network
      In this memo, an IP network/subnet/supernet.  It is possible for
      one physical network to be assigned multiple IP network/subnet
      numbers.  We consider these to be separate networks.  Point-to-
      point physical networks are an exception - they are considered a
      single network no matter how many (if any at all) IP
      network/subnet numbers are assigned to them.

   Network mask
      A 32-bit number indicating the range of IP addresses residing on a
      single IP network/subnet/supernet.  This specification displays
      network masks as hexadecimal numbers.  For example, the network
      mask for a class C IP network is displayed as 0xffffff00.  Such a
      mask is often displayed elsewhere in the literature as
      255.255.255.0.

   Point-to-point networks
      A network that joins a single pair of routers.  A 56Kb serial line
      is an example of a point-to-point network.













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   Broadcast networks
      Networks supporting many (more than two) attached routers,
      together with the capability to address a single physical message
      to all of the attached routers (broadcast).  Neighboring routers
      are discovered dynamically on these nets using OSPF's Hello
      Protocol.  The Hello Protocol itself takes advantage of the
      broadcast capability.  The OSPF protocol makes further use of
      multicast capabilities, if they exist.  Each pair of routers on a
      broadcast network is assumed to be able to communicate directly.
      An ethernet is an example of a broadcast network.

   Non-broadcast networks
      Networks supporting many (more than two) routers, but having no
      broadcast capability.  Neighboring routers are maintained on these
      nets using OSPF's Hello Protocol. However, due to the lack of
      broadcast capability, some configuration information may be
      necessary to aid in the discovery of neighbors. On non-broadcast
      networks, OSPF protocol packets that are normally multicast need
      to be sent to each neighboring router, in turn. An X.25 Public
      Data Network (PDN) is an example of a non-broadcast network.

      OSPF runs in one of two modes over non-broadcast networks.  The
      first mode, called non-broadcast multi-access or NBMA, simulates
      the operation of OSPF on a broadcast network. The second mode,
      called Point-to-MultiPoint, treats the non-broadcast network as a
      collection of point-to-point links.  Non-broadcast networks are
      referred to as NBMA networks or Point-to-MultiPoint networks,
      depending on OSPF's mode of operation over the network.

   Interface
      The connection between a router and one of its attached networks.
      An interface has state information associated with it, which is
      obtained from the underlying lower level protocols and the routing
      protocol itself.  An interface to a network has associated with it
      a single IP address and mask (unless the network is an unnumbered
      point-to-point network).  An interface is sometimes also referred
      to as a link.

   Neighboring routers
      Two routers that have interfaces to a common network.  Neighbor
      relationships are maintained by, and usually dynamically
      discovered by, OSPF's Hello Protocol.

   Adjacency
      A relationship formed between selected neighboring routers for the
      purpose of exchanging routing information.  Not every pair of
      neighboring routers become adjacent.




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   Link state advertisement
      Unit of data describing the local state of a router or network.
      For a router, this includes the state of the router's interfaces
      and adjacencies.  Each link state advertisement is flooded
      throughout the routing domain. The collected link state
      advertisements of all routers and networks forms the protocol's
      link state database.  Throughout this memo, link state
      advertisement is abbreviated as LSA.

   Hello Protocol
      The part of the OSPF protocol used to establish and maintain
      neighbor relationships.  On broadcast networks the Hello Protocol
      can also dynamically discover neighboring routers.

   Flooding
      The part of the OSPF protocol that distributes and synchronizes
      the link-state database between OSPF routers.

   Designated Router
      Each broadcast and NBMA network that has at least two attached
      routers has a Designated Router.  The Designated Router generates
      an LSA for the network and has other special responsibilities in
      the running of the protocol.  The Designated Router is elected by
      the Hello Protocol.

      The Designated Router concept enables a reduction in the number of
      adjacencies required on a broadcast or NBMA network.  This in turn
      reduces the amount of routing protocol traffic and the size of the
      link-state database.

   Lower-level protocols
      The underlying network access protocols that provide services to
      the Internet Protocol and in turn the OSPF protocol.  Examples of
      these are the X.25 packet and frame levels for X.25 PDNs, and the
      ethernet data link layer for ethernets.

1.3.  Brief history of link-state routing technology



   OSPF is a link state routing protocol.  Such protocols are also
   referred to in the literature as SPF-based or distributed-database
   protocols.  This section gives a brief description of the
   developments in link-state technology that have influenced the OSPF
   protocol.

   The first link-state routing protocol was developed for use in the
   ARPANET packet switching network.  This protocol is described in
   [Ref3].  It has formed the starting point for all other link-state
   protocols.  The homogeneous ARPANET environment, i.e., single-vendor



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   packet switches connected by synchronous serial lines, simplified the
   design and implementation of the original protocol.

   Modifications to this protocol were proposed in [Ref4].  These
   modifications dealt with increasing the fault tolerance of the
   routing protocol through, among other things, adding a checksum to
   the LSAs (thereby detecting database corruption).  The paper also
   included means for reducing the routing traffic overhead in a link-
   state protocol.  This was accomplished by introducing mechanisms
   which enabled the interval between LSA originations to be increased
   by an order of magnitude.

   A link-state algorithm has also been proposed for use as an ISO IS-IS
   routing protocol.  This protocol is described in [Ref2].  The
   protocol includes methods for data and routing traffic reduction when
   operating over broadcast networks.  This is accomplished by election
   of a Designated Router for each broadcast network, which then
   originates an LSA for the network.

   The OSPF Working Group of the IETF has extended this work in
   developing the OSPF protocol.  The Designated Router concept has been
   greatly enhanced to further reduce the amount of routing traffic
   required.  Multicast capabilities are utilized for additional routing
   bandwidth reduction.  An area routing scheme has been developed
   enabling information hiding/protection/reduction.  Finally, the
   algorithms have been tailored for efficient operation in TCP/IP
   internets.

1.4.  Organization of this document



   The first three sections of this specification give a general
   overview of the protocol's capabilities and functions.  Sections 4-16
   explain the protocol's mechanisms in detail.  Packet formats,
   protocol constants and configuration items are specified in the
   appendices.

   Labels such as HelloInterval encountered in the text refer to
   protocol constants.  They may or may not be configurable.
   Architectural constants are summarized in Appendix B.  Configurable
   constants are summarized in Appendix C.

   The detailed specification of the protocol is presented in terms of
   data structures.  This is done in order to make the explanation more
   precise.  Implementations of the protocol are required to support the
   functionality described, but need not use the precise data structures
   that appear in this memo.





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1.5.  Acknowledgments



   The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
   Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
   Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui Zhang
   and the rest of the OSPF Working Group for the ideas and support they
   have given to this project.

   The OSPF Point-to-MultiPoint interface is based on work done by Fred
   Baker.

   The OSPF Cryptographic Authentication option was developed by Fred
   Baker and Ran Atkinson.

2.  The Link-state Database: organization and calculations



   The following subsections describe the organization of OSPF's link-
   state database, and the routing calculations that are performed on
   the database in order to produce a router's routing table.

2.1.  Representation of routers and networks



   The Autonomous System's link-state database describes a directed
   graph.  The vertices of the graph consist of routers and networks.  A
   graph edge connects two routers when they are attached via a physical
   point-to-point network.  An edge connecting a router to a network
   indicates that the router has an interface on the network. Networks
   can be either transit or stub networks. Transit networks are those
   capable of carrying data traffic that is neither locally originated
   nor locally destined. A transit network is represented by a graph
   vertex having both incoming and outgoing edges. A stub network's
   vertex has only incoming edges.

   The neighborhood of each network node in the graph depends on the
   network's type (point-to-point, broadcast, NBMA or Point-to-
   MultiPoint) and the number of routers having an interface to the
   network.  Three cases are depicted in Figure 1a.  Rectangles indicate
   routers.  Circles and oblongs indicate networks.  Router names are
   prefixed with the letters RT and network names with the letter N.
   Router interface names are prefixed by the letter I.  Lines between
   routers indicate point-to-point networks.  The left side of the
   figure shows networks with their connected routers, with the
   resulting graphs shown on the right.








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                                                  **FROM**

                                           *      |RT1|RT2|
                +---+Ia    +---+           *   ------------
                |RT1|------|RT2|           T   RT1|   | X |
                +---+    Ib+---+           O   RT2| X |   |
                                           *    Ia|   | X |
                                           *    Ib| X |   |

                    Physical point-to-point networks

                                                  **FROM**
                      +---+                *
                      |RT7|                *      |RT7| N3|
                      +---+                T   ------------
                        |                  O   RT7|   |   |
            +----------------------+       *    N3| X |   |
                       N3                  *

                             Stub networks

                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|N2 |
                +---+      +---+        *  ------------------------
                  |    N2    |          *  RT3|   |   |   |   | X |
            +----------------------+    T  RT4|   |   |   |   | X |
                  |          |          O  RT5|   |   |   |   | X |
                +---+      +---+        *  RT6|   |   |   |   | X |
                |RT5|      |RT6|        *   N2| X | X | X | X |   |
                +---+      +---+

                       Broadcast or NBMA networks

                   Figure 1a: Network map components

   Networks and routers are represented by vertices.  An edge connects
   Vertex A to Vertex B iff the intersection of Column A and Row B is
   marked with an X.

   The top of Figure 1a shows two routers connected by a point-to-point
   link. In the resulting link-state database graph, the two router
   vertices are directly connected by a pair of edges, one in each
   direction. Interfaces to point-to-point networks need not be assigned
   IP addresses.  When interface addresses are assigned, they are
   modelled as stub links, with each router advertising a stub
   connection to the other router's interface address. Optionally, an IP





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RFC 2178                     OSPF Version 2                    July 1997


   subnet can be assigned to the point-to-point network. In this case,
   both routers advertise a stub link to the IP subnet, instead of
   advertising each others' IP interface addresses.

   The middle of Figure 1a shows a network with only one attached router
   (i.e., a stub network). In this case, the network appears on the end
   of a stub connection in the link-state database's graph.

   When multiple routers are attached to a broadcast network, the link-
   state database graph shows all routers bidirectionally connected to
   the network vertex. This is pictured at the bottom of Figure 1a.

   Each network (stub or transit) in the graph has an IP address and
   associated network mask.  The mask indicates the number of nodes on
   the network.  Hosts attached directly to routers (referred to as host
   routes) appear on the graph as stub networks.  The network mask for a
   host route is always 0xffffffff, which indicates the presence of a
   single node.

2.1.1. Representation of non-broadcast networks



   As mentioned previously, OSPF can run over non-broadcast networks in
   one of two modes: NBMA or Point-to-MultiPoint.  The choice of mode
   determines the way that the Hello protocol and flooding work over the
   non-broadcast network, and the way that the network is represented in
   the link-state database.

   In NBMA mode, OSPF emulates operation over a broadcast network: a
   Designated Router is elected for the NBMA network, and the Designated
   Router originates an LSA for the network. The graph representation
   for broadcast networks and NBMA networks is identical. This
   representation is pictured in the middle of Figure 1a.

   NBMA mode is the most efficient way to run OSPF over non-broadcast
   networks, both in terms of link-state database size and in terms of
   the amount of routing protocol traffic.  However, it has one
   significant restriction: it requires all routers attached to the NBMA
   network to be able to communicate directly. This restriction may be
   met on some non-broadcast networks, such as an ATM subnet utilizing
   SVCs. But it is often not met on other non-broadcast networks, such
   as PVC-only Frame Relay networks. On non-broadcast networks where not
   all routers can communicate directly you can break the non-broadcast
   network into logical subnets, with the routers on each subnet being
   able to communicate directly, and then run each separate subnet as an
   NBMA network (see [Ref15]). This however requires quite a bit of
   administrative overhead, and is prone to misconfiguration. It is
   probably better to run such a non-broadcast network in Point-to-
   Multipoint mode.



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RFC 2178                     OSPF Version 2                    July 1997


   In Point-to-MultiPoint mode, OSPF treats all router-to-router
   connections over the non-broadcast network as if they were point-to-
   point links. No Designated Router is elected for the network, nor is
   there an LSA generated for the network. In fact, a vertex for the
   Point-to-MultiPoint network does not appear in the graph of the
   link-state database.

   Figure 1b illustrates the link-state database representation of a
   Point-to-MultiPoint network. On the left side of the figure, a
   Point-to-MultiPoint network is pictured. It is assumed that all
   routers can communicate directly, except for routers RT4 and RT5. I3
   though I6 indicate the routers' IP interface addresses on the Point-
   to-MultiPoint network.  In the graphical representation of the link-
   state database, routers that can communicate directly over the
   Point-to-MultiPoint network are joined by bidirectional edges, and
   each router also has a stub connection to its own IP interface
   address (which is in contrast to the representation of real point-
   to-point links; see Figure 1a).

   On some non-broadcast networks, use of Point-to-MultiPoint mode and
   data-link protocols such as Inverse ARP (see [Ref14]) will allow
   autodiscovery of OSPF neighbors even though broadcast support is not
   available.

2.1.2.  An example link-state database



   Figure 2 shows a sample map of an Autonomous System.  The rectangle
   labelled H1 indicates a host, which has a SLIP connection to Router
   RT12. Router RT12 is therefore advertising a host route.  Lines
   between routers indicate physical point-to-point networks.  The only
   point-to-point network that has been assigned interface addresses is
   the one joining Routers RT6 and RT10.  Routers RT5 and RT7 have BGP
   connections to other Autonomous Systems.  A set of BGP-learned routes
   have been displayed for both of these routers.

   A cost is associated with the output side of each router interface.
   This cost is configurable by the system administrator.  The lower the
   cost,the more likely the interface is to be used to forward data
   traffic.  Costs are also associated with the externally derived
   routing data (e.g., the BGP-learned routes).

   The directed graph resulting from the map in Figure 2 is depicted in
   Figure 3.  Arcs are labelled with the cost of the corresponding
   router output interface. Arcs having no labelled cost have a cost of
   0.  Note that arcs leading from networks to routers always have cost
   0; they are significant nonetheless.  Note also that the externally
   derived routing data appears on the graph as stubs.




Moy                         Standards Track                    [Page 14]

RFC 2178                     OSPF Version 2                    July 1997


                                                  **FROM**
                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|
                +---+      +---+        *  --------------------
                I3|    N2    |I4        *  RT3|   | X | X | X |
            +----------------------+    T  RT4| X |   |   | X |
                I5|          |I6        O  RT5| X |   |   | X |
                +---+      +---+        *  RT6| X | X | X |   |
                |RT5|      |RT6|        *   I3| X |   |   |   |
                +---+      +---+            I4|   | X |   |   |
                                            I5|   |   | X |   |
                                            I6|   |   |   | X |


                   Figure 1b: Network map components
                      Point-to-MultiPoint networks

          All routers can communicate directly over N2, except
             routers RT4 and RT5. I3 through I6 indicate IP
                          interface addresses































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RFC 2178                     OSPF Version 2                    July 1997


                 +
                 | 3+---+                     N12      N14
               N1|--|RT1|\ 1                    \ N13 /
                 |  +---+ \                     8\ |8/8
                 +         \ ____                 \|/
                            /    \   1+---+8    8+---+6
                           *  N3  *---|RT4|------|RT5|--------+
                            \____/    +---+      +---+        |
                  +         /   |                  |7         |
                  | 3+---+ /    |                  |          |
                N2|--|RT2|/1    |1                 |6         |
                  |  +---+    +---+8            6+---+        |
                  +           |RT3|--------------|RT6|        |
                              +---+              +---+        |
                                |2               Ia|7         |
                                |                  |          |
                           +---------+             |          |
                               N4                  |          |
                                                   |          |
                                                   |          |
                       N11                         |          |
                   +---------+                     |          |
                        |                          |          |    N12
                        |3                         |          |6 2/
                      +---+                        |        +---+/
                      |RT9|                        |        |RT7|---N15
                      +---+                        |        +---+ 9
                        |1                   +     |          |1
                       _|__                  |   Ib|5       __|_
                      /    \      1+----+2   |  3+----+1   /    \
                     *  N9  *------|RT11|----|---|RT10|---*  N6  *
                      \____/       +----+    |   +----+    \____/
                        |                    |                |
                        |1                   +                |1
             +--+   10+----+                N8              +---+
             |H1|-----|RT12|                                |RT8|
             +--+SLIP +----+                                +---+
                        |2                                    |4
                        |                                     |
                   +---------+                            +--------+
                       N10                                    N7

                  Figure 2: A sample Autonomous System








Moy                         Standards Track                    [Page 16]

RFC 2178                     OSPF Version 2                    July 1997


                                **FROM**

                 |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
                 |1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
              ----- ---------------------------------------------
              RT1|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT2|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT3|  |  |  |  |  |6 |  |  |  |  |  |  |0 |  |  |  |
              RT4|  |  |  |  |8 |  |  |  |  |  |  |  |0 |  |  |  |
              RT5|  |  |  |8 |  |6 |6 |  |  |  |  |  |  |  |  |  |
              RT6|  |  |8 |  |7 |  |  |  |  |5 |  |  |  |  |  |  |
              RT7|  |  |  |  |6 |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT8|  |  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT9|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          T  RT10|  |  |  |  |  |7 |  |  |  |  |  |  |  |0 |0 |  |
          O  RT11|  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |0 |
          *  RT12|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          *    N1|3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N2|  |3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N3|1 |1 |1 |1 |  |  |  |  |  |  |  |  |  |  |  |  |
               N4|  |  |2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N6|  |  |  |  |  |  |1 |1 |  |1 |  |  |  |  |  |  |
               N7|  |  |  |  |  |  |  |4 |  |  |  |  |  |  |  |  |
               N8|  |  |  |  |  |  |  |  |  |3 |2 |  |  |  |  |  |
               N9|  |  |  |  |  |  |  |  |1 |  |1 |1 |  |  |  |  |
              N10|  |  |  |  |  |  |  |  |  |  |  |2 |  |  |  |  |
              N11|  |  |  |  |  |  |  |  |3 |  |  |  |  |  |  |  |
              N12|  |  |  |  |8 |  |2 |  |  |  |  |  |  |  |  |  |
              N13|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N14|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N15|  |  |  |  |  |  |9 |  |  |  |  |  |  |  |  |  |
               H1|  |  |  |  |  |  |  |  |  |  |  |10|  |  |  |  |


                 Figure 3: The resulting directed graph

           Networks and routers are represented by vertices.
          An edge of cost X connects Vertex A to Vertex B iff
            the intersection of Column A and Row B is marked
                               with an X.

   The link-state database is pieced together from LSAs generated by the
   routers.  In the associated graphical representation, the
   neighborhood of each router or transit network is represented in a
   single, separate LSA.  Figure 4 shows these LSAs graphically. Router
   RT12 has an interface to two broadcast networks and a SLIP line to a
   host.  Network N6 is a broadcast network with three attached routers.
   The cost of all links from Network N6 to its attached routers is 0.



Moy                         Standards Track                    [Page 17]

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   Note that the LSA for Network N6 is actually generated by one of the
   network's attached routers: the router that has been elected
   Designated Router for the network.

2.2.  The shortest-path tree



   When no OSPF areas are configured, each router in the Autonomous
   System has an identical link-state database, leading to an identical
   graphical representation.  A router generates its routing table from
   this graph by calculating a tree of shortest paths with the router
   itself as root.  Obviously, the shortest- path tree depends on the
   router doing the calculation.  The shortest-path tree for Router RT6
   in our example is depicted in Figure 5.

   The tree gives the entire path to any destination network or host.
   However, only the next hop to the destination is used in the
   forwarding process.   Note also that the best route to any router has
   also been calculated.  For the processing of external data, we note
   the next hop and distance to any router advertising external routes.
   The resulting routing table for Router RT6 is pictured in Table 2.
   Note that there is a separate route for each end of a numbered
   point-to-point network (in this case, the serial line between Routers
   RT6 and RT10).


                     **FROM**                       **FROM**

                  |RT12|N9|N10|H1|                 |RT9|RT11|RT12|N9|
           *  --------------------          *  ----------------------
           *  RT12|    |  |   |  |          *   RT9|   |    |    |0 |
           T    N9|1   |  |   |  |          T  RT11|   |    |    |0 |
           O   N10|2   |  |   |  |          O  RT12|   |    |    |0 |
           *    H1|10  |  |   |  |          *    N9|   |    |    |  |
           *                                *
                RT12's router-LSA              N9's network-LSA

               Figure 4: Individual link state components

           Networks and routers are represented by vertices.
          An edge of cost X connects Vertex A to Vertex B iff
            the intersection of Column A and Row B is marked
                               with an X.









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RFC 2178                     OSPF Version 2                    July 1997


                                RT6(origin)
                    RT5 o------------o-----------o Ib
                       /|\    6      |\     7
                     8/8|8\          | \
                     /  |  \        6|  \
                    o   |   o        |   \7
                   N12  o  N14       |    \
                       N13        2  |     \
                            N4 o-----o RT3  \
                                    /        \    5
                                  1/     RT10 o-------o Ia
                                  /           |\
                       RT4 o-----o N3        3| \1
                                /|            |  \ N6     RT7
                               / |         N8 o   o---------o
                              /  |            |   |        /|
                         RT2 o   o RT1        |   |      2/ |9
                            /    |            |   |RT8   /  |
                           /3    |3      RT11 o   o     o   o
                          /      |            |   |    N12 N15
                      N2 o       o N1        1|   |4
                                              |   |
                                           N9 o   o N7
                                             /|
                                            / |
                        N11      RT9       /  |RT12
                         o--------o-------o   o--------o H1
                             3                |   10
                                              |2
                                              |
                                              o N10


                 Figure 5: The SPF tree for Router RT6

  Edges that are not marked with a cost have a cost of of zero (these
 are network-to-router links). Routes to networks N12-N15 are external
             information that is considered in Section 2.3













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           Destination   Next  Hop   Distance
           __________________________________
           N1            RT3         10
           N2            RT3         10
           N3            RT3         7
           N4            RT3         8
           Ib            *           7
           Ia            RT10        12
           N6            RT10        8
           N7            RT10        12
           N8            RT10        10
           N9            RT10        11
           N10           RT10        13
           N11           RT10        14
           H1            RT10        21
           __________________________________
           RT5           RT5         6
           RT7           RT10        8

    Table 2: The portion of Router RT6's routing table listing local
                             destinations.

   Routes to networks belonging to other AS'es (such as N12) appear as
   dashed lines on the shortest path tree in Figure 5.  Use of this
   externally derived routing information is considered in the next
   section.

2.3.  Use of external routing information



   After the tree is created the external routing information is
   examined.  This external routing information may originate from
   another routing protocol such as BGP, or be statically configured
   (static routes).  Default routes can also be included as part of the
   Autonomous System's external routing information.

   External routing information is flooded unaltered throughout the AS.
   In our example, all the routers in the Autonomous System know that
   Router RT7 has two external routes, with metrics 2 and 9.

   OSPF supports two types of external metrics.  Type 1 external metrics
   are expressed in the same units as OSPF interface cost (i.e., in
   terms of the link state metric).  Type 2 external metrics are an
   order of magnitude larger; any Type 2 metric is considered greater
   than the cost of any path internal to the AS.  Use of Type 2 external
   metrics assumes that routing between AS'es is the major cost of
   routing a packet, and eliminates the need for conversion of external
   costs to internal link state metrics.




Moy                         Standards Track                    [Page 20]

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   As an example of Type 1 external metric processing, suppose that the
   Routers RT7 and RT5 in Figure 2 are advertising Type 1 external
   metrics.  For each advertised external route, the total cost from
   Router RT6 is calculated as the sum of the external route's
   advertised cost and the distance from Router RT6 to the advertising
   router.  When two routers are advertising the same external
   destination, RT6 picks the advertising router providing the minimum
   total cost. RT6 then sets the next hop to the external destination
   equal to the next hop that would be used when routing packets to the
   chosen advertising router.

   In Figure 2, both Router RT5 and RT7 are advertising an external
   route to destination Network N12.  Router RT7 is preferred since it
   is advertising N12 at a distance of 10 (8+2) to Router RT6, which is
   better than Router RT5's 14 (6+8).  Table 3 shows the entries that
   are added to the routing table when external routes are examined:



                 Destination   Next  Hop   Distance
                 __________________________________
                 N12           RT10        10
                 N13           RT5         14
                 N14           RT5         14
                 N15           RT10        17


          Table 3: The portion of Router RT6's routing table
                     listing external destinations.

   Processing of Type 2 external metrics is simpler.  The AS boundary
   router advertising the smallest external metric is chosen, regardless
   of the internal distance to the AS boundary router.  Suppose in our
   example both Router RT5 and Router RT7 were advertising Type 2
   external routes.  Then all traffic destined for Network N12 would be
   forwarded to Router RT7, since 2 < 8. When several equal-cost Type 2
   routes exist, the internal distance to the advertising routers is
   used to break the tie.

   Both Type 1 and Type 2 external metrics can be present in the AS at
   the same time.  In that event, Type 1 external metrics always take
   precedence.

   This section has assumed that packets destined for external
   destinations are always routed through the advertising AS boundary
   router.  This is not always desirable.  For example, suppose in
   Figure 2 there is an additional router attached to Network N6, called
   Router RTX. Suppose further that RTX does not participate in OSPF



Moy                         Standards Track                    [Page 21]

RFC 2178                     OSPF Version 2                    July 1997


   routing, but does exchange BGP information with the AS boundary
   router RT7.  Then, Router RT7 would end up advertising OSPF external
   routes for all destinations that should be routed to RTX.  An extra
   hop will sometimes be introduced if packets for these destinations
   need always be routed first to Router RT7 (the advertising router).

