This document is obsolete.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
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.
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.
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.
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.
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.
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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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.
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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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.
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.
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**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 |
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.
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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.
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).
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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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
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.
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.
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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
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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.
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.
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
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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).
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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).
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.
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.
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.
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.
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.
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.
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.
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.
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).
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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).
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.
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.
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.
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).
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].
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.
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
(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).
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).
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).
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.
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
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).
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
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.
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
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).
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.
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.
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.
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.
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.