   To deal with this situation, the OSPF protocol allows an AS boundary
   router to specify a "forwarding address" in its AS- external-LSAs. In
   the above example, Router RT7 would specify RTX's IP address as the
   "forwarding address" for all those destinations whose packets should
   be routed directly to RTX.

   The "forwarding address" has one other application.  It enables
   routers in the Autonomous System's interior to function as "route
   servers".  For example, in Figure 2 the router RT6 could become a
   route server, gaining external routing information through a
   combination of static configuration and external routing protocols.
   RT6 would then start advertising itself as an AS boundary router, and
   would originate a collection of OSPF AS-external-LSAs.  In each AS-
   external-LSA, Router RT6 would specify the correct Autonomous System
   exit point to use for the destination through appropriate setting of
   the LSA's "forwarding address" field.

2.4.  Equal-cost multipath



   The above discussion has been simplified by considering only a single
   route to any destination.  In reality, if multiple equal-cost routes
   to a destination exist, they are all discovered and used.  This
   requires no conceptual changes to the algorithm, and its discussion
   is postponed until we consider the tree-building process in more
   detail.

   With equal cost multipath, a router potentially has several available
   next hops towards any given destination.

3.  Splitting the AS into Areas



   OSPF allows collections of contiguous networks and hosts to be
   grouped together.  Such a group, together with the routers having
   interfaces to any one of the included networks, is called an area.
   Each area runs a separate copy of the basic link-state routing
   algorithm. This means that each area has its own link-state database
   and corresponding graph, as explained in the previous section.

   The topology of an area is invisible from the outside of the area.
   Conversely, routers internal to a given area know nothing of the
   detailed topology external to the area.  This isolation of knowledge
   enables the protocol to effect a marked reduction in routing traffic



Moy                         Standards Track                    [Page 22]

RFC 2178                     OSPF Version 2                    July 1997


   as compared to treating the entire Autonomous System as a single
   link-state domain.

   With the introduction of areas, it is no longer true that all routers
   in the AS have an identical link-state database.  A router actually
   has a separate link-state database for each area it is connected to.
   (Routers connected to multiple areas are called area border routers).
   Two routers belonging to the same area have, for that area, identical
   area link-state databases.

   Routing in the Autonomous System takes place on two levels, depending
   on whether the source and destination of a packet reside in the same
   area (intra-area routing is used) or different areas (inter-area
   routing is used).  In intra-area routing, the packet is routed solely
   on information obtained within the area; no routing information
   obtained from outside the area can be used.  This protects intra-area
   routing from the injection of bad routing information.  We discuss
   inter-area routing in Section 3.2.

3.1.  The backbone of the Autonomous System



   The OSPF backbone is the special OSPF Area 0 (often written as Area
   0.0.0.0, since OSPF Area ID's are typically formatted as IP
   addresses). The OSPF backbone always contains all area border
   routers. The backbone is responsible for distributing routing
   information between non-backbone areas. The backbone must be
   contiguous. However, it need not be physically contiguous; backbone
   connectivity can be established/maintained through the configuration
   of virtual links.

   Virtual links can be configured between any two backbone routers that
   have an interface to a common non-backbone area.  Virtual links
   belong to the backbone.  The protocol treats two routers joined by a
   virtual link as if they were connected by an unnumbered point-to-
   point backbone network.  On the graph of the backbone, two such
   routers are joined by arcs whose costs are the intra-area distances
   between the two routers.  The routing protocol traffic that flows
   along the virtual link uses intra-area routing only.

3.2.  Inter-area routing



   When routing a packet between two non-backbone areas the backbone is
   used.  The path that the packet will travel can be broken up into
   three contiguous pieces: an intra-area path from the source to an
   area border router, a backbone path between the source and
   destination areas, and then another intra-area path to the
   destination.  The algorithm finds the set of such paths that have the
   smallest cost.



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   Looking at this another way, inter-area routing can be pictured as
   forcing a star configuration on the Autonomous System, with the
   backbone as hub and each of the non-backbone areas as spokes.

   The topology of the backbone dictates the backbone paths used between
   areas.  The topology of the backbone can be enhanced by adding
   virtual links.  This gives the system administrator some control over
   the routes taken by inter-area traffic.

   The correct area border router to use as the packet exits the source
   area is chosen in exactly the same way routers advertising external
   routes are chosen.  Each area border router in an area summarizes for
   the area its cost to all networks external to the area.  After the
   SPF tree is calculated for the area, routes to all inter-area
   destinations are calculated by examining the summaries of the area
   border routers.

3.3.  Classification of routers



   Before the introduction of areas, the only OSPF routers having a
   specialized function were those advertising external routing
   information, such as Router RT5 in Figure 2.  When the AS is split
   into OSPF areas, the routers are further divided according to
   function into the following four overlapping categories:


   Internal routers
      A router with all directly connected networks belonging to the
      same area. These routers run a single copy of the basic routing
      algorithm.

   Area border routers
      A router that attaches to multiple areas.  Area border routers run
      multiple copies of the basic algorithm, one copy for each attached
      area. Area border routers condense the topological information of
      their attached areas for distribution to the backbone.  The
      backbone in turn distributes the information to the other areas.

   Backbone routers
      A router that has an interface to the backbone area.  This
      includes all routers that interface to more than one area (i.e.,
      area border routers).  However, backbone routers do not have to be
      area border routers.  Routers with all interfaces connecting to
      the backbone area are supported.







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   AS boundary routers
      A router that exchanges routing information with routers belonging
      to other Autonomous Systems.  Such a router advertises AS external
      routing information throughout the Autonomous System.  The paths
      to each AS boundary router are known by every router in the AS.
      This classification is completely independent of the previous
      classifications: AS boundary routers may be internal or area
      border routers, and may or may not participate in the backbone.

3.4.  A sample area configuration



   Figure 6 shows a sample area configuration.  The first area consists
   of networks N1-N4, along with their attached routers RT1-RT4.  The
   second area consists of networks N6-N8, along with their attached
   routers RT7, RT8, RT10 and RT11.  The third area consists of networks
   N9-N11 and Host H1, along with their attached routers RT9, RT11 and
   RT12.  The third area has been configured so that networks N9-N11 and
   Host H1 will all be grouped into a single route, when advertised
   external to the area (see Section 3.5 for more details).

   In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
   internal routers.  Routers RT3, RT4, RT7, RT10 and RT11 are area
   border routers.  Finally, as before, Routers RT5 and RT7 are AS
   boundary routers.

   Figure 7 shows the resulting link-state database for the Area 1.  The
   figure completely describes that area's intra-area routing.
























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             ...........................
             .   +                     .
             .   | 3+---+              .      N12      N14
             . N1|--|RT1|\ 1           .        \ N13 /
             .   |  +---+ \            .        8\ |8/8
             .   +         \ ____      .          \|/
             .              /    \   1+---+8    8+---+6
             .             *  N3  *---|RT4|------|RT5|--------+
             .              \____/    +---+      +---+        |
             .    +         /      \   .           |7         |
             .    | 3+---+ /        \  .           |          |
             .  N2|--|RT2|/1        1\ .           |6         |
             .    |  +---+            +---+8    6+---+        |
             .    +                   |RT3|------|RT6|        |
             .                        +---+      +---+        |
             .                      2/ .         Ia|7         |
             .                      /  .           |          |
             .             +---------+ .           |          |
             .Area 1           N4      .           |          |
             ...........................           |          |
          ..........................               |          |
          .            N11         .               |          |
          .        +---------+     .               |          |
          .             |          .               |          |    N12
          .             |3         .             Ib|5         |6 2/
          .           +---+        .             +----+     +---+/
          .           |RT9|        .    .........|RT10|.....|RT7|---N15.
          .           +---+        .    .        +----+     +---+ 9    .
          .             |1         .    .    +  /3    1\      |1       .
          .            _|__        .    .    | /        \   __|_       .
          .           /    \      1+----+2   |/          \ /    \      .
          .          *  N9  *------|RT11|----|            *  N6  *     .
          .           \____/       +----+    |             \____/      .
          .             |          .    .    |                |        .
          .             |1         .    .    +                |1       .
          .  +--+   10+----+       .    .   N8              +---+      .
          .  |H1|-----|RT12|       .    .                   |RT8|      .
          .  +--+SLIP +----+       .    .                   +---+      .
          .             |2         .    .                     |4       .
          .             |          .    .                     |        .
          .        +---------+     .    .                 +--------+   .
          .            N10         .    .                     N7       .
          .                        .    .Area 2                        .
          .Area 3                  .    ................................
          ..........................

               Figure 6: A sample OSPF area configuration




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   It also shows the complete view of the internet for the two internal
   routers RT1 and RT2.  It is the job of the area border routers, RT3
   and RT4, to advertise into Area 1 the distances to all destinations
   external to the area.  These are indicated in Figure 7 by the dashed
   stub routes.  Also, RT3 and RT4 must advertise into Area 1 the
   location of the AS boundary routers RT5 and RT7.  Finally, AS-
   external-LSAs from RT5 and RT7 are flooded throughout the entire AS,
   and in particular throughout Area 1.  These LSAs are included in Area
   1's database, and yield routes to Networks N12-N15.

   Routers RT3 and RT4 must also summarize Area 1's topology for
   distribution to the backbone.  Their backbone LSAs are shown in Table
   4.  These summaries show which networks are contained in Area 1
   (i.e., Networks N1-N4), and the distance to these networks from the
   routers RT3 and RT4 respectively.

   The link-state database for the backbone is shown in Figure 8.  The
   set of routers pictured are the backbone routers.  Router RT11 is a
   backbone router because it belongs to two areas.  In order to make
   the backbone connected, a virtual link has been configured between
   Routers R10 and R11.

   The area border routers RT3, RT4, RT7, RT10 and RT11 condense the
   routing information of their attached non-backbone areas for
   distribution via the backbone; these are the dashed stubs that appear
   in Figure 8.  Remember that the third area has been configured to
   condense Networks N9-N11 and Host H1 into a single route.  This
   yields a single dashed line for networks N9-N11 and Host H1 in Figure
   8.  Routers RT5 and RT7 are AS boundary routers; their externally
   derived information also appears on the graph in Figure 8 as stubs.


                     Network   RT3 adv.   RT4 adv.
                     _____________________________
                     N1        4          4
                     N2        4          4
                     N3        1          1
                     N4        2          3

              Table 4: Networks advertised to the backbone
                        by Routers RT3 and RT4.










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                          |RT|RT|RT|RT|RT|RT|
                          |1 |2 |3 |4 |5 |7 |N3|
                       ----- -------------------
                       RT1|  |  |  |  |  |  |0 |
                       RT2|  |  |  |  |  |  |0 |
                       RT3|  |  |  |  |  |  |0 |
                   *   RT4|  |  |  |  |  |  |0 |
                   *   RT5|  |  |14|8 |  |  |  |
                   T   RT7|  |  |20|14|  |  |  |
                   O    N1|3 |  |  |  |  |  |  |
                   *    N2|  |3 |  |  |  |  |  |
                   *    N3|1 |1 |1 |1 |  |  |  |
                        N4|  |  |2 |  |  |  |  |
                     Ia,Ib|  |  |20|27|  |  |  |
                        N6|  |  |16|15|  |  |  |
                        N7|  |  |20|19|  |  |  |
                        N8|  |  |18|18|  |  |  |
                 N9-N11,H1|  |  |29|36|  |  |  |
                       N12|  |  |  |  |8 |2 |  |
                       N13|  |  |  |  |8 |  |  |
                       N14|  |  |  |  |8 |  |  |
                       N15|  |  |  |  |  |9 |  |

                      Figure 7: Area 1's Database.

           Networks and routers are represented by vertices.
          An edge of cost X connects Vertex A to Vertex B iff
            the intersection of Column A and Row B is marked
                               with an X.






















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                                                  **FROM**

                            |RT|RT|RT|RT|RT|RT|RT
                            |3 |4 |5 |6 |7 |10|11|
                         ------------------------
                         RT3|  |  |  |6 |  |  |  |
                         RT4|  |  |8 |  |  |  |  |
                         RT5|  |8 |  |6 |6 |  |  |
                         RT6|8 |  |7 |  |  |5 |  |
                         RT7|  |  |6 |  |  |  |  |
                     *  RT10|  |  |  |7 |  |  |2 |
                     *  RT11|  |  |  |  |  |3 |  |
                     T    N1|4 |4 |  |  |  |  |  |
                     O    N2|4 |4 |  |  |  |  |  |
                     *    N3|1 |1 |  |  |  |  |  |
                     *    N4|2 |3 |  |  |  |  |  |
                          Ia|  |  |  |  |  |5 |  |
                          Ib|  |  |  |7 |  |  |  |
                          N6|  |  |  |  |1 |1 |3 |
                          N7|  |  |  |  |5 |5 |7 |
                          N8|  |  |  |  |4 |3 |2 |
                   N9-N11,H1|  |  |  |  |  |  |11|
                         N12|  |  |8 |  |2 |  |  |
                         N13|  |  |8 |  |  |  |  |
                         N14|  |  |8 |  |  |  |  |
                         N15|  |  |  |  |9 |  |  |

                   Figure 8: The backbone's database.

           Networks and routers are represented by vertices.
          An edge of cost X connects Vertex A to Vertex B iff
            the intersection of Column A and Row B is marked
                               with an X.

   The backbone enables the exchange of summary information between area
   border routers.  Every area border router hears the area summaries
   from all other area border routers.  It then forms a picture of the
   distance to all networks outside of its area by examining the
   collected LSAs, and adding in the backbone distance to each
   advertising router.

   Again using Routers RT3 and RT4 as an example, the procedure goes as
   follows: They first calculate the SPF tree for the backbone.  This
   gives the distances to all other area border routers.  Also noted are
   the distances to networks (Ia and Ib) and AS boundary routers (RT5
   and RT7) that belong to the backbone.  This calculation is shown in
   Table 5.




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   Next, by looking at the area summaries from these area border
   routers, RT3 and RT4 can determine the distance to all networks
   outside their area.  These distances are then advertised internally
   to the area by RT3 and RT4.  The advertisements that Router RT3 and
   RT4 will make into Area 1 are shown in Table 6.  Note that Table 6
   assumes that an area range has been configured for the backbone which
   groups Ia and Ib into a single LSA.

   The information imported into Area 1 by Routers RT3 and RT4 enables
   an internal router, such as RT1, to choose an area border router
   intelligently.  Router RT1 would use RT4 for traffic to Network N6,
   RT3 for traffic to Network N10, and would load share between the two
   for traffic to Network N8.

                              dist  from   dist  from
                              RT3          RT4
                   __________________________________
                   to  RT3    *            21
                   to  RT4    22           *
                   to  RT7    20           14
                   to  RT10   15           22
                   to  RT11   18           25
                   __________________________________
                   to  Ia     20           27
                   to  Ib     15           22
                   __________________________________
                   to  RT5    14           8
                   to  RT7    20           14

                 Table 5: Backbone distances calculated
                        by Routers RT3 and RT4.


                   Destination   RT3 adv.   RT4 adv.
                   _________________________________
                   Ia,Ib         20         27
                   N6            16         15
                   N7            20         19
                   N8            18         18
                   N9-N11,H1     29         36
                   _________________________________
                   RT5           14         8
                   RT7           20         14

              Table 6: Destinations advertised into Area 1
                        by Routers RT3 and RT4.





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   Router RT1 can also determine in this manner the shortest path to the
   AS boundary routers RT5 and RT7.  Then, by looking at RT5 and RT7's
   AS-external-LSAs, Router RT1 can decide between RT5 or RT7 when
   sending to a destination in another Autonomous System (one of the
   networks N12-N15).

   Note that a failure of the line between Routers RT6 and RT10 will
   cause the backbone to become disconnected.  Configuring a virtual
   link between Routers RT7 and RT10 will give the backbone more
   connectivity and more resistance to such failures.

3.5.  IP subnetting support



   OSPF attaches an IP address mask to each advertised route.  The mask
   indicates the range of addresses being described by the particular
   route.  For example, a summary-LSA for the destination 128.185.0.0
   with a mask of 0xffff0000 actually is describing a single route to
   the collection of destinations 128.185.0.0 - 128.185.255.255.
   Similarly, host routes are always advertised with a mask of
   0xffffffff, indicating the presence of only a single destination.

   Including the mask with each advertised destination enables the
   implementation of what is commonly referred to as variable-length
   subnetting.  This means that a single IP class A, B, or C network
   number can be broken up into many subnets of various sizes. For
   example, the network 128.185.0.0 could be broken up into 62
   variable-sized subnets: 15 subnets of size 4K, 15 subnets of size
   256, and 32 subnets of size 8.  Table 7 shows some of the resulting
   network addresses together with their masks.


                  Network address   IP address mask   Subnet size
                  _______________________________________________
                  128.185.16.0      0xfffff000        4K
                  128.185.1.0       0xffffff00        256
                  128.185.0.8       0xfffffff8        8


                   Table 7: Some sample subnet sizes.


   There are many possible ways of dividing up a class A, B, and C
   network into variable sized subnets.  The precise procedure for doing
   so is beyond the scope of this specification.  This specification
   however establishes the following guideline: When an IP packet is
   forwarded, it is always forwarded to the network that is the best
   match for the packet's destination.  Here best match is synonymous
   with the longest or most specific match.  For example, the default



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   route with destination of 0.0.0.0 and mask 0x00000000 is always a
   match for every IP destination.  Yet it is always less specific than
   any other match.  Subnet masks must be assigned so that the best
   match for any IP destination is unambiguous.

   Attaching an address mask to each route also enables the support of
   IP supernetting. For example, a single physical network segment could
   be assigned the [address,mask] pair [192.9.4.0,0xfffffc00]. The
   segment would then be single IP network, containing addresses from
   the four consecutive class C network numbers 192.9.4.0 through
   192.9.7.0. Such addressing is now becoming commonplace with the
   advent of CIDR (see [Ref10]).

   In order to get better aggregation at area boundaries, area address
   ranges can be employed (see Section C.2 for more details).  Each
   address range is defined as an [address,mask] pair.  Many separate
   networks may then be contained in a single address range, just as a
   subnetted network is composed of many separate subnets.  Area border
   routers then summarize the area contents (for distribution to the
   backbone) by advertising a single route for each address range.  The
   cost of the route is the maximum cost to any of the networks falling
   in the specified range.

   For example, an IP subnetted network might be configured as a single
   OSPF area.  In that case, a single address range could be configured:
   a class A, B, or C network number along with its natural IP mask.
   Inside the area, any number of variable sized subnets could be
   defined.  However, external to the area a single route for the entire
   subnetted network would be distributed, hiding even the fact that the
   network is subnetted at all.  The cost of this route is the maximum
   of the set of costs to the component subnets.

3.6.  Supporting stub areas



   In some Autonomous Systems, the majority of the link-state database
   may consist of AS-external-LSAs.  An OSPF AS-external-LSA is usually
   flooded throughout the entire AS.  However, OSPF allows certain areas
   to be configured as "stub areas".  AS-external-LSAs are not flooded
   into/throughout stub areas; routing to AS external destinations in
   these areas is based on a (per-area) default only.  This reduces the
   link-state database size, and therefore the memory requirements, for
   a stub area's internal routers.

   In order to take advantage of the OSPF stub area support, default
   routing must be used in the stub area.  This is accomplished as
   follows.  One or more of the stub area's area border routers must
   advertise a default route into the stub area via summary-LSAs.  These
   summary defaults are flooded throughout the stub area, but no



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   further.  (For this reason these defaults pertain only to the
   particular stub area).  These summary default routes will be used for
   any destination that is not explicitly reachable by an intra-area or
   inter-area path (i.e., AS external destinations).

   An area can be configured as a stub when there is a single exit point
   from the area, or when the choice of exit point need not be made on a
   per-external-destination basis.  For example, Area 3 in Figure 6
   could be configured as a stub area, because all external traffic must
   travel though its single area border router RT11.  If Area 3 were
   configured as a stub, Router RT11 would advertise a default route for
   distribution inside Area 3 (in a summary-LSA), instead of flooding
   the AS-external-LSAs for Networks N12-N15 into/throughout the area.

   The OSPF protocol ensures that all routers belonging to an area agree
   on whether the area has been configured as a stub.  This guarantees
   that no confusion will arise in the flooding of AS-external-LSAs.

   There are a couple of restrictions on the use of stub areas.  Virtual
   links cannot be configured through stub areas.  In addition, AS
   boundary routers cannot be placed internal to stub areas.

3.7.  Partitions of areas



   OSPF does not actively attempt to repair area partitions.  When an
   area becomes partitioned, each component simply becomes a separate
   area.  The backbone then performs routing between the new areas.
   Some destinations reachable via intra-area routing before the
   partition will now require inter-area routing.

   However, in order to maintain full routing after the partition, an
   address range must not be split across multiple components of the
   area partition. Also, the backbone itself must not partition.  If it
   does, parts of the Autonomous System will become unreachable.
   Backbone partitions can be repaired by configuring virtual links (see
   Section 15).

   Another way to think about area partitions is to look at the
   Autonomous System graph that was introduced in Section 2.  Area IDs
   can be viewed as colors for the graph's edges.[1] Each edge of the
   graph connects to a network, or is itself a point-to-point network.
   In either case, the edge is colored with the network's Area ID.

   A group of edges, all having the same color, and interconnected by
   vertices, represents an area.  If the topology of the Autonomous
   System is intact, the graph will have several regions of color, each
   color being a distinct Area ID.




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   When the AS topology changes, one of the areas may become
   partitioned.  The graph of the AS will then have multiple regions of
   the same color (Area ID).  The routing in the Autonomous System will
   continue to function as long as these regions of same color are
   connected by the single backbone region.

4.  Functional Summary



   A separate copy of OSPF's basic routing algorithm runs in each area.
   Routers having interfaces to multiple areas run multiple copies of
   the algorithm.  A brief summary of the routing algorithm follows.

   When a router starts, it first initializes the routing protocol data
   structures.  The router then waits for indications from the lower-
   level protocols that its interfaces are functional.

   A router then uses the OSPF's Hello Protocol to acquire neighbors.
   The router sends Hello packets to its neighbors, and in turn receives
   their Hello packets.  On broadcast and point-to-point networks, the
   router dynamically detects its neighboring routers by sending its
   Hello packets to the multicast address AllSPFRouters.  On non-
   broadcast networks, some configuration information may be necessary
   in order to discover neighbors.  On broadcast and NBMA networks the
   Hello Protocol also elects a Designated router for the network.

   The router will attempt to form adjacencies with some of its newly
   acquired neighbors.  Link-state databases are synchronized between
   pairs of adjacent routers. On broadcast and NBMA networks, the
   Designated Router determines which routers should become adjacent.

   Adjacencies control the distribution of routing information.  Routing
   updates are sent and received only on adjacencies.

   A router periodically advertises its state, which is also called link
   state.  Link state is also advertised when a router's state changes.
   A router's adjacencies are reflected in the contents of its LSAs.
   This relationship between adjacencies and link state allows the
   protocol to detect dead routers in a timely fashion.

   LSAs are flooded throughout the area.  The flooding algorithm is
   reliable, ensuring that all routers in an area have exactly the same
   link-state database.  This database consists of the collection of
   LSAs originated by each router belonging to the area.  From this
   database each router calculates a shortest-path tree, with itself as
   root.  This shortest-path tree in turn yields a routing table for the
   protocol.





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4.1.  Inter-area routing



   The previous section described the operation of the protocol within a
   single area.  For intra-area routing, no other routing information is
   pertinent.  In order to be able to route to destinations outside of
   the area, the area border routers inject additional routing
   information into the area.  This additional information is a
   distillation of the rest of the Autonomous System's topology.

   This distillation is accomplished as follows: Each area border router
   is by definition connected to the backbone.  Each area border router
   summarizes the topology of its attached non-backbone areas for
   transmission on the backbone, and hence to all other area border
   routers. An area border router then has complete topological
   information concerning the backbone, and the area summaries from each
   of the other area border routers.  From this information, the router
   calculates paths to all inter-area destinations.  The router then
   advertises these paths into its attached areas.  This enables the
   area's internal routers to pick the best exit router when forwarding
   traffic inter-area destinations.

4.2.  AS external routes



   Routers that have information regarding other Autonomous Systems can
   flood this information throughout the AS.  This external routing
   information is distributed verbatim to every participating router.
   There is one exception: external routing information is not flooded
   into "stub" areas (see Section 3.6).

   To utilize external routing information, the path to all routers
   advertising external information must be known throughout the AS
   (excepting the stub areas).  For that reason, the locations of these
   AS boundary routers are summarized by the (non-stub) area border
   routers.

4.3.  Routing protocol packets



   The OSPF protocol runs directly over IP, using IP protocol 89.  OSPF
   does not provide any explicit fragmentation/reassembly support.  When
   fragmentation is necessary, IP fragmentation/reassembly is used.
   OSPF protocol packets have been designed so that large protocol
   packets can generally be split into several smaller protocol packets.
   This practice is recommended; IP fragmentation should be avoided
   whenever possible.

   Routing protocol packets should always be sent with the IP TOS field
   set to 0.  If at all possible, routing protocol packets should be
   given preference over regular IP data traffic, both when being sent



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   and received.  As an aid to accomplishing this, OSPF protocol packets
   should have their IP precedence field set to the value Internetwork
   Control (see [Ref5]).

   All OSPF protocol packets share a common protocol header that is
   described in Appendix A.  The OSPF packet types are listed below in
   Table 8.  Their formats are also described in Appendix A.


     Type   Packet  name
           Protocol  function
     __________________________________________________________
     1      Hello                  Discover/maintain  neighbors
     2      Database Description   Summarize database contents
     3      Link State Request     Database download
     4      Link State Update      Database update
     5      Link State Ack         Flooding acknowledgment


                      Table 8: OSPF packet types.

   OSPF's Hello protocol uses Hello packets to discover and maintain
   neighbor relationships.  The Database Description and Link State
   Request packets are used in the forming of adjacencies.  OSPF's
   reliable update mechanism is implemented by the Link State Update and
   Link State Acknowledgment packets.

   Each Link State Update packet carries a set of new link state
   advertisements (LSAs) one hop further away from their point of
   origination.  A single Link State Update packet may contain the LSAs
   of several routers.  Each LSA is tagged with the ID of the
   originating router and a checksum of its link state contents.  Each
   LSA also has a type field; the different types of OSPF LSAs are
   listed below in Table 9.

   OSPF routing packets (with the exception of Hellos) are sent only
   over adjacencies.  This means that all OSPF protocol packets travel a
   single IP hop, except those that are sent over virtual adjacencies.
   The IP source address of an OSPF protocol packet is one end of a
   router adjacency, and the IP destination address is either the other
   end of the adjacency or an IP multicast address.










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        LS     LSA                LSA description
        type   name
        ________________________________________________________
        1      Router-LSAs        Originated by all routers.
                                  This LSA describes
                                  the collected states of the
                                  router's interfaces to an
                                  area. Flooded throughout a
                                  single area only.
        ________________________________________________________
        2      Network-LSAs       Originated for broadcast
                                  and NBMA networks by
                                  the Designated Router. This
                                  LSA contains the
                                  list of routers connected
                                  to the network. Flooded
                                  throughout a single area only.
        ________________________________________________________
        3,4    Summary-LSAs       Originated by area border
                                  routers, and flooded through-
                                  out the LSA's associated
                                  area. Each summary-LSA
                                  describes a route to a
                                  destination outside the area,
                                  yet still inside the AS
                                  (i.e., an inter-area route).
                                  Type 3 summary-LSAs describe
                                  routes to networks. Type 4
                                  summary-LSAs describe
                                  routes to AS boundary routers.
        ________________________________________________________
        5      AS-external-LSAs   Originated by AS boundary
                                  routers, and flooded through-
                                  out the AS. Each
                                  AS-external-LSA describes
                                  a route to a destination in
                                  another Autonomous System.
                                  Default routes for the AS can
                                  also be described by
                                  AS-external-LSAs.

            Table 9: OSPF link state advertisements (LSAs).









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4.4.  Basic implementation requirements



   An implementation of OSPF requires the following pieces of system
   support:

   Timers
      Two different kind of timers are required. The first kind, called
      "single shot timers", fire once and cause a protocol event to be
      processed.  The second kind, called "interval timers", fire at
      continuous intervals.  These are used for the sending of packets
      at regular intervals.  A good example of this is the regular
      broadcast of Hello packets. The granularity of both kinds of
      timers is one second.

      Interval timers should be implemented to avoid drift.  In some
      router implementations, packet processing can affect timer
      execution.  When multiple routers are attached to a single
      network, all doing broadcasts, this can lead to the
      synchronization of routing packets (which should be avoided).  If
      timers cannot be implemented to avoid drift, small random amounts
      should be added to/subtracted from the interval timer at each
      firing.

   IP multicast
      Certain OSPF packets take the form of IP multicast datagrams.
      Support for receiving and sending IP multicast datagrams, along
      with the appropriate lower-level protocol support, is required.
      The IP multicast datagrams used by OSPF never travel more than one
      hop. For this reason, the ability to forward IP multicast
      datagrams is not required.  For information on IP multicast, see
      [Ref7].

   Variable-length subnet support
      The router's IP protocol support must include the ability to
      divide a single IP class A, B, or C network number into many
      subnets of various sizes.  This is commonly called variable-length
      subnetting; see Section 3.5 for details.

   IP supernetting support
      The router's IP protocol support must include the ability to
      aggregate contiguous collections of IP class A, B, and C networks
      into larger quantities called supernets.  Supernetting has been
      proposed as one way to improve the scaling of IP routing in the
      worldwide Internet. For more information on IP supernetting, see
      [Ref10].






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   Lower-level protocol support
      The lower level protocols referred to here are the network access
      protocols, such as the Ethernet data link layer.  Indications must
      be passed from these protocols to OSPF as the network interface
      goes up and down.  For example, on an ethernet it would be
      valuable to know when the ethernet transceiver cable becomes
      unplugged.

   Non-broadcast lower-level protocol support
      On non-broadcast networks, the OSPF Hello Protocol can be aided by
      providing an indication when an attempt is made to send a packet
      to a dead or non-existent router.  For example, on an X.25 PDN a
      dead neighboring router may be indicated by the reception of a
      X.25 clear with an appropriate cause and diagnostic, and this
      information would be passed to OSPF.

   List manipulation primitives
      Much of the OSPF functionality is described in terms of its
      operation on lists of LSAs.  For example, the collection of LSAs
      that will be retransmitted to an adjacent router until
      acknowledged are described as a list.  Any particular LSA may be
      on many such lists.  An OSPF implementation needs to be able to
      manipulate these lists, adding and deleting constituent LSAs as
      necessary.

   Tasking support
      Certain procedures described in this specification invoke other
      procedures.  At times, these other procedures should be executed
      in-line, that is, before the current procedure is finished.  This
      is indicated in the text by instructions to execute a procedure.
      At other times, the other procedures are to be executed only when
      the current procedure has finished.  This is indicated by
      instructions to schedule a task.

4.5.  Optional OSPF capabilities



   The OSPF protocol defines several optional capabilities.  A router
   indicates the optional capabilities that it supports in its OSPF
   Hello packets, Database Description packets and in its LSAs.  This
   enables routers supporting a mix of optional capabilities to coexist
   in a single Autonomous System.

   Some capabilities must be supported by all routers attached to a
   specific area.  In this case, a router will not accept a neighbor's
   Hello Packet unless there is a match in reported capabilities (i.e.,
   a capability mismatch prevents a neighbor relationship from forming).
   An example of this is the ExternalRoutingCapability (see below).




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   Other capabilities can be negotiated during the Database Exchange
   process.  This is accomplished by specifying the optional
   capabilities in Database Description packets.  A capability mismatch
   with a neighbor in this case will result in only a subset of the link
   state database being exchanged between the two neighbors.

   The routing table build process can also be affected by the
   presence/absence of optional capabilities.  For example, since the
   optional capabilities are reported in LSAs, routers incapable of
   certain functions can be avoided when building the shortest path
   tree.

   The OSPF optional capabilities defined in this memo are listed below.
   See Section A.2 for more information.

   ExternalRoutingCapability
      Entire OSPF areas can be configured as "stubs" (see Section 3.6).
      AS-external-LSAs will not be flooded into stub areas.  This
      capability is represented by the E-bit in the OSPF Options field
      (see Section A.2).  In order to ensure consistent configuration of
      stub areas, all routers interfacing to such an area must have the
      E-bit clear in their Hello packets (see Sections 9.5 and 10.5).

5.  Protocol Data Structures



   The OSPF protocol is described herein in terms of its operation on
   various protocol data structures.  The following list comprises the
   top-level OSPF data structures.  Any initialization that needs to be
   done is noted.  OSPF areas, interfaces and neighbors also have
   associated data structures that are described later in this
   specification.

   Router ID
      A 32-bit number that uniquely identifies this router in the AS.
      One possible implementation strategy would be to use the smallest
      IP interface address belonging to the router. If a router's OSPF
      Router ID is changed, the router's OSPF software should be
      restarted before the new Router ID takes effect.  In this case the
      router should flush its self-originated LSAs from the routing
      domain (see Section 14.1) before restarting, or they will persist
      for up to MaxAge minutes.

   Area structures
      Each one of the areas to which the router is connected has its own
      data structure.  This data structure describes the working of the
      basic OSPF algorithm.  Remember that each area runs a separate
      copy of the basic OSPF algorithm.




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   Backbone (area) structure
      The OSPF backbone area is responsible for the dissemination of
      inter-area routing information.

   Virtual links configured
      The virtual links configured with this router as one endpoint.  In
      order to have configured virtual links, the router itself must be
      an area border router.  Virtual links are identified by the Router
      ID of the other endpoint -- which is another area border router.
      These two endpoint routers must be attached to a common area,
      called the virtual link's Transit area.  Virtual links are part of
      the backbone, and behave as if they were unnumbered point-to-point
      networks between the two routers.  A virtual link uses the intra-
      area routing of its Transit area to forward packets.  Virtual
      links are brought up and down through the building of the
      shortest-path trees for the Transit area.

   List of external routes
      These are routes to destinations external to the Autonomous
      System, that have been gained either through direct experience
      with another routing protocol (such as BGP), or through
      configuration information, or through a combination of the two
      (e.g., dynamic external information to be advertised by OSPF with
      configured metric). Any router having these external routes is
      called an AS boundary router.  These routes are advertised by the
      router into the OSPF routing domain via AS-external-LSAs.

   List of AS-external-LSAs
      Part of the link-state database.  These have originated from the
      AS boundary routers.  They comprise routes to destinations
      external to the Autonomous System.  Note that, if the router is
      itself an AS boundary router, some of these AS-external-LSAs have
      been self-originated.

   The routing table
      Derived from the link-state database.  Each entry in the routing
      table is indexed by a destination, and contains the destination's
      cost and a set of paths to use in forwarding packets to the
      destination. A path is described by its type and next hop.  For
      more information, see Section 11.











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   Figure 9 shows the collection of data structures present in a typical
   router.  The router pictured is RT10, from the map in Figure 6.  Note
   that Router RT10 has a virtual link configured to Router RT11, with
   Area 2 as the link's Transit area.  This is indicated by the dashed
   line in Figure 9.  When the virtual link becomes active, through the
   building of the shortest path tree for Area 2, it becomes an
   interface to the backbone (see the two backbone interfaces depicted
   in Figure 9).


                              +----+
                              |RT10|------+
                              +----+       \+-------------+
                             /      \       |Routing Table|
                            /        \      +-------------+
                           /          \
              +------+    /            \    +--------+
              |Area 2|---+              +---|Backbone|
              +------+***********+          +--------+
             /        \           *        /          \
            /          \           *      /            \
       +---------+  +---------+    +------------+       +------------+
       |Interface|  |Interface|    |Virtual Link|       |Interface Ib|
       |  to N6  |  |  to N8  |    |   to RT11  |       +------------+
       +---------+  +---------+    +------------+             |
           /  \           |               |                   |
          /    \          |               |                   |
   +--------+ +--------+  |        +-------------+      +------------+
   |Neighbor| |Neighbor|  |        |Neighbor RT11|      |Neighbor RT6|
   |  RT8   | |  RT7   |  |        +-------------+      +------------+
   +--------+ +--------+  |
                          |
                     +-------------+
                     |Neighbor RT11|
                     +-------------+


                Figure 9: Router RT10's Data structures

6.  The Area Data Structure



   The area data structure contains all the information used to run the
   basic OSPF routing algorithm. Each area maintains its own link-state
   database. A network belongs to a single area, and a router interface
   connects to a single area. Each router adjacency also belongs to a
   single area.





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   The OSPF backbone is the special OSPF area responsible for
   disseminating inter-area routing information.

   The area link-state database consists of the collection of router-
   LSAs, network-LSAs and summary-LSAs that have originated from the
   area's routers.  This information is flooded throughout a single area
   only. The list of AS-external-LSAs (see Section 5) is also considered
   to be part of each area's link-state database.

   Area ID
      A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
      reserved for the backbone.

   List of area address ranges
      In order to aggregate routing information at area boundaries, area
      address ranges can be employed. Each address range is specified by
      an [address,mask] pair and a status indication of either Advertise
      or DoNotAdvertise (see Section 12.4.3).

   Associated router interfaces
      This router's interfaces connecting to the area.  A router
      interface belongs to one and only one area (or the backbone).  For
      the backbone area this list includes all the virtual links.  A
      virtual link is identified by the Router ID of its other endpoint;
      its cost is the cost of the shortest intra-area path through the
      Transit area that exists between the two routers.

   List of router-LSAs
      A router-LSA is generated by each router in the area.  It
      describes the state of the router's interfaces to the area.

   List of network-LSAs
      One network-LSA is generated for each transit broadcast and NBMA
      network in the area.  A network-LSA describes the set of routers
      currently connected to the network.

   List of summary-LSAs
      Summary-LSAs originate from the area's area border routers.  They
      describe routes to destinations internal to the Autonomous System,
      yet external to the area (i.e., inter-area destinations).

   Shortest-path tree
      The shortest-path tree for the area, with this router itself as
      root.  Derived from the collected router-LSAs and network-LSAs by
      the Dijkstra algorithm (see Section 16.1).






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   TransitCapability
      This parameter indicates whether the area can carry data traffic
      that neither originates nor terminates in the area itself. This
      parameter is calculated when the area's shortest-path tree is
      built (see Section 16.1, where TransitCapability is set to TRUE if
      and only if there are one or more fully adjacent virtual links
      using the area as Transit area), and is used as an input to a
      subsequent step of the routing table build process (see Section
      16.3). When an area's TransitCapability is set to TRUE, the area
      is said to be a "transit area".

   ExternalRoutingCapability
      Whether AS-external-LSAs will be flooded into/throughout the area.
      This is a configurable parameter.  If AS-external-LSAs are
      excluded from the area, the area is called a "stub". Within stub
      areas, routing to AS external destinations will be based solely on
      a default summary route.  The backbone cannot be configured as a
      stub area.  Also, virtual links cannot be configured through stub
      areas.  For more information, see Section 3.6.

   StubDefaultCost
      If the area has been configured as a stub area, and the router
      itself is an area border router, then the StubDefaultCost
      indicates the cost of the default summary-LSA that the router
      should advertise into the area. See Section 12.4.3 for more
      information.

   Unless otherwise specified, the remaining sections of this document
   refer to the operation of the OSPF protocol within a single area.

7.  Bringing Up Adjacencies



   OSPF creates adjacencies between neighboring routers for the purpose
   of exchanging routing information. Not every two neighboring routers
   will become adjacent.  This section covers the generalities involved
   in creating adjacencies.  For further details consult Section 10.

7.1.  The Hello Protocol



   The Hello Protocol is responsible for establishing and maintaining
   neighbor relationships.  It also ensures that communication between
   neighbors is bidirectional.  Hello packets are sent periodically out
   all router interfaces.  Bidirectional communication is indicated when
   the router sees itself listed in the neighbor's Hello Packet.  On
   broadcast and NBMA networks, the Hello Protocol elects a Designated
   Router for the network.





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   The Hello Protocol works differently on broadcast networks, NBMA
   networks and Point-to-MultiPoint networks.  On broadcast networks,
   each router advertises itself by periodically multicasting Hello
   Packets.  This allows neighbors to be discovered dynamically.  These
   Hello Packets contain the router's view of the Designated Router's
   identity, and the list of routers whose Hello Packets have been seen
   recently.

   On NBMA networks some configuration information may be necessary for
   the operation of the Hello Protocol.  Each router that may
   potentially become Designated Router has a list of all other routers
   attached to the network.  A router, having Designated Router
   potential, sends Hello Packets to all other potential Designated
   Routers when its interface to the NBMA network first becomes
   operational.  This is an attempt to find the Designated Router for
   the network.  If the router itself is elected Designated Router, it
   begins sending Hello Packets to all other routers attached to the
   network.

   On Point-to-MultiPoint networks, a router sends Hello Packets to all
   neighbors with which it can communicate directly. These neighbors may
   be discovered dynamically through a protocol such as Inverse ARP (see
   [Ref14]), or they may be configured.

   After a neighbor has been discovered, bidirectional communication
   ensured, and (if on a broadcast or NBMA network) a Designated Router
   elected, a decision is made regarding whether or not an adjacency
   should be formed with the neighbor (see Section 10.4). If an
   adjacency is to be formed, the first step is to synchronize the
   neighbors' link-state databases.  This is covered in the next
   section.

7.2.  The Synchronization of Databases



   In a link-state routing algorithm, it is very important for all
   routers' link-state databases to stay synchronized.  OSPF simplifies
   this by requiring only adjacent routers to remain synchronized.  The
   synchronization process begins as soon as the routers attempt to
   bring up the adjacency.  Each router describes its database by
   sending a sequence of Database Description packets to its neighbor.
   Each Database Description Packet describes a set of LSAs belonging to
   the router's database.  When the neighbor sees an LSA that is more
   recent than its own database copy, it makes a note that this newer
   LSA should be requested.

   This sending and receiving of Database Description packets is called
   the "Database Exchange Process".  During this process, the two
   routers form a master/slave relationship.  Each Database Description



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   Packet has a sequence number.  Database Description Packets sent by
   the master (polls) are acknowledged by the slave through echoing of
   the sequence number.  Both polls and their responses contain
   summaries of link state data.  The master is the only one allowed to
   retransmit Database Description Packets.  It does so only at fixed
   intervals, the length of which is the configured per-interface
   constant RxmtInterval.

   Each Database Description contains an indication that there are more
   packets to follow --- the M-bit.  The Database Exchange Process is
   over when a router has received and sent Database Description Packets
   with the M-bit off.

   During and after the Database Exchange Process, each router has a
   list of those LSAs for which the neighbor has more up-to-date
   instances.  These LSAs are requested in Link State Request Packets.
   Link State Request packets that are not satisfied are retransmitted
   at fixed intervals of time RxmtInterval.  When the Database
   Description Process has completed and all Link State Requests have
   been satisfied, the databases are deemed synchronized and the routers
   are marked fully adjacent.  At this time the adjacency is fully
   functional and is advertised in the two routers' router-LSAs.

   The adjacency is used by the flooding procedure as soon as the
   Database Exchange Process begins.  This simplifies database
   synchronization, and guarantees that it finishes in a predictable
   period of time.

7.3.  The Designated Router



   Every broadcast and NBMA network has a Designated Router.  The
   Designated Router performs two main functions for the routing
   protocol:

   o   The Designated Router originates a network-LSA on behalf of
       the network.  This LSA lists the set of routers (including
       the Designated Router itself) currently attached to the
       network.  The Link State ID for this LSA (see Section
       12.1.4) is the IP interface address of the Designated
       Router.  The IP network number can then be obtained by using
       the network's subnet/network mask.

   o   The Designated Router becomes adjacent to all other routers
       on the network.  Since the link state databases are
       synchronized across adjacencies (through adjacency bring-up
       and then the flooding procedure), the Designated Router
       plays a central part in the synchronization process.




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   The Designated Router is elected by the Hello Protocol.  A router's
   Hello Packet contains its Router Priority, which is configurable on a
   per-interface basis.  In general, when a router's interface to a
   network first becomes functional, it checks to see whether there is
   currently a Designated Router for the network.  If there is, it
   accepts that Designated Router, regardless of its Router Priority.
   (This makes it harder to predict the identity of the Designated
   Router, but ensures that the Designated Router changes less often.
   See below.)  Otherwise, the router itself becomes Designated Router
   if it has the highest Router Priority on the network.  A more
   detailed (and more accurate) description of Designated Router
   election is presented in Section 9.4.

   The Designated Router is the endpoint of many adjacencies.  In order
   to optimize the flooding procedure on broadcast networks, the
   Designated Router multicasts its Link State Update Packets to the
   address AllSPFRouters, rather than sending separate packets over each
   adjacency.

   Section 2 of this document discusses the directed graph
   representation of an area.  Router nodes are labelled with their
   Router ID.  Transit network nodes are actually labelled with the IP
   address of their Designated Router.  It follows that when the
   Designated Router changes, it appears as if the network node on the
   graph is replaced by an entirely new node.  This will cause the
   network and all its attached routers to originate new LSAs.  Until
   the link-state databases again converge, some temporary loss of
   connectivity may result.  This may result in ICMP unreachable
   messages being sent in response to data traffic.  For that reason,
   the Designated Router should change only infrequently.  Router
   Priorities should be configured so that the most dependable router on
   a network eventually becomes Designated Router.

7.4.  The Backup Designated Router



   In order to make the transition to a new Designated Router smoother,
   there is a Backup Designated Router for each broadcast and NBMA
   network.  The Backup Designated Router is also adjacent to all
   routers on the network, and becomes Designated Router when the
   previous Designated Router fails.  If there were no Backup Designated
   Router, when a new Designated Router became necessary, new
   adjacencies would have to be formed between the new Designated Router
   and all other routers attached to the network.  Part of the adjacency
   forming process is the synchronizing of link-state databases, which
   can potentially take quite a long time.  During this time, the
   network would not be available for transit data traffic.  The Backup
   Designated obviates the need to form these adjacencies, since they
   already exist.  This means the period of disruption in transit



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   traffic lasts only as long as it takes to flood the new LSAs (which
   announce the new Designated Router).

   The Backup Designated Router does not generate a network-LSA for the
   network.  (If it did, the transition to a new Designated Router would
   be even faster.  However, this is a tradeoff between database size
   and speed of convergence when the Designated Router disappears.)

   The Backup Designated Router is also elected by the Hello Protocol.
   Each Hello Packet has a field that specifies the Backup Designated
   Router for the network.

   In some steps of the flooding procedure, the Backup Designated Router
   plays a passive role, letting the Designated Router do more of the
   work.  This cuts down on the amount of local routing traffic.  See
   Section 13.3 for more information.

7.5.  The graph of adjacencies



   An adjacency is bound to the network that the two routers have in
   common.  If two routers have multiple networks in common, they may
   have multiple adjacencies between them.

   One can picture the collection of adjacencies on a network as forming
   an undirected graph.  The vertices consist of routers, with an edge
   joining two routers if they are adjacent.  The graph of adjacencies
   describes the flow of routing protocol packets, and in particular
   Link State Update Packets, through the Autonomous System.

   Two graphs are possible, depending on whether a Designated Router is
   elected for the network.  On physical point-to-point networks,
   Point-to-MultiPoint networks and virtual links, neighboring routers
   become adjacent whenever they can communicate directly.  In contrast,
   on broadcast and NBMA networks only the Designated Router and the
   Backup Designated Router become adjacent to all other routers
   attached to the network.

   These graphs are shown in Figure 10.  It is assumed that Router RT7
   has become the Designated Router, and Router RT3 the Backup
   Designated Router, for the Network N2.  The Backup Designated Router
   performs a lesser function during the flooding procedure than the
   Designated Router (see Section 13.3).  This is the reason for the
   dashed lines connecting the Backup Designated Router RT3.








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          +---+            +---+
          |RT1|------------|RT2|            o---------------o
          +---+    N1      +---+           RT1             RT2



                                                 RT7
                                                  o---------+
            +---+   +---+   +---+                /|\        |
            |RT7|   |RT3|   |RT4|               / | \       |
            +---+   +---+   +---+              /  |  \      |
              |       |       |               /   |   \     |
         +-----------------------+        RT5o RT6o    oRT4 |
                  |       |     N2            *   *   *     |
                +---+   +---+                  *  *  *      |
                |RT5|   |RT6|                   * * *       |
                +---+   +---+                    ***        |
                                                  o---------+
                                                 RT3


                  Figure 10: The graph of adjacencies

8.  Protocol Packet Processing



   This section discusses the general processing of OSPF routing
   protocol packets.  It is very important that the router link-state
   databases remain synchronized.  For this reason, routing protocol
   packets should get preferential treatment over ordinary data packets,
   both in sending and receiving.

   Routing protocol packets are sent along adjacencies only (with the
   exception of Hello packets, which are used to discover the
   adjacencies).  This means that all routing protocol packets travel a
   single IP hop, except those sent over virtual links.

   All routing protocol packets begin with a standard header. The
   sections below provide details on how to fill in and verify this
   standard header.  Then, for each packet type, the section giving more
   details on that particular packet type's processing is listed.

8.1.  Sending protocol packets



   When a router sends a routing protocol packet, it fills in the fields
   of the standard OSPF packet header as follows.  For more details on
   the header format consult Section A.3.1:





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   Version #
      Set to 2, the version number of the protocol as documented in this
      specification.

   Packet type
      The type of OSPF packet, such as Link state Update or Hello
      Packet.

   Packet length
      The length of the entire OSPF packet in bytes, including the
      standard OSPF packet header.

   Router ID
      The identity of the router itself (who is originating the packet).

   Area ID
      The OSPF area that the packet is being sent into.

   Checksum
      The standard IP 16-bit one's complement checksum of the entire
      OSPF packet, excluding the 64-bit authentication field.  This
      checksum is calculated as part of the appropriate authentication
      procedure; for some OSPF authentication types, the checksum
      calculation is omitted.  See Section D.4 for details.

   AuType and Authentication
      Each OSPF packet exchange is authenticated.  Authentication types
      are assigned by the protocol and are documented in Appendix D.  A
      different authentication procedure can be used for each IP
      network/subnet.  Autype indicates the type of authentication
      procedure in use.  The 64-bit authentication field is then for use
      by the chosen authentication procedure.  This procedure should be
      the last called when forming the packet to be sent.  See Section
      D.4 for details.

















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   The IP destination address for the packet is selected as follows.  On
   physical point-to-point networks, the IP destination is always set to
   the address AllSPFRouters.  On all other network types (including
   virtual links), the majority of OSPF packets are sent as unicasts,
   i.e., sent directly to the other end of the adjacency.  In this case,
   the IP destination is just the Neighbor IP address associated with
   the other end of the adjacency (see Section 10).  The only packets
   not sent as unicasts are on broadcast networks; on these networks
   Hello packets are sent to the multicast destination AllSPFRouters,
   the Designated Router and its Backup send both Link State Update
   Packets and Link State Acknowledgment Packets to the multicast
   address AllSPFRouters, while all other routers send both their Link
   State Update and Link State Acknowledgment Packets to the multicast
   address AllDRouters.

   Retransmissions of Link State Update packets are ALWAYS sent as
   unicasts.

   The IP source address should be set to the IP address of the sending
   interface.  Interfaces to unnumbered point-to-point networks have no
   associated IP address.  On these interfaces, the IP source should be
   set to any of the other IP addresses belonging to the router.  For
   this reason, there must be at least one IP address assigned to the
   router.[2] Note that, for most purposes, virtual links act precisely
   the same as unnumbered point-to-point networks.  However, each
   virtual link does have an IP interface address (discovered during the
   routing table build process) which is used as the IP source when
   sending packets over the virtual link.

   For more information on the format of specific OSPF packet types,
   consult the sections listed in Table 10.


             Type   Packet name            detailed section (transmit)
             _________________________________________________________
             1      Hello                  Section  9.5
             2      Database description   Section 10.8
             3      Link state request     Section 10.9
             4      Link state update      Section 13.3
             5      Link state ack         Section 13.5

    Table 10: Sections describing OSPF protocol packet transmission.

8.2.  Receiving protocol packets



   Whenever a protocol packet is received by the router it is marked
   with the interface it was received on.  For routers that have virtual
   links configured, it may not be immediately obvious which interface



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   to associate the packet with.  For example, consider the Router RT11
   depicted in Figure 6.  If RT11 receives an OSPF protocol packet on
   its interface to Network N8, it may want to associate the packet with
   the interface to Area 2, or with the virtual link to Router RT10
   (which is part of the backbone).  In the following, we assume that
   the packet is initially associated with the non-virtual  link.[3]

   In order for the packet to be accepted at the IP level, it must pass
   a number of tests, even before the packet is passed to OSPF for
   processing:

   o   The IP checksum must be correct.

   o   The packet's IP destination address must be the IP address
       of the receiving interface, or one of the IP multicast
       addresses AllSPFRouters or AllDRouters.

   o   The IP protocol specified must be OSPF (89).

   o   Locally originated packets should not be passed on to OSPF.
       That is, the source IP address should be examined to make
       sure this is not a multicast packet that the router itself
       generated.

   Next, the OSPF packet header is verified.  The fields specified
   in the header must match those configured for the receiving
   interface.  If they do not, the packet should be discarded:


   o   The version number field must specify protocol version 2.

   o   The Area ID found in the OSPF header must be verified.  If
       both of the following cases fail, the packet should be
       discarded.  The Area ID specified in the header must either:

       (1) Match the Area ID of the receiving interface.  In this
           case, the packet has been sent over a single hop.
           Therefore, the packet's IP source address is required to
           be on the same network as the receiving interface.  This
           can be verified by comparing the packet's IP source
           address to the interface's IP address, after masking
           both addresses with the interface mask.  This comparison
           should not be performed on point-to-point networks. On
           point-to-point networks, the interface addresses of each
           end of the link are assigned independently, if they are
           assigned at all.





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       (2) Indicate the backbone.  In this case, the packet has
           been sent over a virtual link.  The receiving router
           must be an area border router, and the Router ID
           specified in the packet (the source router) must be the
           other end of a configured virtual link.  The receiving
           interface must also attach to the virtual link's
           configured Transit area.  If all of these checks
           succeed, the packet is accepted and is from now on
           associated with the virtual link (and the backbone
           area).

   o   Packets whose IP destination is AllDRouters should only be
       accepted if the state of the receiving interface is DR or
       Backup (see Section 9.1).

   o   The AuType specified in the packet must match the AuType
       specified for the associated area.

   o   The packet must be authenticated.  The authentication
       procedure is indicated by the setting of AuType (see
       Appendix D).  The authentication procedure may use one or
       more Authentication keys, which can be configured on a per-
       interface basis.  The authentication procedure may also
       verify the checksum field in the OSPF packet header (which,
       when used, is set to the standard IP 16-bit one's complement
       checksum of the OSPF packet's contents after excluding the
       64-bit authentication field).  If the authentication
       procedure fails, the packet should be discarded.

   If the packet type is Hello, it should then be further processed by
   the Hello Protocol (see Section 10.5).  All other packet types are
   sent/received only on adjacencies.  This means that the packet must
   have been sent by one of the router's active neighbors.  If the
   receiving interface connects to a broadcast network, Point-to-
   MultiPoint network or NBMA network the sender is identified by the IP
   source address found in the packet's IP header.  If the receiving
   interface connects to a point-to-point network or a virtual link, the
   sender is identified by the Router ID (source router) found in the
   packet's OSPF header.  The data structure associated with the
   receiving interface contains the list of active neighbors.  Packets
   not matching any active neighbor are discarded.

   At this point all received protocol packets are associated with an
   active neighbor.  For the further input processing of specific packet
   types, consult the sections listed in Table 11.






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      Type   Packet name            detailed section (receive)
      ________________________________________________________
      1      Hello                  Section 10.5
      2      Database description   Section 10.6
      3      Link state request     Section 10.7
      4      Link state update      Section 13
      5      Link state ack         Section 13.7

     Table 11: Sections describing OSPF protocol packet reception.

9.  The Interface Data Structure



   An OSPF interface is the connection between a router and a network.
   We assume a single OSPF interface to each attached network/subnet,
   although supporting multiple interfaces on a single network is
   considered in Appendix F. Each interface structure has at most one IP
   interface address.

   An OSPF interface can be considered to belong to the area that
   contains the attached network.  All routing protocol packets
   originated by the router over this interface are labelled with the
   interface's Area ID.  One or more router adjacencies may develop over
   an interface. A router's LSAs reflect the state of its interfaces and
   their associated adjacencies.

   The following data items are associated with an interface. Note that
   a number of these items are actually configuration for the attached
   network; such items must be the same for all routers connected to the
   network.

   Type
      The OSPF interface type is either point-to-point, broadcast, NBMA,
      Point-to-MultiPoint or virtual link.

   State
      The functional level of an interface.  State determines whether or
      not full adjacencies are allowed to form over the interface.
      State is also reflected in the router's LSAs.

   IP interface address
      The IP address associated with the interface.  This appears as the
      IP source address in all routing protocol packets originated over
      this interface.  Interfaces to unnumbered point-to-point networks
      do not have an associated IP address.

   IP interface mask
      Also referred to as the subnet mask, this indicates the portion of
      the IP interface address that identifies the attached network.



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      Masking the IP interface address with the IP interface mask yields
      the IP network number of the attached network.  On point-to-point
      networks and virtual links, the IP interface mask is not defined.
      On these networks, the link itself is not assigned an IP network
      number, and so the addresses of each side of the link are assigned
      independently, if they are assigned at all.

   Area ID
      The Area ID of the area to which the attached network belongs.
      All routing protocol packets originating from the interface are
      labelled with this Area ID.

   HelloInterval
      The length of time, in seconds, between the Hello packets that the
      router sends on the interface.  Advertised in Hello packets sent
      out this interface.

   RouterDeadInterval
      The number of seconds before the router's neighbors will declare
      it down, when they stop hearing the router's Hello Packets.
      Advertised in Hello packets sent out this interface.

   InfTransDelay
      The estimated number of seconds it takes to transmit a Link State
      Update Packet over this interface.  LSAs contained in the Link
      State Update packet will have their age incremented by this amount
      before transmission.  This value should take into account
      transmission and propagation delays; it must be greater than zero.

   Router Priority
      An 8-bit unsigned integer.  When two routers attached to a network
      both attempt to become Designated Router, the one with the highest
      Router Priority takes precedence.  A router whose Router Priority
      is set to 0 is ineligible to become Designated Router on the
      attached network.  Advertised in Hello packets sent out this
      interface.

   Hello Timer
      An interval timer that causes the interface to send a Hello
      packet.  This timer fires every HelloInterval seconds.  Note that
      on non-broadcast networks a separate Hello packet is sent to each
      qualified neighbor.

   Wait Timer
      A single shot timer that causes the interface to exit the Waiting
      state, and as a consequence select a Designated Router on the
      network.  The length of the timer is RouterDeadInterval seconds.




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   List of neighboring routers
      The other routers attached to this network.  This list is formed
      by the Hello Protocol.  Adjacencies will be formed to some of
      these neighbors.  The set of adjacent neighbors can be determined
      by an examination of all of the neighbors' states.

   Designated Router
      The Designated Router selected for the attached network.  The
      Designated Router is selected on all broadcast and NBMA networks
      by the Hello Protocol.  Two pieces of identification are kept for
      the Designated Router: its Router ID and its IP interface address
      on the network.  The Designated Router advertises link state for
      the network; this network-LSA is labelled with the Designated
      Router's IP address.  The Designated Router is initialized to
      0.0.0.0, which indicates the lack of a Designated Router.

   Backup Designated Router
      The Backup Designated Router is also selected on all broadcast and
      NBMA networks by the Hello Protocol.  All routers on the attached
      network become adjacent to both the Designated Router and the
      Backup Designated Router.  The Backup Designated Router becomes
      Designated Router when the current Designated Router fails. The
      Backup Designated Router is initialized to 0.0.0.0, indicating the
      lack of a Backup Designated Router.

   Interface output cost(s)
      The cost of sending a data packet on the interface, expressed in
      the link state metric.  This is advertised as the link cost for
      this interface in the router-LSA. The cost of an interface must be
      greater than zero.

   RxmtInterval
      The number of seconds between LSA retransmissions, for adjacencies
      belonging to this interface.  Also used when retransmitting
      Database Description and Link State Request Packets.

   AuType
      The type of authentication used on the attached network/subnet.
      Authentication types are defined in Appendix D.  All OSPF packet
      exchanges are authenticated.  Different authentication schemes may
      be used on different networks/subnets.










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   Authentication key
      This configured data allows the authentication procedure to
      generate and/or verify OSPF protocol packets.  The Authentication
      key can be configured on a per-interface basis.  For example, if
      the AuType indicates simple password, the Authentication key would
      be a 64-bit clear password which is inserted into the OSPF packet
      header. If instead Autype indicates Cryptographic authentication,
      then the Authentication key is a shared secret which enables the
      generation/verification of message digests which are appended to
      the OSPF protocol packets. When Cryptographic authentication is
      used, multiple simultaneous keys are supported in order to achieve
      smooth key transition (see Section D.3).

9.1.  Interface states



   The various states that router interfaces may attain is documented in
   this section.  The states are listed in order of progressing
   functionality.  For example, the inoperative state is listed first,
   followed by a list of intermediate states before the final, fully
   functional state is achieved.  The specification makes use of this
   ordering by sometimes making references such as "those interfaces in
   state greater than X".  Figure 11 shows the graph of interface state
   changes.  The arcs of the graph are labelled with the event causing
   the state change.  These events are documented in Section 9.2.  The
   interface state machine is described in more detail in Section 9.3.

   Down
      This is the initial interface state.  In this state, the lower-
      level protocols have indicated that the interface is unusable.  No
      protocol traffic at all will be sent or received on such a
      interface.  In this state, interface parameters should be set to
      their initial values.



















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                                  +----+   UnloopInd   +--------+
                                  |Down|<--------------|Loopback|
                                  +----+               +--------+
                                     |
                                     |InterfaceUp
                          +-------+  |               +--------------+
                          |Waiting|<-+-------------->|Point-to-point|
                          +-------+                  +--------------+
                              |
                     WaitTimer|BackupSeen
                              |
                              |
                              |   NeighborChange
          +------+           +-+<---------------- +-------+
          |Backup|<----------|?|----------------->|DROther|
          +------+---------->+-+<-----+           +-------+
                    Neighbor  |       |
                    Change    |       |Neighbor
                              |       |Change
                              |     +--+
                              +---->|DR|
                                    +--+

                   Figure 11: Interface State changes

             In addition to the state transitions pictured,
           Event InterfaceDown always forces Down State, and
               Event LoopInd always forces Loopback State

      All interface timers should be disabled, and there should be no
      adjacencies associated with the interface.

   Loopback
      In this state, the router's interface to the network is looped
      back.  The interface may be looped back in hardware or software.
      The interface will be unavailable for regular data traffic.
      However, it may still be desirable to gain information on the
      quality of this interface, either through sending ICMP pings to
      the interface or through something like a bit error test.  For
      this reason, IP packets may still be addressed to an interface in
      Loopback state.  To facilitate this, such interfaces are
      advertised in router-LSAs as single host routes, whose destination
      is the IP interface address.[4]








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   Waiting
      In this state, the router is trying to determine the identity of
      the (Backup) Designated Router for the network.  To do this, the
      router monitors the Hello Packets it receives.  The router is not
      allowed to elect a Backup Designated Router nor a Designated
      Router until it transitions out of Waiting state.  This prevents
      unnecessary changes of (Backup) Designated Router.

   Point-to-point
      In this state, the interface is operational, and connects either
      to a physical point-to-point network or to a virtual link.  Upon
      entering this state, the router attempts to form an adjacency with
      the neighboring router.  Hello Packets are sent to the neighbor
      every HelloInterval seconds.

   DR Other
      The interface is to a broadcast or NBMA network on which another
      router has been selected to be the Designated Router.  In this
      state, the router itself has not been selected Backup Designated
      Router either.  The router forms adjacencies to both the
      Designated Router and the Backup Designated Router (if they
      exist).

   Backup
      In this state, the router itself is the Backup Designated Router
      on the attached network.  It will be promoted to Designated Router
      when the present Designated Router fails.  The router establishes
      adjacencies to all other routers attached to the network.  The
      Backup Designated Router performs slightly different functions
      during the Flooding Procedure, as compared to the Designated
      Router (see Section 13.3).  See Section 7.4 for more details on
      the functions performed by the Backup Designated Router.

   DR  In this state, this router itself is the Designated Router
      on the attached network.  Adjacencies are established to all other
      routers attached to the network.  The router must also originate a
      network-LSA for the network node.  The network-LSA will contain
      links to all routers (including the Designated Router itself)
      attached to the network.  See Section 7.3 for more details on the
      functions performed by the Designated Router.

9.2.  Events causing interface state changes



   State changes can be effected by a number of events.  These events
   are pictured as the labelled arcs in Figure 11.  The label
   definitions are listed below.  For a detailed explanation of the
   effect of these events on OSPF protocol operation, consult Section
   9.3.



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   InterfaceUp
      Lower-level protocols have indicated that the network interface is
      operational.  This enables the interface to transition out of Down
      state.  On virtual links, the interface operational indication is
      actually a result of the shortest path calculation (see Section
      16.7).

   WaitTimer
      The Wait Timer has fired, indicating the end of the waiting period
      that is required before electing a (Backup) Designated Router.

   BackupSeen
      The router has detected the existence or non-existence of a Backup
      Designated Router for the network.  This is done in one of two
      ways.  First, an Hello Packet may be received from a neighbor
      claiming to be itself the Backup Designated Router.
      Alternatively, an Hello Packet may be received from a neighbor
      claiming to be itself the Designated Router, and indicating that
      there is no Backup Designated Router.  In either case there must
      be bidirectional communication with the neighbor, i.e., the router
      must also appear in the neighbor's Hello Packet.  This event
      signals an end to the Waiting state.

   NeighborChange
      There has been a change in the set of bidirectional neighbors
      associated with the interface.  The (Backup) Designated Router
      needs to be recalculated.  The following neighbor changes lead to
      the NeighborChange event. For an explanation of neighbor states,
      see Section 10.1.

       o   Bidirectional communication has been established to a
           neighbor.  In other words, the state of the neighbor has
           transitioned to 2-Way or higher.

       o   There is no longer bidirectional communication with a
           neighbor.  In other words, the state of the neighbor has
           transitioned to Init or lower.

       o   One of the bidirectional neighbors is newly declaring
           itself as either Designated Router or Backup Designated
           Router.  This is detected through examination of that
           neighbor's Hello Packets.

       o   One of the bidirectional neighbors is no longer
           declaring itself as Designated Router, or is no longer
           declaring itself as Backup Designated Router.  This is
           again detected through examination of that neighbor's
           Hello Packets.



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       o   The advertised Router Priority for a bidirectional
           neighbor has changed.  This is again detected through
           examination of that neighbor's Hello Packets.

   LoopInd
      An indication has been received that the interface is now looped
      back to itself.  This indication can be received either from
      network management or from the lower level protocols.

   UnloopInd
      An indication has been received that the interface is no longer
      looped back.  As with the LoopInd event, this indication can be
      received either from network management or from the lower level
      protocols.

   InterfaceDown
      Lower-level protocols indicate that this interface is no longer
      functional. No matter what the current interface state is, the new
      interface state will be Down.

9.3.  The Interface state machine



   A detailed description of the interface state changes follows.  Each
   state change is invoked by an event (Section 9.2).  This event may
   produce different effects, depending on the current state of the
   interface.  For this reason, the state machine below is organized by
   current interface state and received event. Each entry in the state
   machine describes the resulting new interface state and the required
   set of additional actions.

   When an interface's state changes, it may be necessary to originate a
   new router-LSA.  See Section 12.4 for more details.

   Some of the required actions below involve generating events for the
   neighbor state machine.  For example, when an interface becomes
   inoperative, all neighbor connections associated with the interface
   must be destroyed.  For more information on the neighbor state
   machine, see Section 10.3.













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    State(s):  Down

       Event:  InterfaceUp

   New state:  Depends upon action routine

      Action:  Start the interval Hello Timer, enabling the
               periodic sending of Hello packets out the interface.
               If the attached network is a physical point-to-point
               network, Point-to-MultiPoint network or virtual
               link, the interface state transitions to Point-to-
               Point.  Else, if the router is not eligible to
               become Designated Router the interface state
               transitions to DR Other.

               Otherwise, the attached network is a broadcast or
               NBMA network and the router is eligible to become
               Designated Router.  In this case, in an attempt to
               discover the attached network's Designated Router
               the interface state is set to Waiting and the single
               shot Wait Timer is started.  Additionally, if the
               network is an NBMA network examine the configured
               list of neighbors for this interface and generate
               the neighbor event Start for each neighbor that is
               also eligible to become Designated Router.

    State(s):  Waiting

       Event:  BackupSeen

   New state:  Depends upon action routine.

      Action:  Calculate the attached network's Backup Designated
               Router and Designated Router, as shown in Section
               9.4.  As a result of this calculation, the new state
               of the interface will be either DR Other, Backup or
               DR.


    State(s):  Waiting

       Event:  WaitTimer

   New state:  Depends upon action routine.







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      Action:  Calculate the attached network's Backup Designated
               Router and Designated Router, as shown in Section
               9.4.  As a result of this calculation, the new state
               of the interface will be either DR Other, Backup or
               DR.


    State(s):  DR Other, Backup or DR

       Event:  NeighborChange

   New state:  Depends upon action routine.

      Action:  Recalculate the attached network's Backup Designated
               Router and Designated Router, as shown in Section
               9.4.  As a result of this calculation, the new state
               of the interface will be either DR Other, Backup or
               DR.


    State(s):  Any State

       Event:  InterfaceDown

   New state:  Down

      Action:  All interface variables are reset, and interface
               timers disabled.  Also, all neighbor connections
               associated with the interface are destroyed.  This
               is done by generating the event KillNbr on all
               associated neighbors (see Section 10.2).


    State(s):  Any State

       Event:  LoopInd

   New state:  Loopback

      Action:  Since this interface is no longer connected to the
               attached network the actions associated with the
               above InterfaceDown event are executed.


    State(s):  Loopback

       Event:  UnloopInd




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   New state:  Down

      Action:  No actions are necessary.  For example, the
               interface variables have already been reset upon
               entering the Loopback state.  Note that reception of
               an InterfaceUp event is necessary before the
               interface again becomes fully functional.

9.4.  Electing the Designated Router



   This section describes the algorithm used for calculating a network's
   Designated Router and Backup Designated Router.  This algorithm is
   invoked by the Interface state machine.  The initial time a router
   runs the election algorithm for a network, the network's Designated
   Router and Backup Designated Router are initialized to 0.0.0.0.  This
   indicates the lack of both a Designated Router and a Backup
   Designated Router.

   The Designated Router election algorithm proceeds as follows: Call
   the router doing the calculation Router X.  The list of neighbors
   attached to the network and having established bidirectional
   communication with Router X is examined.  This list is precisely the
   collection of Router X's neighbors (on this network) whose state is
   greater than or equal to 2-Way (see Section 10.1).  Router X itself
   is also considered to be on the list.  Discard all routers from the
   list that are ineligible to become Designated Router.  (Routers
   having Router Priority of 0 are ineligible to become Designated
   Router.)  The following steps are then executed, considering only
   those routers that remain on the list:

   (1) Note the current values for the network's Designated Router
       and Backup Designated Router.  This is used later for
       comparison purposes.

   (2) Calculate the new Backup Designated Router for the network
       as follows.  Only those routers on the list that have not
       declared themselves to be Designated Router are eligible to
       become Backup Designated Router.  If one or more of these
       routers have declared themselves Backup Designated Router
       (i.e., they are currently listing themselves as Backup
       Designated Router, but not as Designated Router, in their
       Hello Packets) the one having highest Router Priority is
       declared to be Backup Designated Router.  In case of a tie,
       the one having the highest Router ID is chosen.  If no
       routers have declared themselves Backup Designated Router,






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       choose the router having highest Router Priority, (again
       excluding those routers who have declared themselves
       Designated Router), and again use the Router ID to break
       ties.

   (3) Calculate the new Designated Router for the network as
       follows.  If one or more of the routers have declared
       themselves Designated Router (i.e., they are currently
       listing themselves as Designated Router in their Hello
       Packets) the one having highest Router Priority is declared
       to be Designated Router.  In case of a tie, the one having
       the highest Router ID is chosen.  If no routers have
       declared themselves Designated Router, assign the Designated
       Router to be the same as the newly elected Backup Designated
       Router.

   (4) If Router X is now newly the Designated Router or newly the
       Backup Designated Router, or is now no longer the Designated
       Router or no longer the Backup Designated Router, repeat
       steps 2 and 3, and then proceed to step 5.  For example, if
       Router X is now the Designated Router, when step 2 is
       repeated X will no longer be eligible for Backup Designated
       Router election.  Among other things, this will ensure that
       no router will declare itself both Backup Designated Router
       and Designated Router.[5]

   (5) As a result of these calculations, the router itself may now
       be Designated Router or Backup Designated Router.  See
       Sections 7.3 and 7.4 for the additional duties this would
       entail.  The router's interface state should be set
       accordingly.  If the router itself is now Designated Router,
       the new interface state is DR.  If the router itself is now
       Backup Designated Router, the new interface state is Backup.
       Otherwise, the new interface state is DR Other.

   (6) If the attached network is an NBMA network, and the router
       itself has just become either Designated Router or Backup
       Designated Router, it must start sending Hello Packets to
       those neighbors that are not eligible to become Designated
       Router (see Section 9.5.1).  This is done by invoking the
       neighbor event Start for each neighbor having a Router
       Priority of 0.

   (7) If the above calculations have caused the identity of either
       the Designated Router or Backup Designated Router to change,
       the set of adjacencies associated with this interface will
       need to be modified.  Some adjacencies may need to be
       formed, and others may need to be broken.  To accomplish



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       this, invoke the event AdjOK?  on all neighbors whose state
       is at least 2-Way.  This will cause their eligibility for
       adjacency to be reexamined (see Sections 10.3 and 10.4).


   The reason behind the election algorithm's complexity is the desire
   for an orderly transition from Backup Designated Router to Designated
   Router, when the current Designated Router fails.  This orderly
   transition is ensured through the introduction of hysteresis: no new
   Backup Designated Router can be chosen until the old Backup accepts
   its new Designated Router responsibilities.

   The above procedure may elect the same router to be both Designated
   Router and Backup Designated Router, although that router will never
   be the calculating router (Router X) itself.  The elected Designated
   Router may not be the router having the highest Router Priority, nor
   will the Backup Designated Router necessarily have the second highest
   Router Priority.  If Router X is not itself eligible to become
   Designated Router, it is possible that neither a Backup Designated
   Router nor a Designated Router will be selected in the above
   procedure.  Note also that if Router X is the only attached router
   that is eligible to become Designated Router, it will select itself
   as Designated Router and there will be no Backup Designated Router
   for the network.

9.5.  Sending Hello packets



   Hello packets are sent out each functioning router interface.  They
   are used to discover and maintain neighbor relationships.[6] On
   broadcast and NBMA networks, Hello Packets are also used to elect the
   Designated Router and Backup Designated Router.

   The format of an Hello packet is detailed in Section A.3.2.  The
   Hello Packet contains the router's Router Priority (used in choosing
   the Designated Router), and the interval between Hello Packets sent
   out the interface (HelloInterval).  The Hello Packet also indicates
   how often a neighbor must be heard from to remain active
   (RouterDeadInterval).  Both HelloInterval and RouterDeadInterval must
   be the same for all routers attached to a common network.  The Hello
   packet also contains the IP address mask of the attached network
   (Network Mask).  On unnumbered point-to-point networks and on virtual
   links this field should be set to 0.0.0.0.

   The Hello packet's Options field describes the router's optional OSPF
   capabilities.  One optional capability is defined in this
   specification (see Sections 4.5 and A.2).  The E-bit of the Options
   field should be set if and only if the attached area is capable of
   processing AS-external-LSAs (i.e., it is not a stub area). If the E-



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   bit is set incorrectly the neighboring routers will refuse to accept
   the Hello Packet (see Section 10.5).  Unrecognized bits in the Hello
   Packet's Options field should be set to zero.

   In order to ensure two-way communication between adjacent routers,
   the Hello packet contains the list of all routers on the network from
   which Hello Packets have been seen recently.  The Hello packet also
   contains the router's current choice for Designated Router and Backup
   Designated Router.  A value of 0.0.0.0 in these fields means that one
   has not yet been selected.

   On broadcast networks and physical point-to-point networks, Hello
   packets are sent every HelloInterval seconds to the IP multicast
   address AllSPFRouters.  On virtual links, Hello packets are sent as
   unicasts (addressed directly to the other end of the virtual link)
   every HelloInterval seconds. On Point-to-MultiPoint networks,
   separate Hello packets are sent to each attached neighbor every
   HelloInterval seconds. Sending of Hello packets on NBMA networks is
   covered in the next section.

9.5.1.  Sending Hello packets on NBMA networks



   Static configuration information may be necessary in order for the
   Hello Protocol to function on non-broadcast networks (see Sections
   C.5 and C.6).  On NBMA networks, every attached router which is
   eligible to become Designated Router becomes aware of all of its
   neighbors on the network (either through configuration or by some
   unspecified mechanism).  Each neighbor is labelled with the
   neighbor's Designated Router eligibility.

   The interface state must be at least Waiting for any Hello Packets to
   be sent out the NBMA interface. Hello Packets are then sent directly
   (as unicasts) to some subset of a router's neighbors.  Sometimes an
   Hello Packet is sent periodically on a timer; at other times it is
   sent as a response to a received Hello Packet.  A router's hello-
   sending behavior varies depending on whether the router itself is
   eligible to become Designated Router.

   If the router is eligible to become Designated Router, it must
   periodically send Hello Packets to all neighbors that are also
   eligible. In addition, if the router is itself the Designated Router
   or Backup Designated Router, it must also send periodic Hello Packets
   to all other neighbors.  This means that any two eligible routers are
   always exchanging Hello Packets, which is necessary for the correct
   operation of the Designated Router election algorithm.  To minimize
   the number of Hello Packets sent, the number of eligible routers on
   an NBMA network should be kept small.




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   If the router is not eligible to become Designated Router, it must
   periodically send Hello Packets to both the Designated Router and the
   Backup Designated Router (if they exist).  It must also send an Hello
   Packet in reply to an Hello Packet received from any eligible
   neighbor (other than the current Designated Router and Backup
   Designated Router).  This is needed to establish an initial
   bidirectional relationship with any potential Designated Router.

   When sending Hello packets periodically to any neighbor, the interval
   between Hello Packets is determined by the neighbor's state.  If the
   neighbor is in state Down, Hello Packets are sent every PollInterval
   seconds.  Otherwise, Hello Packets are sent every HelloInterval
   seconds.

10.  The Neighbor Data Structure



   An OSPF router converses with its neighboring routers.  Each separate
   conversation is described by a "neighbor data structure".  Each
   conversation is bound to a particular OSPF router interface, and is
   identified either by the neighboring router's OSPF Router ID or by
   its Neighbor IP address (see below). Thus if the OSPF router and
   another router have multiple attached networks in common, multiple
   conversations ensue, each described by a unique neighbor data
   structure.  Each separate conversation is loosely referred to in the
   text as being a separate "neighbor".

   The neighbor data structure contains all information pertinent to the
   forming or formed adjacency between the two neighbors.  (However,
   remember that not all neighbors become adjacent.)  An adjacency can
   be viewed as a highly developed conversation between two routers.

   State
      The functional level of the neighbor conversation.  This is
      described in more detail in Section 10.1.

   Inactivity Timer
      A single shot timer whose firing indicates that no Hello Packet
      has been seen from this neighbor recently.  The length of the
      timer is RouterDeadInterval seconds.

   Master/Slave
      When the two neighbors are exchanging databases, they form a
      master/slave relationship.  The master sends the first Database
      Description Packet, and is the only part that is allowed to
      retransmit.  The slave can only respond to the master's Database
      Description Packets.  The master/slave relationship is negotiated
      in state ExStart.




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   DD Sequence Number
      The DD Sequence number of the Database Description packet that is
      currently being sent to the neighbor.

   Last received Database Description packet
      The initialize(I), more (M) and master(MS) bits, Options field,
      and DD sequence number contained in the last Database Description
      packet received from the neighbor. Used to determine whether the
      next Database Description packet received from the neighbor is a
      duplicate.

   Neighbor ID
      The OSPF Router ID of the neighboring router.  The Neighbor ID is
      learned when Hello packets are received from the neighbor, or is
      configured if this is a virtual adjacency (see Section C.4).

   Neighbor Priority
      The Router Priority of the neighboring router. Contained in the
      neighbor's Hello packets, this item is used when selecting the
      Designated Router for the attached network.

   Neighbor IP address
      The IP address of the neighboring router's interface to the
      attached network.  Used as the Destination IP address when
      protocol packets are sent as unicasts along this adjacency.  Also
      used in router-LSAs as the Link ID for the attached network if the
      neighboring router is selected to be Designated Router (see
      Section 12.4.1).  The Neighbor IP address is learned when Hello
      packets are received from the neighbor.  For virtual links, the
      Neighbor IP address is learned during the routing table build
      process (see Section 15).

   Neighbor Options
      The optional OSPF capabilities supported by the neighbor.  Learned
      during the Database Exchange process (see Section 10.6).  The
      neighbor's optional OSPF capabilities are also listed in its Hello
      packets. This enables received Hello Packets to be rejected (i.e.,
      neighbor relationships will not even start to form) if there is a
      mismatch in certain crucial OSPF capabilities (see Section 10.5).
      The optional OSPF capabilities are documented in Section 4.5.

   Neighbor's Designated Router
      The neighbor's idea of the Designated Router.  If this is the
      neighbor itself, this is important in the local calculation of the
      Designated Router. Defined only on broadcast and NBMA networks.






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   Neighbor's Backup Designated Router
      The neighbor's idea of the Backup Designated Router.  If this is
      the neighbor itself, this is important in the local calculation of
      the Backup Designated Router.  Defined only on broadcast and NBMA
      networks.

   The next set of variables are lists of LSAs.  These lists describe
   subsets of the area link-state database.  This memo defines five
   distinct types of LSAs, all of which may be present in an area link-
   state database: router-LSAs, network-LSAs, and Type 3 and 4 summary-
   LSAs (all stored in the area data structure), and AS- external-LSAs
   (stored in the global data structure).

   Link state retransmission list
      The list of LSAs that have been flooded but not acknowledged on
      this adjacency.  These will be retransmitted at intervals until
      they are acknowledged, or until the adjacency is destroyed.

   Database summary list
      The complete list of LSAs that make up the area link-state
      database, at the moment the neighbor goes into Database Exchange
      state.  This list is sent to the neighbor in Database Description
      packets.

   Link state request list
      The list of LSAs that need to be received from this neighbor in
      order to synchronize the two neighbors' link-state databases.
      This list is created as Database Description packets are received,
      and is then sent to the neighbor in Link State Request packets.
      The list is depleted as appropriate Link State Update packets are
      received.

10.1.  Neighbor states



   The state of a neighbor (really, the state of a conversation being
   held with a neighboring router) is documented in the following
   sections.  The states are listed in order of progressing
   functionality.  For example, the inoperative state is listed first,
   followed by a list of intermediate states before the final, fully
   functional state is achieved.  The specification makes use of this
   ordering by sometimes making references such as "those
   neighbors/adjacencies in state greater than X".  Figures 12 and 13
   show the graph of neighbor state changes.  The arcs of the graphs are
   labelled with the event causing the state change.  The neighbor
   events are documented in Section 10.2.






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   The graph in Figure 12 shows the state changes effected by the Hello
   Protocol.  The Hello Protocol is responsible for neighbor acquisition
   and maintenance, and for ensuring two way communication between
   neighbors.

   The graph in Figure 13 shows the forming of an adjacency.  Not every
   two neighboring routers become adjacent (see Section 10.4).  The
   adjacency starts to form when the neighbor is in state ExStart.
   After the two routers discover their master/slave status, the state
   transitions to Exchange.  At this point the neighbor starts to be
   used in the flooding procedure, and the two neighboring routers begin
   synchronizing their databases.  When this synchronization is
   finished, the neighbor is in state Full and we say that the two
   routers are fully adjacent.  At this point the adjacency is listed in
   LSAs.

   For a more detailed description of neighbor state changes, together
   with the additional actions involved in each change, see Section
   10.3.

   Down
      This is the initial state of a neighbor conversation.  It
      indicates that there has been no recent information received from
      the neighbor. On NBMA networks, Hello packets may still be sent to
      "Down" neighbors, although at a reduced frequency (see Section
      9.5.1).

























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                                   +----+
                                   |Down|
                                   +----+
                                     |\
                                     | \Start
                                     |  \      +-------+
                             Hello   |   +---->|Attempt|
                            Received |         +-------+
                                     |             |
                             +----+<-+             |HelloReceived
                             |Init|<---------------+
                             +----+<--------+
                                |           |
                                |2-Way      |1-Way
                                |Received   |Received
                                |           |
              +-------+         |        +-----+
              |ExStart|<--------+------->|2-Way|
              +-------+                  +-----+

           Figure 12: Neighbor state changes (Hello Protocol)

             In addition to the state transitions pictured,
                Event KillNbr always forces Down State,
            Event Inactivity Timer always forces Down State,
                 Event LLDown always forces Down State

























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                                  +-------+
                                  |ExStart|
                                  +-------+
                                    |
                     NegotiationDone|
                                    +->+--------+
                                       |Exchange|
                                    +--+--------+
                                    |
                            Exchange|
                              Done  |
                    +----+          |      +-------+
                    |Full|<---------+----->|Loading|
                    +----+<-+              +-------+
                            |  LoadingDone     |
                            +------------------+

         Figure 13: Neighbor state changes (Database Exchange)

             In addition to the state transitions pictured,
             Event SeqNumberMismatch forces ExStart state,
                  Event BadLSReq forces ExStart state,
                     Event 1-Way forces Init state,
                Event KillNbr always forces Down State,
            Event InactivityTimer always forces Down State,
                 Event LLDown always forces Down State,
            Event AdjOK? leads to adjacency forming/breaking
























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   Attempt
      This state is only valid for neighbors attached to NBMA networks.
      It indicates that no recent information has been received from the
      neighbor, but that a more concerted effort should be made to
      contact the neighbor.  This is done by sending the neighbor Hello
      packets at intervals of HelloInterval (see Section 9.5.1).

   Init
      In this state, an Hello packet has recently been seen from the
      neighbor.  However, bidirectional communication has not yet been
      established with the neighbor (i.e., the router itself did not
      appear in the neighbor's Hello packet).  All neighbors in this
      state (or higher) are listed in the Hello packets sent from the
      associated interface.

   2-Way
      In this state, communication between the two routers is
      bidirectional.  This has been assured by the operation of the
      Hello Protocol.  This is the most advanced state short of
      beginning adjacency establishment.  The (Backup) Designated Router
      is selected from the set of neighbors in state 2-Way or greater.

   ExStart
      This is the first step in creating an adjacency between the two
      neighboring routers.  The goal of this step is to decide which
      router is the master, and to decide upon the initial DD sequence
      number.  Neighbor conversations in this state or greater are
      called adjacencies.

   Exchange
      In this state the router is describing its entire link state
      database by sending Database Description packets to the neighbor.
      Each Database Description Packet has a DD sequence number, and is
      explicitly acknowledged.  Only one Database Description Packet is
      allowed outstanding at any one time.  In this state, Link State
      Request Packets may also be sent asking for the neighbor's more
      recent LSAs.  All adjacencies in Exchange state or greater are
      used by the flooding procedure.  In fact, these adjacencies are
      fully capable of transmitting and receiving all types of OSPF
      routing protocol packets.

   Loading
      In this state, Link State Request packets are sent to the neighbor
      asking for the more recent LSAs that have been discovered (but not
      yet received) in the Exchange state.






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   Full
      In this state, the neighboring routers are fully adjacent.  These
      adjacencies will now appear in router-LSAs and network-LSAs.

10.2.  Events causing neighbor state changes



   State changes can be effected by a number of events.  These events
   are shown in the labels of the arcs in Figures 12 and 13.  The label
   definitions are as follows:

   HelloReceived
      An Hello packet has been received from the neighbor.

   Start
      This is an indication that Hello Packets should now be sent to the
      neighbor at intervals of HelloInterval seconds.  This event is
      generated only for neighbors associated with NBMA networks.

   2-WayReceived
      Bidirectional communication has been realized between the two
      neighboring routers.  This is indicated by the router seeing
      itself in the neighbor's Hello packet.

   NegotiationDone
      The Master/Slave relationship has been negotiated, and DD sequence
      numbers have been exchanged.  This signals the start of the
      sending/receiving of Database Description packets.  For more
      information on the generation of this event, consult Section 10.8.

   ExchangeDone
      Both routers have successfully transmitted a full sequence of
      Database Description packets.  Each router now knows what parts of
      its link state database are out of date.  For more information on
      the generation of this event, consult Section 10.8.

   BadLSReq
      A Link State Request has been received for an LSA not contained in
      the database. This indicates an error in the Database Exchange
      process.

   Loading Done
      Link State Updates have been received for all out-of-date portions
      of the database.  This is indicated by the Link state request list
      becoming empty after the Database Exchange process has completed.







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   AdjOK?
      A decision must be made as to whether an adjacency should be
      established/maintained with the neighbor.  This event will start
      some adjacencies forming, and destroy others.

   The following events cause well developed neighbors to revert to
   lesser states.  Unlike the above events, these events may occur when
   the neighbor conversation is in any of a number of states.

   SeqNumberMismatch
      A Database Description packet has been received that either a) has
      an unexpected DD sequence number, b) unexpectedly has the Init bit
      set or c) has an Options field differing from the last Options
      field received in a Database Description packet.  Any of these
      conditions indicate that some error has occurred during adjacency
      establishment.

   1-Way
      An Hello packet has been received from the neighbor, in which the
      router is not mentioned. This indicates that communication with
      the neighbor is not bidirectional.

   KillNbr
      This is an indication that all communication with the neighbor is
      now impossible, forcing the neighbor to revert to Down state.

   InactivityTimer
      The inactivity Timer has fired.  This means that no Hello packets
      have been seen recently from the neighbor. The neighbor reverts to
      Down state.

   LLDown
      This is an indication from the lower level protocols that the
      neighbor is now unreachable.  For example, on an X.25 network this
      could be indicated by an X.25 clear indication with appropriate
      cause and diagnostic fields.  This event forces the neighbor into
      Down state.

10.3.  The Neighbor state machine



   A detailed description of the neighbor state changes follows.  Each
   state change is invoked by an event (Section 10.2).  This event may
   produce different effects, depending on the current state of the
   neighbor.  For this reason, the state machine below is organized by
   current neighbor state and received event.  Each entry in the state
   machine describes the resulting new neighbor state and the required
   set of additional actions.




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   When a neighbor's state changes, it may be necessary to rerun the
   Designated Router election algorithm.  This is determined by whether
   the interface NeighborChange event is generated (see Section 9.2).
   Also, if the Interface is in DR state (the router is itself
   Designated Router), changes in neighbor state may cause a new
   network-LSA to be originated (see Section 12.4).

   When the neighbor state machine needs to invoke the interface state
   machine, it should be done as a scheduled task (see Section 4.4).
   This simplifies things, by ensuring that neither state machine will
   be executed recursively.


    State(s):  Down

       Event:  Start

   New state:  Attempt

      Action:  Send an Hello Packet to the neighbor (this neighbor
               is always associated with an NBMA network) and start
               the Inactivity Timer for the neighbor.  The timer's
               later firing would indicate that communication with
               the neighbor was not attained.


    State(s):  Attempt

       Event:  HelloReceived

   New state:  Init

      Action:  Restart the Inactivity Timer for the neighbor, since
               the neighbor has now been heard from.


    State(s):  Down

       Event:  HelloReceived

   New state:  Init

      Action:  Start the Inactivity Timer for the neighbor.  The
               timer's later firing would indicate that the neighbor
               is dead.






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    State(s):  Init or greater

       Event:  HelloReceived

   New state:  No state change.

      Action:  Restart the Inactivity Timer for the neighbor, since
               the neighbor has again been heard from.


    State(s):  Init

       Event:  2-WayReceived

   New state:  Depends upon action routine.

      Action:  Determine whether an adjacency should be established
               with the neighbor (see Section 10.4).  If not, the
               new neighbor state is 2-Way.

               Otherwise (an adjacency should be established) the
               neighbor state transitions to ExStart.  Upon
               entering this state, the router increments the DD
               sequence number in the neighbor data structure.  If
               this is the first time that an adjacency has been
               attempted, the DD sequence number should be assigned
               some unique value (like the time of day clock).  It
               then declares itself master (sets the master/slave
               bit to master), and starts sending Database
               Description Packets, with the initialize (I), more
               (M) and master (MS) bits set.  This Database
               Description Packet should be otherwise empty.  This
               Database Description Packet should be retransmitted
               at intervals of RxmtInterval until the next state is
               entered (see Section 10.8).


    State(s):  ExStart

       Event:  NegotiationDone

   New state:  Exchange

      Action:  The router must list the contents of its entire area
               link state database in the neighbor Database summary
               list.  The area link state database consists of the
               router-LSAs, network-LSAs and summary-LSAs contained
               in the area structure, along with the AS-external-



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               LSAs contained in the global structure.  AS-
               external-LSAs are omitted from a virtual neighbor's
               Database summary list.  AS-external-LSAs are omitted
               from the Database summary list if the area has been
               configured as a stub (see Section 3.6).  LSAs whose
               age is equal to MaxAge are instead added to the
               neighbor's Link state retransmission list.  A
               summary of the Database summary list will be sent to
               the neighbor in Database Description packets.  Each
               Database Description Packet has a DD sequence
               number, and is explicitly acknowledged.  Only one
               Database Description Packet is allowed outstanding
               at any one time.  For more detail on the sending and
               receiving of Database Description packets, see
               Sections 10.8 and 10.6.


    State(s):  Exchange

       Event:  ExchangeDone

   New state:  Depends upon action routine.

      Action:  If the neighbor Link state request list is empty,
               the new neighbor state is Full.  No other action is
               required.  This is an adjacency's final state.

               Otherwise, the new neighbor state is Loading.  Start
               (or continue) sending Link State Request packets to
               the neighbor (see Section 10.9).  These are requests
               for the neighbor's more recent LSAs (which were
               discovered but not yet received in the Exchange
               state).  These LSAs are listed in the Link state
               request list associated with the neighbor.


    State(s):  Loading

       Event:  Loading Done

   New state:  Full

      Action:  No action required.  This is an adjacency's final
               state.







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    State(s):  2-Way

       Event:  AdjOK?

   New state:  Depends upon action routine.

      Action:  Determine whether an adjacency should be formed with
               the neighboring router (see Section 10.4).  If not,
               the neighbor state remains at 2-Way.  Otherwise,
               transition the neighbor state to ExStart and perform
               the actions associated with the above state machine
               entry for state Init and event 2-WayReceived.


    State(s):  ExStart or greater

       Event:  AdjOK?

   New state:  Depends upon action routine.

      Action:  Determine whether the neighboring router should
               still be adjacent.  If yes, there is no state change
               and no further action is necessary.

               Otherwise, the (possibly partially formed) adjacency
               must be destroyed.  The neighbor state transitions
               to 2-Way.  The Link state retransmission list,
               Database summary list and Link state request list
               are cleared of LSAs.


    State(s):  Exchange or greater

       Event:  SeqNumberMismatch

   New state:  ExStart

      Action:  The (possibly partially formed) adjacency is torn
               down, and then an attempt is made at
               reestablishment.  The neighbor state first
               transitions to ExStart.  The Link state
               retransmission list, Database summary list and Link
               state request list are cleared of LSAs.  Then the
               router increments the DD sequence number in the
               neighbor data structure, declares itself master
               (sets the master/slave bit to master), and starts
               sending Database Description Packets, with the
               initialize (I), more (M) and master (MS) bits set.



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               This Database Description Packet should be otherwise
               empty (see Section 10.8).


    State(s):  Exchange or greater

       Event:  BadLSReq

   New state:  ExStart

      Action:  The action for event BadLSReq is exactly the same as
               for the neighbor event SeqNumberMismatch.  The
               (possibly partially formed) adjacency is torn down,
               and then an attempt is made at reestablishment.  For
               more information, see the neighbor state machine
               entry that is invoked when event SeqNumberMismatch
               is generated in state Exchange or greater.


    State(s):  Any state

       Event:  KillNbr

   New state:  Down

      Action:  The Link state retransmission list, Database summary
               list and Link state request list are cleared of
               LSAs.  Also, the Inactivity Timer is disabled.


    State(s):  Any state

       Event:  LLDown

   New state:  Down

      Action:  The Link state retransmission list, Database summary
               list and Link state request list are cleared of
               LSAs.  Also, the Inactivity Timer is disabled.












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    State(s):  Any state

       Event:  InactivityTimer

   New state:  Down

      Action:  The Link state retransmission list, Database summary
               list and Link state request list are cleared of
               LSAs.


    State(s):  2-Way or greater

       Event:  1-WayReceived

   New state:  Init

      Action:  The Link state retransmission list, Database summary
               list and Link state request list are cleared of
               LSAs.


    State(s):  2-Way or greater

       Event:  2-WayReceived

   New state:  No state change.

      Action:  No action required.


    State(s):  Init

       Event:  1-WayReceived

   New state:  No state change.

      Action:  No action required.


10.4.  Whether to become adjacent



   Adjacencies are established with some subset of the router's
   neighbors.  Routers connected by point-to-point networks, Point-to-
   MultiPoint networks and virtual links always become adjacent.  On
   broadcast and NBMA networks, all routers become adjacent to both the
   Designated Router and the Backup Designated Router.




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   The adjacency-forming decision occurs in two places in the neighbor
   state machine.  First, when bidirectional communication is initially
   established with the neighbor, and secondly, when the identity of the
   attached network's (Backup) Designated Router changes.  If the
   decision is made to not attempt an adjacency, the state of the
   neighbor communication stops at 2-Way.

   An adjacency should be established with a bidirectional neighbor when
   at least one of the following conditions holds:

   o   The underlying network type is point-to-point

   o   The underlying network type is Point-to-MultiPoint

   o   The underlying network type is virtual link

   o   The router itself is the Designated Router

   o   The router itself is the Backup Designated Router

   o   The neighboring router is the Designated Router

   o   The neighboring router is the Backup Designated Router

10.5.  Receiving Hello Packets



   This section explains the detailed processing of a received Hello
   Packet.  (See Section A.3.2 for the format of Hello packets.)  The
   generic input processing of OSPF packets will have checked the
   validity of the IP header and the OSPF packet header.  Next, the
   values of the Network Mask, HelloInterval, and RouterDeadInterval
   fields in the received Hello packet must be checked against the
   values configured for the receiving interface.  Any mismatch causes
   processing to stop and the packet to be dropped.  In other words, the
   above fields are really describing the attached network's
   configuration.  However, there is one exception to the above rule: on
   point-to-point networks and on virtual links, the Network Mask in the
   received Hello Packet should be ignored.

   The receiving interface attaches to a single OSPF area (this could be
   the backbone).  The setting of the E-bit found in the Hello Packet's
   Options field must match this area's ExternalRoutingCapability.  If
   AS-external-LSAs are not flooded into/throughout the area (i.e, the
   area is a "stub") the E-bit must be clear in received Hello Packets,
   otherwise the E-bit must be set.  A mismatch causes processing to
   stop and the packet to be dropped.  The setting of the rest of the
   bits in the Hello Packet's Options field should be ignored.




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   At this point, an attempt is made to match the source of the Hello
   Packet to one of the receiving interface's neighbors.  If the
   receiving interface connects to a broadcast, Point-to-MultiPoint or
   NBMA network the source is identified by the IP source address found
   in the Hello's IP header.  If the receiving interface connects to a
   point-to-point link or a virtual link, the source is identified by
   the Router ID found in the Hello's OSPF packet header.  The
   interface's current list of neighbors is contained in the interface's
   data structure.  If a matching neighbor structure cannot be found,
   (i.e., this is the first time the neighbor has been detected), one is
   created.  The initial state of a newly created neighbor is set to
   Down.

   When receiving an Hello Packet from a neighbor on a broadcast,
   Point-to-MultiPoint or NBMA network, set the neighbor structure's
   Neighbor ID equal to the Router ID found in the packet's OSPF header.
   When receiving an Hello on a point-to-point network (but not on a
   virtual link) set the neighbor structure's Neighbor IP address to the
   packet's IP source address.

   Now the rest of the Hello Packet is examined, generating events to be
   given to the neighbor and interface state machines.  These state
   machines are specified either to be executed or scheduled (see
   Section 4.4).  For example, by specifying below that the neighbor
   state machine be executed in line, several neighbor state transitions
   may be effected by a single received Hello:

   o   Each Hello Packet causes the neighbor state machine to be
       executed with the event HelloReceived.

   o   Then the list of neighbors contained in the Hello Packet is
       examined.  If the router itself appears in this list, the
       neighbor state machine should be executed with the event 2-
       WayReceived.  Otherwise, the neighbor state machine should
       be executed with the event 1-WayReceived, and the processing
       of the packet stops.

   o   Next, the Hello Packet's Router Priority field is examined.
       If this field is different than the one previously received
       from the neighbor, the receiving interface's state machine
       is scheduled with the event NeighborChange.  In any case,
       the Router Priority field in the neighbor data structure
       should be updated accordingly.

   o   Next the Designated Router field in the Hello Packet is
       examined.  If the neighbor is both declaring itself to be
       Designated Router (Designated Router field = Neighbor IP
       address) and the Backup Designated Router field in the



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       packet is equal to 0.0.0.0 and the receiving interface is in
       state Waiting, the receiving interface's state machine is
       scheduled with the event BackupSeen.  Otherwise, if the
       neighbor is declaring itself to be Designated Router and it
       had not previously, or the neighbor is not declaring itself
       Designated Router where it had previously, the receiving
       interface's state machine is scheduled with the event
       NeighborChange.  In any case, the Neighbors' Designated
       Router item in the neighbor structure is updated
       accordingly.

   o   Finally, the Backup Designated Router field in the Hello
       Packet is examined.  If the neighbor is declaring itself to
       be Backup Designated Router (Backup Designated Router field
       = Neighbor IP address) and the receiving interface is in
       state Waiting, the receiving interface's state machine is
       scheduled with the event BackupSeen.  Otherwise, if the
       neighbor is declaring itself to be Backup Designated Router
       and it had not previously, or the neighbor is not declaring
       itself Backup Designated Router where it had previously, the
       receiving interface's state machine is scheduled with the
       event NeighborChange.  In any case, the Neighbor's Backup
       Designated Router item in the neighbor structure is updated
       accordingly.

   On NBMA networks, receipt of an Hello Packet may also cause an Hello
   Packet to be sent back to the neighbor in response. See Section 9.5.1
   for more details.

10.6.  Receiving Database Description Packets



   This section explains the detailed processing of a received Database
   Description Packet.  The incoming Database Description Packet has
   already been associated with a neighbor and receiving interface by
   the generic input packet processing (Section 8.2).  Whether the
   Database Description packet should be accepted, and if so, how it
   should be further processed depends upon the neighbor state.

   If a Database Description packet is accepted, the following packet
   fields should be saved in the corresponding neighbor data structure
   under "last received Database Description packet": the packet's
   initialize(I), more (M) and master(MS) bits, Options field, and DD
   sequence number. If these fields are set identically in two
   consecutive Database Description packets received from the neighbor,
   the second Database Description packet is considered to be a
   "duplicate" in the processing described below.





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   If the Interface MTU field in the Database Description packet
   indicates an IP datagram size that is larger than the router can
   accept on the receiving interface without fragmentation, the Database
   Description packet is rejected.  Otherwise, if the neighbor state is:

   Down
      The packet should be rejected.

   Attempt
      The packet should be rejected.

   Init
      The neighbor state machine should be executed with the event 2-
      WayReceived.  This causes an immediate state change to either
      state 2-Way or state ExStart. If the new state is ExStart, the
      processing of the current packet should then continue in this new
      state by falling through to case ExStart below.

   2-Way
      The packet should be ignored.  Database Description Packets are
      used only for the purpose of bringing up adjacencies.[7]

   ExStart
      If the received packet matches one of the following cases, then
      the neighbor state machine should be executed with the event
      NegotiationDone (causing the state to transition to Exchange), the
      packet's Options field should be recorded in the neighbor
      structure's Neighbor Options field and the packet should be
      accepted as next in sequence and processed further (see below).
      Otherwise, the packet should be ignored.

       o   The initialize(I), more (M) and master(MS) bits are set,
           the contents of the packet are empty, and the neighbor's
           Router ID is larger than the router's own.  In this case
           the router is now Slave.  Set the master/slave bit to
           slave, and set the neighbor data structure's DD sequence
           number to that specified by the master.

       o   The initialize(I) and master(MS) bits are off, the
           packet's DD sequence number equals the neighbor data
           structure's DD sequence number (indicating
           acknowledgment) and the neighbor's Router ID is smaller
           than the router's own.  In this case the router is
           Master.







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   Exchange
      Duplicate Database Description packets are discarded by the
      master, and cause the slave to retransmit the last Database
      Description packet that it had sent. Otherwise (the packet is not
      a duplicate):

       o   If the state of the MS-bit is inconsistent with the
           master/slave state of the connection, generate the
           neighbor event SeqNumberMismatch and stop processing the
           packet.

       o   If the initialize(I) bit is set, generate the neighbor
           event SeqNumberMismatch and stop processing the packet.

       o   If the packet's Options field indicates a different set
           of optional OSPF capabilities than were previously
           received from the neighbor (recorded in the Neighbor
           Options field of the neighbor structure), generate the
           neighbor event SeqNumberMismatch and stop processing the
           packet.

       o   Database Description packets must be processed in
           sequence, as indicated by the packets' DD sequence
           numbers. If the router is master, the next packet
           received should have DD sequence number equal to the DD
           sequence number in the neighbor data structure. If the
           router is slave, the next packet received should have DD
           sequence number equal to one more than the DD sequence
           number stored in the neighbor data structure. In either
           case, if the packet is the next in sequence it should be
           accepted and its contents processed as specified below.

       o   Else, generate the neighbor event SeqNumberMismatch and
           stop processing the packet.

   Loading or Full
      In this state, the router has sent and received an entire sequence
      of Database Description Packets.  The only packets received should
      be duplicates (see above). In particular, the packet's Options
      field should match the set of optional OSPF capabilities
      previously indicated by the neighbor (stored in the neighbor
      structure's Neighbor Options field).  Any other packets received,
      including the reception of a packet with the Initialize(I) bit
      set, should generate the neighbor event SeqNumberMismatch.[8]
      Duplicates should be discarded by the master.  The slave must
      respond to duplicates by repeating the last Database Description
      packet that it had sent.




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   When the router accepts a received Database Description Packet as the
   next in sequence the packet contents are processed as follows.  For
   each LSA listed, the LSA's LS type is checked for validity.  If the
   LS type is unknown (e.g., not one of the LS types 1-5 defined by this
   specification), or if this is an AS-external-LSA (LS type = 5) and
   the neighbor is associated with a stub area, generate the neighbor
   event SeqNumberMismatch and stop processing the packet.  Otherwise,
   the router looks up the LSA in its database to see whether it also
   has an instance of the LSA.  If it does not, or if the database copy
   is less recent (see Section 13.1), the LSA is put on the Link state
   request list so that it can be requested (immediately or at some
   later time) in Link State Request Packets.

   When the router accepts a received Database Description Packet as the
   next in sequence, it also performs the following actions, depending
   on whether it is master or slave:

   Master
      Increments the DD sequence number in the neighbor data structure.
      If the router has already sent its entire sequence of Database
      Description Packets, and the just accepted packet has the more bit
      (M) set to 0, the neighbor event ExchangeDone is generated.
      Otherwise, it should send a new Database Description to the slave.

   Slave
      Sets the DD sequence number in the neighbor data structure to the
      DD sequence number appearing in the received packet.  The slave
      must send a Database Description Packet in reply.  If the received
      packet has the more bit (M) set to 0, and the packet to be sent by
      the slave will also have the M-bit set to 0, the neighbor event
      ExchangeDone is generated.  Note that the slave always generates
      this event before the master.

10.7.  Receiving Link State Request Packets



   This section explains the detailed processing of received Link State
   Request packets.  Received Link State Request Packets specify a list
   of LSAs that the neighbor wishes to receive.  Link State Request
   Packets should be accepted when the neighbor is in states Exchange,
   Loading, or Full.  In all other states Link State Request Packets
   should be ignored.










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   Each LSA specified in the Link State Request packet should be located
   in the router's database, and copied into Link State Update packets
   for transmission to the neighbor.  These LSAs should NOT be placed on
   the Link state retransmission list for the neighbor.  If an LSA
   cannot be found in the database, something has gone wrong with the
   Database Exchange process, and neighbor event BadLSReq should be
   generated.

10.8.  Sending Database Description Packets



   This section describes how Database Description Packets are sent to a
   neighbor. The Database Description packet's Interface MTU field is
   set to the size of the largest IP datagram that can be sent out the
   sending interface, without fragmentation.  Common MTUs in use in the
   Internet can be found in Table 7-1 of [Ref22]. Interface MTU should
   be set to 0 in Database Description packets sent over virtual links.

   The router's optional OSPF capabilities (see Section 4.5) are
   transmitted to the neighbor in the Options field of the Database
   Description packet.  The router should maintain the same set of
   optional capabilities throughout the Database Exchange and flooding
   procedures.  If for some reason the router's optional capabilities
   change, the Database Exchange procedure should be restarted by
   reverting to neighbor state ExStart.  One optional capability is
   defined in this specification (see Sections 4.5 and A.2). The E-bit
   should be set if and only if the attached network belongs to a non-
   stub area. Unrecognized bits in the Options field should be set to
   zero.  The sending of Database Description packets depends on the
   neighbor's state.  In state ExStart the router sends empty Database
   Description packets, with the initialize (I), more (M) and master
   (MS) bits set.  These packets are retransmitted every RxmtInterval
   seconds.

   In state Exchange the Database Description Packets actually contain
   summaries of the link state information contained in the router's
   database.  Each LSA in the area's link-state database (at the time
   the neighbor transitions into Exchange state) is listed in the
   neighbor Database summary list.  Each new Database Description Packet
   copies its DD sequence number from the neighbor data structure and
   then describes the current top of the Database summary list.  Items
   are removed from the Database summary list when the previous packet
   is acknowledged.

   In state Exchange, the determination of when to send a Database
   Description packet depends on whether the router is master or slave:






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   Master
      Database Description packets are sent when either a) the slave
      acknowledges the previous Database Description packet by echoing
      the DD sequence number or b) RxmtInterval seconds elapse without
      an acknowledgment, in which case the previous Database Description
      packet is retransmitted.

   Slave
      Database Description packets are sent only in response to Database
      Description packets received from the master.  If the Database
      Description packet received from the master is new, a new Database
      Description packet is sent, otherwise the previous Database
      Description packet is resent.

   In states Loading and Full the slave must resend its last Database
   Description packet in response to duplicate Database Description
   packets received from the master.  For this reason the slave must
   wait RouterDeadInterval seconds before freeing the last Database
   Description packet.  Reception of a Database Description packet from
   the master after this interval will generate a SeqNumberMismatch
   neighbor event.

10.9.  Sending Link State Request Packets



   In neighbor states Exchange or Loading, the Link state request list
   contains a list of those LSAs that need to be obtained from the
   neighbor.  To request these LSAs, a router sends the neighbor the
   beginning of the Link state request list, packaged in a Link State
   Request packet.

   When the neighbor responds to these requests with the proper Link
   State Update packet(s), the Link state request list is truncated and
   a new Link State Request packet is sent.  This process continues
   until the Link state request list becomes empty.  Unsatisfied Link
   State Request packets are retransmitted at intervals of RxmtInterval.
   There should be at most one Link State Request packet outstanding at
   any one time.

   When the Link state request list becomes empty, and the neighbor
   state is Loading (i.e., a complete sequence of Database Description
   packets has been sent to and received from the neighbor), the Loading
   Done neighbor event is generated.









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10.10.  An Example



   Figure 14 shows an example of an adjacency forming.  Routers RT1 and
   RT2 are both connected to a broadcast network.  It is assumed that
   RT2 is the Designated Router for the network, and that RT2 has a
   higher Router ID than Router RT1.

   The neighbor state changes realized by each router are listed on the
   sides of the figure.

   At the beginning of Figure 14, Router RT1's interface to the network
   becomes operational.  It begins sending Hello Packets, although it
   doesn't know the identity of the Designated Router or of any other
   neighboring routers.  Router RT2 hears this hello (moving the
   neighbor to Init state), and in its next Hello Packet indicates that
   it is itself the Designated Router and that it has heard Hello
   Packets from RT1.  This in turn causes RT1 to go to state ExStart, as
   it starts to bring up the adjacency.

   RT1 begins by asserting itself as the master.  When it sees that RT2
   is indeed the master (because of RT2's higher Router ID), RT1
   transitions to slave state and adopts its neighbor's DD sequence
   number.  Database Description packets are then exchanged, with polls
   coming from the master (RT2) and responses from the slave (RT1).
   This sequence of Database Description Packets ends when both the poll
   and associated response has the M-bit off.

   In this example, it is assumed that RT2 has a completely up to date
   database.  In that case, RT2 goes immediately into Full state.  RT1
   will go into Full state after updating the necessary parts of its
   database.  This is done by sending Link State Request Packets, and
   receiving Link State Update Packets in response.  Note that, while
   RT1 has waited until a complete set of Database Description Packets
   has been received (from RT2) before sending any Link State Request
   Packets, this need not be the case.  RT1 could have interleaved the
   sending of Link State Request Packets with the reception of Database
   Description Packets.














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            +---+                                         +---+
            |RT1|                                         |RT2|
            +---+                                         +---+

            Down                                          Down
                            Hello(DR=0,seen=0)
                       ------------------------------>
                         Hello (DR=RT2,seen=RT1,...)      Init
                       <------------------------------
            ExStart        D-D (Seq=x,I,M,Master)
                       ------------------------------>
                           D-D (Seq=y,I,M,Master)         ExStart
                       <------------------------------
            Exchange       D-D (Seq=y,M,Slave)
                       ------------------------------>
                           D-D (Seq=y+1,M,Master)         Exchange
                       <------------------------------
                           D-D (Seq=y+1,M,Slave)
                       ------------------------------>
                                     ...
                                     ...
                                     ...
                           D-D (Seq=y+n, Master)
                       <------------------------------
                           D-D (Seq=y+n, Slave)
             Loading   ------------------------------>
                                 LS Request                Full
                       ------------------------------>
                                 LS Update
                       <------------------------------
                                 LS Request
                       ------------------------------>
                                 LS Update
                       <------------------------------
             Full


                Figure 14: An adjacency bring-up example













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11.  The Routing Table Structure



   The routing table data structure contains all the information
   necessary to forward an IP data packet toward its destination.  Each
   routing table entry describes the collection of best paths to a
   particular destination.  When forwarding an IP data packet, the
   routing table entry providing the best match for the packet's IP
   destination is located.  The matching routing table entry then
   provides the next hop towards the packet's destination.  OSPF also
   provides for the existence of a default route (Destination ID =
   DefaultDestination, Address Mask = 0x00000000).  When the default
   route exists, it matches all IP destinations (although any other
   matching entry is a better match). Finding the routing table entry
   that best matches an IP destination is further described in Section
   11.1.

   There is a single routing table in each router.  Two sample routing
   tables are described in Sections 11.2 and 11.3.  The building of the
   routing table is discussed in Section 16.

   The rest of this section defines the fields found in a routing table
   entry.  The first set of fields describes the routing table entry's
   destination.

   Destination Type
      Destination type is either "network" or "router". Only network entries
      are actually used when forwarding IP data traffic.  Router routing
      table entries are used solely as intermediate steps in the routing
      table build process.

      A network is a range of IP addresses, to which IP data traffic may be
      forwarded.  This includes IP networks (class A, B, or C), IP subnets,
      IP supernets and single IP hosts.  The default route also falls into
      this category.

      Router entries are kept for area border routers and AS boundary
      routers.  Routing table entries for area border routers are used when
      calculating the inter-area routes (see Section 16.2), and when
      maintaining configured virtual links (see Section 15).  Routing table
      entries for AS boundary routers are used when calculating the AS
      external routes (see Section 16.4).

   Destination ID
      The destination's identifier or name.  This depends on the
      Destination Type.  For networks, the identifier is their associated IP
      address.  For routers, the identifier is the OSPF Router ID.[9]





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   Address Mask
      Only defined for networks.  The network's IP address together with its
      address mask defines a range of IP addresses.  For IP subnets, the
      address mask is referred to as the subnet mask.  For host routes, the
      mask is "all ones" (0xffffffff).

   Optional Capabilities
      When the destination is a router this field indicates the optional
      OSPF capabilities supported by the destination router.  The only
      optional capability defined by this specification is the ability to
      process AS-external-LSAs.  For a further discussion of OSPF's optional
      capabilities, see Section 4.5.

   The set of paths to use for a destination may vary based on the OSPF
   area to which the paths belong.  This means that there may be
   multiple routing table entries for the same destination, depending on
   the values of the next field.

   Area
      This field indicates the area whose link state information has led
      to the routing table entry's collection of paths.  This is called
      the entry's associated area.  For sets of AS external paths, this
      field is not defined.  For destinations of type "router", there
      may be separate sets of paths (and therefore separate routing
      table entries) associated with each of several areas. For example,
      this will happen when two area border routers share multiple areas
      in common.  For destinations of type "network", only the set of
      paths associated with the best area (the one providing the
      preferred route) is kept.

   The rest of the routing table entry describes the set of paths to the
   destination.  The following fields pertain to the set of paths as a
   whole.  In other words, each one of the paths contained in a routing
   table entry is of the same path-type and cost (see below).

   Path-type
      There are four possible types of paths used to route traffic to
      the destination, listed here in order of preference: intra-area,
      inter-area, type 1 external or type 2 external.  Intra-area paths
      indicate destinations belonging to one of the router's attached
      areas.  Inter-area paths are paths to destinations in other OSPF
      areas.  These are discovered through the examination of received
      summary-LSAs.  AS external paths are paths to destinations
      external to the AS.  These are detected through the examination of
      received AS-external-LSAs.






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   Cost
      The link state cost of the path to the destination.  For all paths
      except type 2 external paths this describes the entire path's
      cost.  For Type 2 external paths, this field describes the cost of
      the portion of the path internal to the AS.  This cost is
      calculated as the sum of the costs of the path's constituent
      links.

   Type 2 cost
      Only valid for type 2 external paths.  For these paths, this field
      indicates the cost of the path's external portion.  This cost has
      been advertised by an AS boundary router, and is the most
      significant part of the total path cost.  For example, a type 2
      external path with type 2 cost of 5 is always preferred over a
      path with type 2 cost of 10, regardless of the cost of the two
      paths' internal components.

   Link State Origin
      Valid only for intra-area paths, this field indicates the LSA
      (router-LSA or network-LSA) that directly references the
      destination.  For example, if the destination is a transit
      network, this is the transit network's network-LSA.  If the
      destination is a stub network, this is the router-LSA for the
      attached router.  The LSA is discovered during the shortest-path
      tree calculation (see Section 16.1).  Multiple LSAs may reference
      the destination, however a tie-breaking scheme always reduces the
      choice to a single LSA. The Link State Origin field is not used by
      the OSPF protocol, but it is used by the routing table calculation
      in OSPF's Multicast routing extensions (MOSPF).

   When multiple paths of equal path-type and cost exist to a
   destination (called elsewhere "equal-cost" paths), they are stored in
   a single routing table entry.  Each one of the "equal-cost" paths is
   distinguished by the following fields:

   Next hop
      The outgoing router interface to use when forwarding traffic to
      the destination.  On broadcast, Point-to-MultiPoint and NBMA
      networks, the next hop also includes the IP address of the next
      router (if any) in the path towards the destination.

   Advertising router
      Valid only for inter-area and AS external paths.  This field
      indicates the Router ID of the router advertising the summary-LSA
      or AS-external-LSA that led to this path.






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11.1.  Routing table lookup



   When an IP data packet is received, an OSPF router finds the routing
   table entry that best matches the packet's destination.  This routing
   table entry then provides the outgoing interface and next hop router
   to use in forwarding the packet. This section describes the process
   of finding the best matching routing table entry. The process
   consists of a number of steps, wherein the collection of routing
   table entries is progressively pruned.  In the end, the single
   routing table entry remaining is called the "best match".

   Before the lookup begins, "discard" routing table entries should be
   inserted into the routing table for each of the router's active area
   address ranges (see Section 3.5).  (An area range is considered
   "active" if the range contains one or more networks reachable by
   intra-area paths.) The destination of a "discard" entry is the set of
   addresses described by its associated active area address range, and
   the path type of each "discard" entry is set to "inter-area".[10]

   Note that the steps described below may fail to produce a best match
   routing table entry (i.e., all existing routing table entries are
   pruned for some reason or another), or the best match routing table
   entry may be one of the above "discard" routing table entries. In
   these cases, the packet's IP destination is considered unreachable.
   Instead of being forwarded, the packet should be discarded and an
   ICMP destination unreachable message should be returned to the
   packet's source.

   (1) Select the complete set of "matching" routing table entries
       from the routing table.  Each routing table entry describes
       a (set of) path(s) to a range of IP addresses. If the data
       packet's IP destination falls into an entry's range of IP
       addresses, the routing table entry is called a match. (It is
       quite likely that multiple entries will match the data
       packet.  For example, a default route will match all
       packets.)

   (2) Reduce the set of matching entries to those having the most
       preferential path-type (see Section 11). OSPF has a four
       level hierarchy of paths. Intra-area paths are the most
       preferred, followed in order by inter-area, type 1 external
       and type 2 external paths.

   (3) Select the remaining routing table entry that provides the
       most specific (longest) match. Another way of saying this is
       to choose the remaining entry that specifies the narrowest
       range of IP addresses.[11] For example, the entry for the
       address/mask pair of (128.185.1.0, 0xffffff00) is more



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       specific than an entry for the pair (128.185.0.0,
       0xffff0000). The default route is the least specific match,
       since it matches all destinations.

11.2.  Sample routing table, without areas



   Consider the Autonomous System pictured in Figure 2.  No OSPF areas
   have been configured.  A single metric is shown per outbound
   interface.  The calculation of Router RT6's routing table proceeds as
   described in Section 2.2.  The resulting routing table is shown in
   Table 12.  Destination types are abbreviated: Network as "N", Router
   as "R".

   There are no instances of multiple equal-cost shortest paths in this
   example.  Also, since there are no areas, there are no inter-area
   paths.

   Routers RT5 and RT7 are AS boundary routers.  Intra-area routes have
   been calculated to Routers RT5 and RT7.  This allows external routes
   to be calculated to the destinations advertised by RT5 and RT7 (i.e.,
   Networks N12, N13, N14 and N15).  It is assumed all AS-external-LSAs
   originated by RT5 and RT7 are advertising type 1 external metrics.
   This results in type 1 external paths being calculated to
   destinations N12-N15.

11.3.  Sample routing table, with areas



   Consider the previous example, this time split into OSPF areas.  An
   OSPF area configuration is pictured in Figure 6.  Router RT4's
   routing table will be described for this area configuration.  Router
   RT4 has a connection to Area 1 and a backbone connection.  This
   causes Router RT4 to view the AS as the concatenation of the two
   graphs shown in Figures 7 and 8.  The resulting routing table is
   displayed in Table 13.

















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      Type   Dest   Area   Path  Type    Cost   Next     Adv.
                                                Hop(s)   Router(s)
      ____________________________________________________________
      N      N1     0      intra-area    10     RT3      *
      N      N2     0      intra-area    10     RT3      *
      N      N3     0      intra-area    7      RT3      *
      N      N4     0      intra-area    8      RT3      *
      N      Ib     0      intra-area    7      *        *
      N      Ia     0      intra-area    12     RT10     *
      N      N6     0      intra-area    8      RT10     *
      N      N7     0      intra-area    12     RT10     *
      N      N8     0      intra-area    10     RT10     *
      N      N9     0      intra-area    11     RT10     *
      N      N10    0      intra-area    13     RT10     *
      N      N11    0      intra-area    14     RT10     *
      N      H1     0      intra-area    21     RT10     *
      R      RT5    0      intra-area    6      RT5      *
      R      RT7    0      intra-area    8      RT10     *
      ____________________________________________________________
      N      N12    *      type 1 ext.   10     RT10     RT7
      N      N13    *      type 1 ext.   14     RT5      RT5
      N      N14    *      type 1 ext.   14     RT5      RT5
      N      N15    *      type 1 ext.   17     RT10     RT7


               Table 12: The routing table for Router RT6
                         (no configured areas).

   Again, Routers RT5 and RT7 are AS boundary routers.  Routers RT3,
   RT4, RT7, RT10 and RT11 are area border routers.  Note that there are
   two routing entries for the area border router RT3, since it has two
   areas in common with RT4 (Area 1 and the backbone).

   Backbone paths have been calculated to all area border routers.
   These are used when determining the inter-area routes.  Note that all
   of the inter-area routes are associated with the backbone; this is
   always the case when the calculating router is itself an area border
   router.  Routing information is condensed at area boundaries.  In
   this example, we assume that Area 3 has been defined so that networks
   N9-N11 and the host route to H1 are all condensed to a single route
   when advertised into the backbone (by Router RT11).  Note that the
   cost of this route is the maximum of the set of costs to its
   individual components.








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   There is a virtual link configured between Routers RT10 and RT11.
   Without this configured virtual link, RT11 would be unable to
   advertise a route for networks N9-N11 and Host H1 into the backbone,
   and there would not be an entry for these networks in Router RT4's
   routing table.

   In this example there are two equal-cost paths to Network N12.
   However, they both use the same next hop (Router RT5).


   Type   Dest        Area   Path  Type    Cost   Next      Adv.
                                                  Hops(s)   Router(s)
   __________________________________________________________________
   N      N1          1      intra-area    4      RT1       *
   N      N2          1      intra-area    4      RT2       *
   N      N3          1      intra-area    1      *         *
   N      N4          1      intra-area    3      RT3       *
   R      RT3         1      intra-area    1      *         *
   __________________________________________________________________
   N      Ib          0      intra-area    22     RT5       *
   N      Ia          0      intra-area    27     RT5       *
   R      RT3         0      intra-area    21     RT5       *
   R      RT5         0      intra-area    8      *         *
   R      RT7         0      intra-area    14     RT5       *
   R      RT10        0      intra-area    22     RT5       *
   R      RT11        0      intra-area    25     RT5       *
   __________________________________________________________________
   N      N6          0      inter-area    15     RT5       RT7
   N      N7          0      inter-area    19     RT5       RT7
   N      N8          0      inter-area    18     RT5       RT7
   N      N9-N11,H1   0      inter-area    36     RT5       RT11
   __________________________________________________________________
   N      N12         *      type 1 ext.   16     RT5       RT5,RT7
   N      N13         *      type 1 ext.   16     RT5       RT5
   N      N14         *      type 1 ext.   16     RT5       RT5
   N      N15         *      type 1 ext.   23     RT5       RT7

                  Table 13: Router RT4's routing table
                       in the presence of areas.


   Router RT4's routing table would improve (i.e., some of the paths in
   the routing table would become shorter) if an additional virtual link
   were configured between Router RT4 and Router RT3.  The new virtual
   link would itself be associated with the first entry for area border
   router RT3 in Table 13 (an intra-area path through Area 1).  This
   would yield a cost of 1 for the virtual link.  The routing table
   entries changes that would be caused by the addition of this virtual



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   link are shown in Table 14.

12.  Link State Advertisements (LSAs)



   Each router in the Autonomous System originates one or more link
   state advertisements (LSAs).  This memo defines five distinct types
   of LSAs, which are described in Section 4.3.  The collection of LSAs
   forms the link-state database.  Each separate type of LSA has a
   separate function. Router-LSAs and network-LSAs describe how an
   area's routers and networks are interconnected.  Summary-LSAs provide
   a way of condensing an area's routing information. AS-external-LSAs
   provide a way of transparently advertising externally-derived routing
   information throughout the Autonomous System.

   Each LSA begins with a standard 20-byte header.  This LSA header is
   discussed below.

    Type   Dest        Area   Path  Type   Cost   Next     Adv.
                                                  Hop(s)   Router(s)
    ________________________________________________________________
    N      Ib          0      intra-area   16     RT3      *
    N      Ia          0      intra-area   21     RT3      *
    R      RT3         0      intra-area   1      *        *
    R      RT10        0      intra-area   16     RT3      *
    R      RT11        0      intra-area   19     RT3      *
    ________________________________________________________________
    N      N9-N11,H1   0      inter-area   30     RT3      RT11


                  Table 14: Changes resulting from an
                        additional virtual link.

12.1.  The LSA Header



   The LSA header contains the LS type, Link State ID and Advertising
   Router fields.  The combination of these three fields uniquely
   identifies the LSA.

   There may be several instances of an LSA present in the Autonomous
   System, all at the same time.  It must then be determined which
   instance is more recent.  This determination is made by examining the
   LS sequence, LS checksum and LS age fields.  These fields are also
   contained in the 20-byte LSA header.

   Several of the OSPF packet types list LSAs.  When the instance is not
   important, an LSA is referred to by its LS type, Link State ID and
   Advertising Router (see Link State Request Packets).  Otherwise, the
   LS sequence number, LS age and LS checksum fields must also be



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   referenced.

   A detailed explanation of the fields contained in the LSA header
   follows.

12.1.1.  LS age



   This field is the age of the LSA in seconds.  It should be processed
   as an unsigned 16-bit integer.  It is set to 0 when the LSA is
   originated.  It must be incremented by InfTransDelay on every hop of
   the flooding procedure.  LSAs are also aged as they are held in each
   router's database.

   The age of an LSA is never incremented past MaxAge.  LSAs having age
   MaxAge are not used in the routing table calculation.  When an LSA's
   age first reaches MaxAge, it is reflooded. An LSA of age MaxAge is
   finally flushed from the database when it is no longer needed to
   ensure database synchronization.  For more information on the aging
   of LSAs, consult Section 14.

   The LS age field is examined when a router receives two instances of
   an LSA, both having identical LS sequence numbers and LS checksums.
   An instance of age MaxAge is then always accepted as most recent;
   this allows old LSAs to be flushed quickly from the routing domain.
   Otherwise, if the ages differ by more than MaxAgeDiff, the instance
   having the smaller age is accepted as most recent.[12] See Section
   13.1 for more details.

12.1.2.  Options



   The Options field in the LSA header indicates which optional
   capabilities are associated with the LSA.  OSPF's optional
   capabilities are described in Section 4.5. One optional capability is
   defined by this specification, represented by the E-bit found in the
   Options field.  The unrecognized bits in the Options field should be
   set to zero.  The E-bit represents OSPF's ExternalRoutingCapability.
   This bit should be set in all LSAs associated with the backbone, and
   all LSAs associated with non-stub areas (see Section 3.6).  It should
   also be set in all AS-external-LSAs.  It should be reset in all
   router-LSAs, network-LSAs and summary-LSAs associated with a stub
   area.  For all LSAs, the setting of the E-bit is for informational
   purposes only; it does not affect the routing table calculation.









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12.1.3.  LS type



   The LS type field dictates the format and function of the LSA.  LSAs
   of different types have different names (e.g., router-LSAs or
   network-LSAs).  All LSA types defined by this memo, except the AS-
   external-LSAs (LS type = 5), are flooded throughout a single area
   only.  AS-external-LSAs are flooded throughout the entire Autonomous
   System, excepting stub areas (see Section 3.6).  Each separate LSA
   type is briefly described below in Table 15.

12.1.4.  Link State ID



   This field identifies the piece of the routing domain that is being
   described by the LSA.  Depending on the LSA's LS type, the Link State
   ID takes on the values listed in Table 16.

   Actually, for Type 3 summary-LSAs (LS type = 3) and AS-external-LSAs
   (LS type = 5), the Link State ID may additionally have one or more of
   the destination network's "host" bits set. For example, when
   originating an AS-external-LSA for the network 10.0.0.0 with mask of
   255.0.0.0, the Link State ID can be set to anything in the range
   10.0.0.0 through 10.255.255.255 inclusive (although 10.0.0.0 should
   be used whenever possible). The freedom to set certain host bits
   allows a router to originate separate LSAs for two networks having
   the same address but different masks. See Appendix E for details.


























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            LS Type   LSA description
            ________________________________________________
            1         These are the router-LSAs.
                      They describe the collected
                       states of the router's
                      interfaces. For more information,
                      consult Section 12.4.1.
            ________________________________________________
            2         These are the network-LSAs.
                      They describe the set of routers
                      attached to the network. For
                      more information, consult
                      Section 12.4.2.
            ________________________________________________
            3 or 4    These are the summary-LSAs.
                      They describe inter-area routes,
                      and enable the condensation of
                      routing information at area
                      borders. Originated by area border
                      routers, the Type 3 summary-LSAs
                      describe routes to networks while the
                      Type 4 summary-LSAs describe routes to
                      AS boundary routers.
            ________________________________________________
            5         These are the AS-external-LSAs.
                      Originated by AS boundary routers,
                      they describe routes
                      to destinations external to the
                      Autonomous System. A default route for
                      the Autonomous System can also be
                      described by an AS-external-LSA.

            Table 15: OSPF link state advertisements (LSAs).

            LS Type   Link State ID
            _______________________________________________
            1         The originating router's Router ID.
            2         The IP interface address of the
                      network's Designated Router.
            3         The destination network's IP address.
            4         The Router ID of the described AS
                      boundary router.
            5         The destination network's IP address.


                   Table 16: The LSA's Link State ID.





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   When the LSA is describing a network (LS type = 2, 3 or 5), the
   network's IP address is easily derived by masking the Link State ID
   with the network/subnet mask contained in the body of the LSA.  When
   the LSA is describing a router (LS type = 1 or 4), the Link State ID
   is always the described router's OSPF Router ID.

   When an AS-external-LSA (LS Type = 5) is describing a default route,
   its Link State ID is set to DefaultDestination (0.0.0.0).

12.1.5.  Advertising Router



   This field specifies the OSPF Router ID of the LSA's originator.  For
   router-LSAs, this field is identical to the Link State ID field.
   Network-LSAs are originated by the network's Designated Router.
   Summary-LSAs originated by area border routers.  AS-external-LSAs are
   originated by AS boundary routers.

12.1.6.  LS sequence number




   The sequence number field is a signed 32-bit integer.  It is used to
   detect old and duplicate LSAs.  The space of sequence numbers is
   linearly ordered.  The larger the sequence number (when compared as
   signed 32-bit integers) the more recent the LSA.  To describe to
   sequence number space more precisely, let N refer in the discussion
   below to the constant 2**31.

   The sequence number -N (0x80000000) is reserved (and unused).  This
   leaves -N + 1 (0x80000001) as the smallest (and therefore oldest)
   sequence number; this sequence number is referred to as the constant
   InitialSequenceNumber. A router uses InitialSequenceNumber the first
   time it originates any LSA.  Afterwards, the LSA's sequence number is
   incremented each time the router originates a new instance of the
   LSA.  When an attempt is made to increment the sequence number past
   the maximum value of N - 1 (0x7fffffff; also referred to as
   MaxSequenceNumber), the current instance of the LSA must first be
   flushed from the routing domain.  This is done by prematurely aging
   the LSA (see Section 14.1) and reflooding it.  As soon as this flood
   has been acknowledged by all adjacent neighbors, a new instance can
   be originated with sequence number of InitialSequenceNumber.

   The router may be forced to promote the sequence number of one of its
   LSAs when a more recent instance of the LSA is unexpectedly received
   during the flooding process. This should be a rare event.  This may
   indicate that an out-of-date LSA, originated by the router itself
   before its last restart/reload, still exists in the Autonomous
   System.  For more information see Section 13.4.




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12.1.7.  LS checksum



   This field is the checksum of the complete contents of the LSA,
   excepting the LS age field.  The LS age field is excepted so that an
   LSA's age can be incremented without updating the checksum.  The
   checksum used is the same that is used for ISO connectionless
   datagrams; it is commonly referred to as the Fletcher checksum.  It
   is documented in Annex B of [Ref6]. The LSA header also contains the
   length of the LSA in bytes; subtracting the size of the LS age field
   (two bytes) yields the amount of data to checksum.

   The checksum is used to detect data corruption of an LSA.  This
   corruption can occur while an LSA is being flooded, or while it is
   being held in a router's memory.  The LS checksum field cannot take
   on the value of zero; the occurrence of such a value should be
   considered a checksum failure.  In other words, calculation of the
   checksum is not optional.

   The checksum of an LSA is verified in two cases: a) when it is
   received in a Link State Update Packet and b) at times during the
   aging of the link state database.  The detection of a checksum
   failure leads to separate actions in each case.  See Sections 13 and
   14 for more details.

   Whenever the LS sequence number field indicates that two instances of
   an LSA are the same, the LS checksum field is examined.  If there is
   a difference, the instance with the larger LS checksum is considered
   to be most recent.[13] See Section 13.1 for more details.

12.2.  The link state database



   A router has a separate link state database for every area to which
   it belongs. All routers belonging to the same area have identical
   link state databases for the area.

   The databases for each individual area are always dealt with
   separately.  The shortest path calculation is performed separately
   for each area (see Section 16).  Components of the area link-state
   database are flooded throughout the area only.  Finally, when an
   adjacency (belonging to Area A) is being brought up, only the
   database for Area A is synchronized between the two routers.

   The area database is composed of router-LSAs, network-LSAs and
   summary-LSAs (all listed in the area data structure).  In addition,
   external routes (AS-external-LSAs) are included in all non-stub area
   databases (see Section 3.6).





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   An implementation of OSPF must be able to access individual pieces of
   an area database.  This lookup function is based on an LSA's LS type,
   Link State ID and Advertising Router.[14] There will be a single
   instance (the most up-to-date) of each LSA in the database.  The
   database lookup function is invoked during the LSA flooding procedure
   (Section 13) and the routing table calculation (Section 16).  In
   addition, using this lookup function the router can determine whether
   it has itself ever originated a particular LSA, and if so, with what
   LS sequence number.

   An LSA is added to a router's database when either a) it is received
   during the flooding process (Section 13) or b) it is originated by
   the router itself (Section 12.4).  An LSA is deleted from a router's
   database when either a) it has been overwritten by a newer instance
   during the flooding process (Section 13) or b) the router originates
   a newer instance of one of its self-originated LSAs (Section 12.4) or
   c) the LSA ages out and is flushed from the routing domain (Section
   14).

   Whenever an LSA is deleted from the database it must also be removed
   from all neighbors' Link state retransmission lists (see Section 10).

12.3.  Representation of TOS



   For backward compatibility with previous versions of the OSPF
   specification ([Ref9]), TOS-specific information can be included in
   router-LSAs, summary-LSAs and AS-external-LSAs.  The encoding of TOS
   in OSPF LSAs is specified in Table 17. That table relates the OSPF
   encoding to the IP packet header's TOS field (defined in [Ref12]).
   The OSPF encoding is expressed as a decimal integer, and the IP
   packet header's TOS field is expressed in the binary TOS values used
   in [Ref12].



















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                    OSPF encoding   RFC 1349 TOS values
                    ___________________________________________
                    0               0000 normal service
                    2               0001 minimize monetary cost
                    4               0010 maximize reliability
                    6               0011
                    8               0100 maximize throughput
                    10              0101
                    12              0110
                    14              0111
                    16              1000 minimize delay
                    18              1001
                    20              1010
                    22              1011
                    24              1100
                    26              1101
                    28              1110
                    30              1111

                  Table 17: Representing TOS in OSPF.

12.4.  Originating LSAs



   Into any given OSPF area, a router will originate several LSAs.  Each
   router originates a router-LSA.  If the router is also the Designated
   Router for any of the area's networks, it will originate network-LSAs
   for those networks.

   Area border routers originate a single summary-LSA for each known
   inter-area destination.  AS boundary routers originate a single AS-
   external-LSA for each known AS external destination.  Destinations
   are advertised one at a time so that the change in any single route
   can be flooded without reflooding the entire collection of routes.
   During the flooding procedure, many LSAs can be carried by a single
   Link State Update packet.

   As an example, consider Router RT4 in Figure 6.  It is an area border
   router, having a connection to Area 1 and the backbone.  Router RT4
   originates 5 distinct LSAs into the backbone (one router-LSA, and one
   summary-LSA for each of the networks N1-N4).  Router RT4 will also
   originate 8 distinct LSAs into Area 1 (one router-LSA and seven
   summary-LSAs as pictured in Figure 7).  If RT4 has been selected as
   Designated Router for Network N3, it will also originate a network-
   LSA for N3 into Area 1.

   In this same figure, Router RT5 will be originating 3 distinct AS-
   external-LSAs (one for each of the networks N12-N14).  These will be
   flooded throughout the entire AS, assuming that none of the areas



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   have been configured as stubs.  However, if area 3 has been
   configured as a stub area, the AS-external-LSAs for networks N12-N14
   will not be flooded into area 3 (see Section 3.6).  Instead, Router
   RT11 would originate a default summary- LSA that would be flooded
   throughout area 3 (see Section 12.4.3).  This instructs all of area
   3's internal routers to send their AS external traffic to RT11.

   Whenever a new instance of an LSA is originated, its LS sequence
   number is incremented, its LS age is set to 0, its LS checksum is
   calculated, and the LSA is added to the link state database and
   flooded out the appropriate interfaces.  See Section 13.2 for details
   concerning the installation of the LSA into the link state database.
   See Section 13.3 for details concerning the flooding of newly
   originated LSAs.

   The ten events that can cause a new instance of an LSA to be
   originated are:

   (1) The LS age field of one of the router's self-originated LSAs
       reaches the value LSRefreshTime. In this case, a new
       instance of the LSA is originated, even though the contents
       of the LSA (apart from the LSA header) will be the same.
       This guarantees periodic originations of all LSAs.  This
       periodic updating of LSAs adds robustness to the link state
       algorithm.  LSAs that solely describe unreachable
       destinations should not be refreshed, but should instead be
       flushed from the routing domain (see Section 14.1).

   When whatever is being described by an LSA changes, a new LSA is
   originated.  However, two instances of the same LSA may not be
   originated within the time period MinLSInterval.  This may require
   that the generation of the next instance be delayed by up to
   MinLSInterval.  The following events may cause the contents of an LSA
   to change.  These events should cause new originations if and only if
   the contents of the new LSA would be different:

   (2) An interface's state changes (see Section 9.1).  This may
       mean that it is necessary to produce a new instance of the
       router-LSA.

   (3) An attached network's Designated Router changes.  A new
       router-LSA should be originated.  Also, if the router itself
       is now the Designated Router, a new network-LSA should be
       produced.  If the router itself is no longer the Designated
       Router, any network-LSA that it might have originated for
       the network should be flushed from the routing domain (see
       Section 14.1).




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   (4) One of the neighboring routers changes to/from the FULL
       state.  This may mean that it is necessary to produce a new
       instance of the router-LSA.  Also, if the router is itself
       the Designated Router for the attached network, a new
       network-LSA should be produced.

   The next four events concern area border routers only:

   (5) An intra-area route has been added/deleted/modified in the
       routing table.  This may cause a new instance of a summary-
       LSA (for this route) to be originated in each attached area
       (possibly including the backbone).

   (6) An inter-area route has been added/deleted/modified in the
       routing table.  This may cause a new instance of a summary-
       LSA (for this route) to be originated in each attached area
       (but NEVER for the backbone).

   (7) The router becomes newly attached to an area.  The router
       must then originate summary-LSAs into the newly attached
       area for all pertinent intra-area and inter-area routes in
       the router's routing table.  See Section 12.4.3 for more
       details.

   (8) When the state of one of the router's configured virtual
       links changes, it may be necessary to originate a new
       router-LSA into the virtual link's Transit area (see the
       discussion of the router-LSA's bit V in Section 12.4.1), as
       well as originating a new router-LSA into the backbone.

   The last two events concern AS boundary routers (and former AS
   boundary routers) only:

   (9) An external route gained through direct experience with an
       external routing protocol (like BGP) changes.  This will
       cause an AS boundary router to originate a new instance of
       an AS-external-LSA.

   (10)
       A router ceases to be an AS boundary router, perhaps after
       restarting. In this situation the router should flush all
       AS-external-LSAs that it had previously originated.  These
       LSAs can be flushed via the premature aging procedure
       specified in Section 14.1.







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   The construction of each type of LSA is explained in detail below. In
   general, these sections describe the contents of the LSA body (i.e.,
   the part coming after the 20-byte LSA header).  For information
   concerning the building of the LSA header, see Section 12.1.

12.4.1.  Router-LSAs



   A router originates a router-LSA for each area that it belongs to.
   Such an LSA describes the collected states of the router's links to
   the area.  The LSA is flooded throughout the particular area, and no
   further.  The format of a router-LSA is shown in Appendix A (Section
   A.4.2).  The first 20 bytes of the LSA consist of the generic LSA
   header that was discussed in Section 12.1.  router-LSAs have LS type
   = 1.

   A router also indicates whether it is an area border router, or an AS
   boundary router, by setting the appropriate bits

                  ....................................
                  . 192.1.2                   Area 1 .
                  .     +                            .
                  .     |                            .
                  .     | 3+---+1                    .
                  .  N1 |--|RT1|-----+               .
                  .     |  +---+      \              .
                  .     |              \  _______N3  .
                  .     +               \/       \   .  1+---+
                  .                     * 192.1.1 *------|RT4|
                  .     +               /\_______/   .   +---+
                  .     |              /     |       .
                  .     | 3+---+1     /      |       .
                  .  N2 |--|RT2|-----+      1|       .
                  .     |  +---+           +---+8    .         6+---+
                  .     |                  |RT3|----------------|RT6|
                  .     +                  +---+     .          +---+
                  . 192.1.3                  |2      .   18.10.0.6|7
                  .                          |       .            |
                  .                   +------------+ .
                  .                     192.1.4 (N4) .
                  ....................................

               Figure 15: Area 1 with IP addresses shown

   (bit B and bit E, respectively) in its router-LSAs. This enables
   paths to those types of routers to be saved in the routing table, for
   later processing of summary-LSAs and AS-external-LSAs.  Bit B should
   be set whenever the router is actively attached to two or more areas,
   even if the router is not currently attached to the OSPF backbone



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   area.  Bit E should never be set in a router-LSA for a stub area
   (stub areas cannot contain AS boundary routers).

   In addition, the router sets bit V in its router-LSA for Area A if
   and only if the router is the endpoint of one or more fully adjacent
   virtual links having Area A as their Transit area. The setting of bit
   V enables other routers in Area A to discover whether the area
   supports transit traffic (see TransitCapability in Section 6).

   The router-LSA then describes the router's working connections (i.e.,
   interfaces or links) to the area.  Each link is typed according to
   the kind of attached network.  Each link is also labelled with its
   Link ID.  This Link ID gives a name to the entity that is on the
   other end of the link.  Table 18 summarizes the values used for the
   Type and Link ID fields.

           Link type   Description       Link ID
           __________________________________________________
           1           Point-to-point    Neighbor Router ID
                       link
           2           Link to transit   Interface address of
                       network           Designated Router
           3           Link to stub      IP network number
                       network
           4           Virtual link      Neighbor Router ID

                   Table 18: Link descriptions in the
                              router-LSA.

   In addition, the Link Data field is specified for each link.  This
   field gives 32 bits of extra information for the link.  For links to
   transit networks, numbered point-to-point links and virtual links,
   this field specifies the IP interface address of the associated
   router interface (this is needed by the routing table calculation,
   see Section 16.1.1).  For links to stub networks, this field
   specifies the stub network's IP address mask. For unnumbered point-
   to-point links, the Link Data field should be set to the unnumbered
   interface's MIB-II [Ref8] ifIndex value.

   Finally, the cost of using the link for output is specified.  The
   output cost of a link is configurable. With the exception of links to
   stub networks, the output cost must always be non-zero.

   To further describe the process of building the list of link
   descriptions, suppose a router wishes to build a router-LSA for Area
   A.  The router examines its collection of interface data structures.
   For each interface, the following steps are taken:




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   o    If the attached network does not belong to Area A, no
       links are added to the LSA, and the next interface should be
       examined.

   o    If the state of the interface is Down, no links are added.

   o    If the state of the interface is Loopback, add a Type 3
       link (stub network) as long as this is not an interface to an
       unnumbered point-to-point network.  The Link ID should be set to
       the IP interface address, the Link Data set to the
       mask 0xffffffff (indicating a host route), and the cost set to 0.

   o   Otherwise, the link descriptions added to the router-LSA
       depend on the OSPF interface type. Link descriptions used for
       point-to-point interfaces are specified in Section 12.4.1.1, for
       virtual links in Section 12.4.1.2, for broadcast and NBMA
       interfaces in 12.4.1.3, and for Point-to-MultiPoint interfaces in
       12.4.1.4.

   After consideration of all the router interfaces, host links are
   added to the router-LSA by examining the list of attached hosts
   belonging to Area A.  A host route is represented as a Type 3 link
   (stub network) whose Link ID is the host's IP address, Link Data is
   the mask of all ones (0xffffffff), and cost the host's configured
   cost (see Section C.7).

12.4.1.1.  Describing point-to-point interfaces



   For point-to-point interfaces, one or more link descriptions are
   added to the router-LSA as follows:

   o   If the neighboring router is fully adjacent, add a
       Type 1 link (point-to-point). The Link ID should be set to the
       Router ID of the neighboring router. For numbered point-to-point
       networks, the Link Data should specify the IP interface address.
       For unnumbered point-to-point networks, the Link Data field
       should specify the interface's MIB-II [Ref8] ifIndex value. The
       cost should be set to the output cost of the point-to-point
       interface.

   o   In addition, as long as the state of the interface
       is "Point-to-Point" (and regardless of the neighboring router
       state), a Type 3 link (stub network) should be added. There are
       two forms that this stub link can take:







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   Option 1
      Assuming that the neighboring router's IP address is known, set
      the Link ID of the Type 3 link to the neighbor's IP address, the
      Link Data to the mask 0xffffffff (indicating a host route), and
      the cost to the interface's configured output cost.[15]

   Option 2
      If a subnet has been assigned to the point-to-point link, set the
      Link ID of the Type 3 link to the subnet's IP address, the Link
      Data to the subnet's mask, and the cost to the interface's
      configured output cost.[16]

12.4.1.2.  Describing broadcast and NBMA interfaces



   For operational broadcast and NBMA interfaces, a single link
   description is added to the router-LSA as follows:

   o   If the state of the interface is Waiting, add a Type
       3 link (stub network) with Link ID set to the IP network number
       of the attached network, Link Data set to the attached network's
       address mask, and cost equal to the interface's configured output
       cost.

   o   Else, there has been a Designated Router elected for
       the attached network.  If the router is fully adjacent to the
       Designated Router, or if the router itself is Designated Router
       and is fully adjacent to at least one other router, add a single
       Type 2 link (transit network) with Link ID set to the IP
       interface address of the attached network's Designated Router
       (which may be the router itself), Link Data set to the router's
       own IP interface address, and cost equal to the interface's
       configured output cost.  Otherwise, add a link as if the
       interface state were Waiting (see above).

12.4.1.3.  Describing virtual links



   For virtual links, a link description is added to the router-LSA only
   when the virtual neighbor is fully adjacent. In this case, add a Type
   4 link (virtual link) with Link ID set to the Router ID of the
   virtual neighbor, Link Data set to the IP interface address
   associated with the virtual link and cost set to the cost calculated
   for the virtual link during the routing table calculation (see
   Section 15).








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12.4.1.4.  Describing Point-to-MultiPoint interfaces



   For operational Point-to-MultiPoint interfaces, one or more link
   descriptions are added to the router-LSA as follows:

   o   A single Type 3 link (stub network) is added with
       Link ID set to the router's own IP interface address, Link Data
       set to the mask 0xffffffff (indicating a host route), and cost
       set to 0.

   o   For each fully adjacent neighbor associated with the
       interface, add an additional Type 1 link (point-to-point) with
       Link ID set to the Router ID of the neighboring router, Link Data
       set to the IP interface address and cost equal to the interface's
       configured output cost.

12.4.1.5.  Examples of router-LSAs



   Consider the router-LSAs generated by Router RT3, as pictured in
   Figure 6.  The area containing Router RT3 (Area 1) has been redrawn,
   with actual network addresses, in Figure 15.  Assume that the last
   byte of all of RT3's interface addresses is 3, giving it the
   interface addresses 192.1.1.3 and 192.1.4.3, and that the other
   routers have similar addressing schemes.  In addition, assume that
   all links are functional, and that Router IDs are assigned as the
   smallest IP interface address.

   RT3 originates two router-LSAs, one for Area 1 and one for the
   backbone.  Assume that Router RT4 has been selected as the Designated
   router for network 192.1.1.0.  RT3's router-LSA for Area 1 is then
   shown below.  It indicates that RT3 has two connections to Area 1,
   the first a link to the transit network 192.1.1.0 and the second a
   link to the stub network 192.1.4.0.  Note that the transit network is
   identified by the IP interface of its Designated Router (i.e., the
   Link ID = 192.1.1.4 which is the Designated Router RT4's IP interface
   to 192.1.1.0).  Note also that RT3 has indicated that it is an area
   border router.














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     ; RT3's router-LSA for Area 1

     LS age = 0                     ;always true on origination
     Options = (E-bit)              ;
     LS type = 1                    ;indicates router-LSA
     Link State ID = 192.1.1.3      ;RT3's Router ID
     Advertising Router = 192.1.1.3 ;RT3's Router ID
     bit E = 0                      ;not an AS boundary router
     bit B = 1                      ;area border router
     #links = 2
            Link ID = 192.1.1.4     ;IP address of Desig. Rtr.
            Link Data = 192.1.1.3   ;RT3's IP interface to net
            Type = 2                ;connects to transit network
            # TOS metrics = 0
            metric = 1

            Link ID = 192.1.4.0     ;IP Network number
            Link Data = 0xffffff00  ;Network mask
            Type = 3                ;connects to stub network
            # TOS metrics = 0
            metric = 2

   Next RT3's router-LSA for the backbone is shown.  It indicates that
   RT3 has a single attachment to the backbone.  This attachment is via
   an unnumbered point-to-point link to Router RT6.  RT3 has again
   indicated that it is an area border router.


     ; RT3's router-LSA for the backbone

     LS age = 0                     ;always true on origination
     Options = (E-bit)              ;
     LS type = 1                    ;indicates router-LSA
     Link State ID = 192.1.1.3      ;RT3's router ID
     Advertising Router = 192.1.1.3 ;RT3's router ID
     bit E = 0                      ;not an AS boundary router
     bit B = 1                      ;area border router
     #links = 1
            Link ID = 18.10.0.6     ;Neighbor's Router ID
            Link Data = 0.0.0.3     ;MIB-II ifIndex of P-P link
            Type = 1                ;connects to router
            # TOS metrics = 0
            metric = 8








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12.4.2.  Network-LSAs



   A network-LSA is generated for every transit broadcast or NBMA
   network.  (A transit network is a network having two or more attached
   routers).  The network-LSA describes all the routers that are
   attached to the network.

   The Designated Router for the network originates the LSA.  The
   Designated Router originates the LSA only if it is fully adjacent to
   at least one other router on the network.  The network-LSA is flooded
   throughout the area that contains the transit network, and no
   further.  The network-LSA lists those routers that are fully adjacent
   to the Designated Router; each fully adjacent router is identified by
   its OSPF Router ID. The Designated Router includes itself in this
   list.

   The Link State ID for a network-LSA is the IP interface address of
   the Designated Router.  This value, masked by the network's address
   mask (which is also contained in the network-LSA) yields the
   network's IP address.

   A router that has formerly been the Designated Router for a network,
   but is no longer, should flush the network-LSA that it had previously
   originated.  This LSA is no longer used in the routing table
   calculation.  It is flushed by prematurely incrementing the LSA's age
   to MaxAge and reflooding (see Section 14.1). In addition, in those
   rare cases where a router's Router ID has changed, any network-LSAs
   that were originated with the router's previous Router ID must be
   flushed. Since the router may have no idea what it's previous Router
   ID might have been, these network-LSAs are indicated by having their
   Link State ID equal to one of the router's IP interface addresses and
   their Advertising Router equal to some value other than the router's
   current Router ID (see Section 13.4 for more details).

12.4.2.1.  Examples of network-LSAs



   Again consider the area configuration in Figure 6.  Network-LSAs are
   originated for Network N3 in Area 1, Networks N6 and N8 in Area 2,
   and Network N9 in Area 3.  Assuming that Router RT4 has been selected
   as the Designated Router for Network N3, the following network-LSA is
   generated by RT4 on behalf of Network N3 (see Figure 15 for the
   address assignments):









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     ; Network-LSA for Network N3

     LS age = 0                     ;always true on origination
     Options = (E-bit)              ;
     LS type = 2                    ;indicates network-LSA
     Link State ID = 192.1.1.4      ;IP address of Desig. Rtr.
     Advertising Router = 192.1.1.4 ;RT4's Router ID
     Network Mask = 0xffffff00
            Attached Router = 192.1.1.4    ;Router ID
            Attached Router = 192.1.1.1    ;Router ID
            Attached Router = 192.1.1.2    ;Router ID
            Attached Router = 192.1.1.3    ;Router ID

12.4.3.  Summary-LSAs



   The destination described by a summary-LSA is either an IP network,
   an AS boundary router or a range of IP addresses.  Summary-LSAs are
   flooded throughout a single area only.  The destination described is
   one that is external to the area, yet still belongs to the Autonomous
   System.

   Summary-LSAs are originated by area border routers.  The precise
   summary routes to advertise into an area are determined by examining
   the routing table structure (see Section 11) in accordance with the
   algorithm described below. Note that only intra-area routes are
   advertised into the backbone, while both intra-area and inter-area
   routes are advertised into the other areas.

   To determine which routes to advertise into an attached Area A, each
   routing table entry is processed as follows.  Remember that each
   routing table entry describes a set of equal-cost best paths to a
   particular destination:

   o  Only Destination Types of network and AS boundary router
      are advertised in summary-LSAs.  If the routing table entry's
      Destination Type is area border router, examine the next routing
      table entry.

   o  AS external routes are never advertised in summary-LSAs.
      If the routing table entry has Path-type of type 1 external or
      type 2 external, examine the next routing table entry.

   o  Else, if the area associated with this set of paths is
      the Area A itself, do not generate a summary-LSA for the
      route.[17]






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   o  Else, if the next hops associated with this set of paths
      belong to Area A itself, do not generate a summary-LSA for the
      route.[18] This is the logical equivalent of a Distance Vector
      protocol's split horizon logic.

   o  Else, if the routing table cost equals or exceeds the
      value LSInfinity, a summary-LSA cannot be generated for this
      route.

   o  Else, if the destination of this route is an AS boundary
      router, a summary-LSA should be originated if and only if the
      routing table entry describes the preferred path to the AS
      boundary router (see Step 3 of Section 16.4).  If so, a Type 4
      summary-LSA is originated for the destination, with Link State ID
      equal to the AS boundary router's Router ID and metric equal to
      the routing table entry's cost. Note: these LSAs should not be
      generated if Area A has been configured as a stub area.

   o  Else, the Destination type is network. If this is an
      inter-area route, generate a Type 3 summary-LSA for the
      destination, with Link State ID equal to the network's address (if
      necessary, the Link State ID can also have one or more of the
      network's host bits set; see Appendix E for details) and metric
      equal to the routing table cost.

   o  The one remaining case is an intra-area route to a network.  This
      means that the network is contained in one of the router's
      directly attached areas.  In general, this information must be
      condensed before appearing in summary-LSAs.  Remember that an area
      has a configured list of address ranges, each range consisting of
      an [address,mask] pair and a status indication of either Advertise
      or DoNotAdvertise.  At most a single Type 3 summary-LSA is
      originated for each range. When the range's status indicates
      Advertise, a Type 3 summary-LSA is generated with Link State ID
      equal to the range's address (if necessary, the Link State ID can
      also have one or more of the range's "host" bits set; see Appendix
      E for details) and cost equal to the largest cost of any of the
      component networks. When the range's status indicates
      DoNotAdvertise, the Type 3 summary-LSA is suppressed and the
      component networks remain hidden from other areas.

   By default, if a network is not contained in any explicitly
   configured address range, a Type 3 summary-LSA is generated with Link
   State ID equal to the network's address (if necessary, the Link State
   ID can also have one or more of the network's "host" bits set; see
   Appendix E for details) and metric equal to the network's routing
   table cost.




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   If an area is capable of carrying transit traffic (i.e., its
   TransitCapability is set to TRUE), routing information concerning
   backbone networks should not be condensed before being summarized
   into the area.  Nor should the advertisement of backbone networks
   into transit areas be suppressed.  In other words, the backbone's
   configured ranges should be ignored when originating summary-LSAs
   into transit areas.

   If a router advertises a summary-LSA for a destination which then
   becomes unreachable, the router must then flush the LSA from the
   routing domain by setting its age to MaxAge and reflooding (see
   Section 14.1).  Also, if the destination is still reachable, yet can
   no longer be advertised according to the above procedure (e.g., it is
   now an inter-area route, when it used to be an intra-area route
   associated with some non-backbone area; it would thus no longer be
   advertisable to the backbone), the LSA should also be flushed from
   the routing domain.

12.4.3.1.  Originating summary-LSAs into stub areas



   The algorithm in Section 12.4.3 is optional when Area A is an OSPF
   stub area. Area border routers connecting to a stub area can
   originate summary-LSAs into the area according to the Section
   12.4.3's algorithm, or can choose to originate only a subset of the
   summary-LSAs, possibly under configuration control.  The fewer LSAs
   originated, the smaller the stub area's link state database, further
   reducing the demands on its routers' resources. However, omitting
   LSAs may also lead to sub-optimal inter-area routing, although
   routing will continue to function.

   As specified in Section 12.4.3, Type 4 summary-LSAs (ASBR-summary-
   LSAs) are never originated into stub areas.

   In a stub area, instead of importing external routes each area border
   router originates a "default summary-LSA" into the area. The Link
   State ID for the default summary-LSA is set to DefaultDestination,
   and the metric set to the (per-area) configurable parameter
   StubDefaultCost.  Note that StubDefaultCost need not be configured
   identically in all of the stub area's area border routers.

12.4.3.2.  Examples of summary-LSAs



   Consider again the area configuration in Figure 6.  Routers RT3, RT4,
   RT7, RT10 and RT11 are all area border routers, and therefore are
   originating summary-LSAs.  Consider in particular Router RT4.  Its
   routing table was calculated as the example in Section 11.3. RT4
   originates summary-LSAs into both the backbone and Area 1.  Into the
   backbone, Router RT4 originates separate LSAs for each of the



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   networks N1-N4.  Into Area 1, Router RT4 originates separate LSAs for
   networks N6-N8 and the AS boundary routers RT5,RT7.  It also
   condenses host routes Ia and Ib into a single summary-LSA.  Finally,
   the routes to networks N9,N10,N11 and Host H1 are advertised by a
   single summary-LSA.  This condensation was originally performed by
   the router RT11.

   These LSAs are illustrated graphically in Figures 7 and 8.  Two of
   the summary-LSAs originated by Router RT4 follow.  The actual IP
   addresses for the networks and routers in question have been assigned
   in Figure 15.

     ; Summary-LSA for Network N1,
     ; originated by Router RT4 into the backbone

     LS age = 0                  ;always true on origination
     Options = (E-bit)           ;
     LS type = 3                 ;Type 3 summary-LSA
     Link State ID = 192.1.2.0   ;N1's IP network number
     Advertising Router = 192.1.1.4       ;RT4's ID
     metric = 4

     ; Summary-LSA for AS boundary router RT7
     ; originated by Router RT4 into Area 1

     LS age = 0                  ;always true on origination
     Options = (E-bit)           ;
     LS type = 4                 ;Type 4 summary-LSA
     Link State ID = Router RT7's ID
     Advertising Router = 192.1.1.4       ;RT4's ID
     metric = 14

12.4.4.  AS-external-LSAs



   AS-external-LSAs describe routes to destinations external to the
   Autonomous System.  Most AS-external-LSAs describe routes to specific
   external destinations; in these cases the LSA's Link State ID is set
   to the destination network's IP address (if necessary, the Link State
   ID can also have one or more of the network's "host" bits set; see
   Appendix E for details).  However, a default route for the Autonomous
   System can be described in an AS-external-LSA by setting the LSA's
   Link State ID to DefaultDestination (0.0.0.0).  AS-external-LSAs are
   originated by AS boundary routers.  An AS boundary router originates
   a single AS-external-LSA for each external route that it has learned,
   either through another routing protocol (such as BGP), or through
   configuration information.





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   AS-external-LSAs are the only type of LSAs that are flooded
   throughout the entire Autonomous System; all other types of LSAs are
   specific to a single area.  However, AS-external-LSAs are not flooded
   into/throughout stub areas (see Section 3.6).  This enables a
   reduction in link state database size for routers internal to stub
   areas.

   The metric that is advertised for an external route can be one of two
   types.  Type 1 metrics are comparable to the link state metric.  Type
   2 metrics are assumed to be larger than the cost of any intra-AS
   path.

   If a router advertises an AS-external-LSA for a destination which
   then becomes unreachable, the router must then flush the LSA from the
   routing domain by setting its age to MaxAge and reflooding (see
   Section 14.1).

12.4.4.1.  Examples of AS-external-LSAs



   Consider once again the AS pictured in Figure 6.  There are two AS
   boundary routers: RT5 and RT7.  Router RT5 originates three AS-
   external-LSAs, for networks N12-N14.  Router RT7 originates two AS-
   external-LSAs, for networks N12 and N15.  Assume that RT7 has learned
   its route to N12 via BGP, and that it wishes to advertise a Type 2
   metric to the AS.  RT7 would then originate the following LSA for
   N12:

     ; AS-external-LSA for Network N12,
     ; originated by Router RT7

     LS age = 0                  ;always true on origination
     Options = (E-bit)           ;
     LS type = 5                 ;AS-external-LSA
     Link State ID = N12's IP network number
     Advertising Router = Router RT7's ID
     bit E = 1                   ;Type 2 metric
     metric = 2
     Forwarding address = 0.0.0.0

   In the above example, the forwarding address field has been set to
   0.0.0.0, indicating that packets for the external destination should
   be forwarded to the advertising OSPF router (RT7). This is not always
   desirable.  Consider the example pictured in Figure 16.  There are
   three OSPF routers (RTA, RTB and RTC) connected to a common network.
   Only one of these routers, RTA, is exchanging BGP information with
   the non-OSPF router RTX.  RTA must then originate AS- external-LSAs
   for those destinations it has learned from RTX.  By using the AS-
   external-LSA's forwarding address field, RTA can specify that packets



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   for these destinations be forwarded directly to RTX.  Without this
   feature, Routers RTB and RTC would take an extra hop to get to these
   destinations.

   Note that when the forwarding address field is non-zero, it should
   point to a router belonging to another Autonomous System.

   A forwarding address can also be specified for the default route. For
   example, in figure 16 RTA may want to specify that all externally-
   destined packets should by default be forwarded to its BGP peer RTX.
   The resulting AS-external-LSA is pictured below.  Note that the Link
   State ID is set to DefaultDestination.

     ; Default route, originated by Router RTA
     ; Packets forwarded through RTX

     LS age = 0                  ;always true on origination
     Options = (E-bit)           ;
     LS type = 5                 ;AS-external-LSA
     Link State ID = DefaultDestination  ; default route
     Advertising Router = Router RTA's ID
     bit E = 1                   ;Type 2 metric
     metric = 1
     Forwarding address = RTX's IP address

   In figure 16, suppose instead that both RTA and RTB exchange BGP
   information with RTX.  In this case, RTA and RTB would originate the
   same set of AS-external-LSAs.  These LSAs, if they specify the same
   metric, would be functionally equivalent since they would specify the
   same destination and forwarding address (RTX). This leads to a clear
   duplication of effort.  If only one of RTA or RTB originated the set
   of AS-external-LSAs, the routing would remain the same, and the size
   of the link state database would decrease.  However, it must be
   unambiguously defined as to which router originates the LSAs
   (otherwise neither may, or the identity of the originator may
   oscillate). The following rule is thereby established: if two
   routers, both reachable from one another, originate functionally
   equivalent AS-external-LSAs (i.e., same destination, cost and non-
   zero forwarding address), then the LSA originated by the router
   having the highest OSPF Router ID is used.  The router having the
   lower OSPF Router ID can then flush its LSA.  Flushing an LSA is
   discussed in Section 14.1.

13.  The Flooding Procedure



   Link State Update packets provide the mechanism for flooding LSAs.  A
   Link State Update packet may contain several distinct LSAs, and
   floods each LSA one hop further from its point of origination.  To



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   make the flooding procedure reliable, each LSA must be acknowledged
   separately.  Acknowledgments are transmitted in Link State
   Acknowledgment packets.  Many separate acknowledgments can also be
   grouped together into a single packet.

   The flooding procedure starts when a Link State Update packet has
   been received.  Many consistency checks have been made on the
   received packet before being handed to the flooding procedure (see
   Section 8.2).  In particular, the Link State Update packet has been
   associated with a particular neighbor, and a particular area.  If the
   neighbor is in a lesser state than Exchange, the packet should be
   dropped without further processing.

                                +
                                |
                      +---+.....|.BGP
                      |RTA|-----|.....+---+
                      +---+     |-----|RTX|
                                |     +---+
                      +---+     |
                      |RTB|-----|
                      +---+     |
                                |
                      +---+     |
                      |RTC|-----|
                      +---+     |
                                |
                                +

                 Figure 16: Forwarding address example

   All types of LSAs, other than AS-external-LSAs, are associated with a
   specific area.  However, LSAs do not contain an area field.  An LSA's
   area must be deduced from the Link State Update packet header.

   For each LSA contained in a Link State Update packet, the following
   steps are taken:


    (1) Validate the LSA's LS checksum.  If the checksum turns out to be
        invalid, discard the LSA and get the next one from the Link
        State Update packet.

    (2) Examine the LSA's LS type.  If the LS type is unknown, discard
        the LSA and get the next one from the Link State Update Packet.
        This specification defines LS types 1-5 (see Section 4.3).

    (3) Else if this is an AS-external-LSA (LS type = 5), and the area



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        has been configured as a stub area, discard the LSA and get the
        next one from the Link State Update Packet.  AS-external-LSAs
        are not flooded into/throughout stub areas (see Section 3.6).

    (4) Else if the LSA's LS age is equal to MaxAge, and there is
        currently no instance of the LSA in the router's link state
        database, then take the following actions:

        (a) Acknowledge the receipt of the LSA by sending a Link State
            Acknowledgment packet back to the sending neighbor (see
            Section 13.5).

        (b) Purge all outstanding requests for equal or previous
            instances of the LSA from the sending neighbor's Link State
            Request list (see Section 10).

        (c) If the sending neighbor is in state Exchange or in state
            Loading, then install the MaxAge LSA in the link state
            database.  Otherwise, simply discard the LSA.  In either
            case, examine the next LSA (if any) listed in the Link State
            Update packet.

    (5) Otherwise, find the instance of this LSA that is currently
        contained in the router's link state database.  If there is no
        database copy, or the received LSA is more recent than the
        database copy (see Section 13.1 below for the determination of
        which LSA is more recent) the following steps must be performed:

        (a) If there is already a database copy, and if the database
            copy was installed less than MinLSArrival seconds ago,
            discard the new LSA (without acknowledging it) and examine
            the next LSA (if any) listed in the Link State Update
            packet.

        (b) Otherwise immediately flood the new LSA out some subset of
            the router's interfaces (see Section 13.3).  In some cases
            (e.g., the state of the receiving interface is DR and the
            LSA was received from a router other than the Backup DR) the
            LSA will be flooded back out the receiving interface.  This
            occurrence should be noted for later use by the
            acknowledgment process (Section 13.5).

        (c) Remove the current database copy from all neighbors' Link
            state retransmission lists.

        (d) Install the new LSA in the link state database (replacing
            the current database copy).  This may cause the routing
            table calculation to be scheduled.  In addition, timestamp



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            the new LSA with the current time (i.e., the time it was
            received).  The flooding procedure cannot overwrite the
            newly installed LSA until MinLSArrival seconds have elapsed.
            The LSA installation process is discussed further in Section
            13.2.

        (e) Possibly acknowledge the receipt of the LSA by sending a
            Link State Acknowledgment packet back out the receiving
            interface.  This is explained below in Section 13.5.

        (f) If this new LSA indicates that it was originated by the
            receiving router itself (i.e., is considered a self-
            originated LSA), the router must take special action, either
            updating the LSA or in some cases flushing it from the
            routing domain. For a description of how self-originated
            LSAs are detected and subsequently handled, see Section
            13.4.

    (6) Else, if there is an instance of the LSA on the sending
        neighbor's Link state request list, an error has occurred in the
        Database Exchange process.  In this case, restart the Database
        Exchange process by generating the neighbor event BadLSReq for
        the sending neighbor and stop processing the Link State Update
        packet.

    (7) Else, if the received LSA is the same instance as the database
        copy (i.e., neither one is more recent) the following two steps
        should be performed:

        (a) If the LSA is listed in the Link state retransmission list
            for the receiving adjacency, the router itself is expecting
            an acknowledgment for this LSA.  The router should treat the
            received LSA as an acknowledgment by removing the LSA from
            the Link state retransmission list.  This is termed an
            "implied acknowledgment".  Its occurrence should be noted
            for later use by the acknowledgment process (Section 13.5).

        (b) Possibly acknowledge the receipt of the LSA by sending a
            Link State Acknowledgment packet back out the receiving
            interface.  This is explained below in Section 13.5.

    (8) Else, the database copy is more recent.  If the database copy
        has LS age equal to MaxAge and LS sequence number equal to
        MaxSequenceNumber, simply discard the received LSA without
        acknowledging it. (In this case, the LSA's LS sequence number is
        wrapping, and the MaxSequenceNumber LSA must be completely
        flushed before any new LSA instance can be introduced).
        Otherwise, send the database copy back to the sending neighbor,



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        encapsulated within a Link State Update Packet. The Link State
        Update Packet should be unicast to the neighbor. In so doing, do
        not put the database copy of the LSA on the neighbor's link
        state retransmission list, and do not acknowledge the received
        (less recent) LSA instance.

13.1.  Determining which LSA is newer



   When a router encounters two instances of an LSA, it must determine
   which is more recent.  This occurred above when comparing a received
   LSA to its database copy. This comparison must also be done during
   the Database Exchange procedure which occurs during adjacency bring-
   up.

   An LSA is identified by its LS type, Link State ID and Advertising
   Router.  For two instances of the same LSA, the LS sequence number,
   LS age, and LS checksum fields are used to determine which instance
   is more recent:

   o   The LSA having the newer LS sequence number is more recent.
       See Section 12.1.6 for an explanation of the LS sequence number
       space.  If both instances have the same LS sequence number, then:

   o   If the two instances have different LS checksums, then the
       instance having the larger LS checksum (when considered as a 16-
       bit unsigned integer) is considered more recent.

   o   Else, if only one of the instances has its LS age field set
       to MaxAge, the instance of age MaxAge is considered to be more
       recent.

   o   Else, if the LS age fields of the two instances differ by
       more than MaxAgeDiff, the instance having the smaller (younger)
       LS age is considered to be more recent.

   o   Else, the two instances are considered to be identical.















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13.2.  Installing LSAs in the database



   Installing a new LSA in the database, either as the result of
   flooding or a newly self-originated LSA, may cause the OSPF routing
   table structure to be recalculated.  The contents of the new LSA
   should be compared to the old instance, if present.  If there is no
   difference, there is no need to recalculate the routing table. When
   comparing an LSA to its previous instance, the following are all
   considered to be differences in contents:

   o   The LSA's Options field has changed.

   o   One of the LSA instances has LS age set to MaxAge, and
       the other does not.

   o   The length field in the LSA header has changed.

   o   The body of the LSA