RFC 1586

Network Working Group                                         O. deSouza
Request for Comments: 1586                                  M. Rodrigues
Category: Informational                           AT&T Bell Laboratories
                                                              March 1994

                      Guidelines for Running OSPF
                       Over Frame Relay Networks

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.


   This memo specifies guidelines for implementors and users of the Open
   Shortest Path First (OSPF) routing protocol to bring about
   improvements in how the protocol runs over frame relay networks.  We
   show how to configure frame relay interfaces in a way that obviates
   the "full-mesh" connectivity required by current OSPF
   implementations. This allows for simpler, more economic network
   designs.  These guidelines do not require any protocol changes; they
   only provide recommendations for how OSPF should be implemented and
   configured to use frame relay networks efficiently.


   This memo is the result of work done in the OSPF Working Group of the
   IETF.  Comments and contributions from several sources, especially
   Fred Baker of ACC, John Moy of Proteon, and Bala Rajagopalan of AT&T
   Bell Laboratories are included in this work.

1.  Introduction

   A frame relay (FR) network provides virtual circuits (VCs) to
   interconnect attached devices. Each VC is uniquely identified at each
   FR interface by a Data Link Connection Identifier (DLCI).  RFC 1294
   specifies the encapsulation of multiprotocol traffic over FR [1].
   The devices on a FR network may either be fully interconnected with a
   "mesh" of VCs, or partially interconnected.  OSPF characterizes FR
   networks as non-broadcast multiple access (NBMA) because they can
   support more than two attached routers, but do not have a broadcast
   capability [2].  Under the NBMA model, the physical FR interface on a
   router corresponds to a single OSPF interface through which the
   router is connected to one or more neighbors on the FR network; all
   the neighboring routers must also be directly connected to each other

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RFC 1586                 OSPF over Frame Relay                March 1994

   over the FR network.  Hence OSPF implementations that use the NBMA
   model for FR do not work when the routers are partially
   interconnected.  Further, the topological representation of a
   multiple access network has each attached router bi-directionally
   connected to the network vertex with a single link metric assigned to
   the edge directed into the vertex.

   We see that the NBMA model becomes more restrictive as the number of
   routers connected to the network increases. First, the number of VCs
   required for full-mesh connectivity increases quadratically with the
   number of routers. Public FR services typically offer performance
   guarantees for each VC provisioned by the service. This means that
   real physical resources in the FR network are devoted to each VC, and
   for this the customer eventually pays. The expense for full-mesh
   connectivity thus grows quadratically with the number of
   interconnected routers.  We need to build OSPF implementations that
   allow for partial connectivity over FR.  Second, using a single link
   metric (per TOS) for the FR interface does not allow OSPF to weigh
   some VCs more heavily than others according to the performance
   characteristics of each connection. To make efficient use of the FR
   network resources, it should be possible to assign different link
   metrics to different VCs.

   These limitations of the current OSPF model for FR become more severe
   as the network size increases, and render FR technology less cost
   effective than it could be for large networks. We propose solutions
   to these problems that do not increase complexity by much and do not
   require any changes to the OSPF protocol.

2.  Summary of Recommendations

   We propose expanding the general view of an OSPF interface to account
   for its functional type (point-to-point, broadcast, NBMA) rather than
   its physical type. In most instances, the physical network can only
   serve one function and can only be defined as one type of OSPF
   interface. For multiplexed interfaces such as FR however, logical
   connections between routers can serve different functions. Hence one
   VC on a FR interface can be viewed distintly from other VCs on the
   same physical interface.  The solution requires that OSPF be able to
   support logical interfaces (networks) as well as physical interfaces.
   Each logical network can be either point-to-point, that is, a single
   VC, or NBMA, that is, a collection of VCs.  It is not necessary to
   define new interface types for logical networks, since the operation
   of the protocol over logical point-to-point networks and logical NBMA
   networks remains the same as for the corresponding physical networks.
   For instance, logical point-to-point links could be numbered or
   unnumbered.  It is only necessary for implementations to provide the
   hooks that give users the ability to configure an individual VC as a

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RFC 1586                 OSPF over Frame Relay                March 1994

   logical point-to-point network or a collection of VCs as a logical
   NBMA network.

   The NBMA model does provide some economy in OSPF protocol processing
   and overhead and is the recommended mode of operation for small
   homogeneous networks. Other than the Designated Router (DR) and the
   backup Designated Router (BDR), each router maintains only two
   adjacencies, one each with the DR and BDR, regardless of the size of
   the NBMA network.  When FR VCs are configured as point-to-point
   links, a router would have many more adjacencies to maintain,
   resulting in increased protocol overhead. If all VCs were to have
   comparable performance characteristics as well, there may not be
   compelling reasons to assign a different link metric to each VC.

3.  Implementing OSPF over FR

   We recommend that OSPF router implementations be built so that
   administrators can configure network layer interfaces that consist of
   one or more FR VCs within a single physical interface.  Each logical
   network interface could then be configured as the appropriate type of
   OSPF interface, that is, point-to-point for a single VC, or NBMA for
   a collection of VCs.  This capability would allow a router to belong
   to one or more distinct IP subnets on a single physical FR interface.
   Thus, it is necessary that the router be able to support multiple IP
   addresses on a single physical FR interface.  As with physical NBMA
   networks, logical NBMA networks must be full-mesh connected. While
   logical point-to-point links can be either numbered or unnumbered, we
   show that it is easier to implement routers to handle numbered
   logical point-to-point links.

3.1  Numbered Logical Interfaces

   The router administrator should be able to configure numbered logical
   interfaces over FR as follows:

     STEP 1: Configure the physical interface specifying relevant
             parameters such as the slot, connector, and port numbers,
             physical frame format, encoding, and clock mode. In its
             internal interface MIB [3], the router should create a new
             ifEntry in the ifTable, assign the physical interface an
             ifIndex, and increment the ifNumber by one.

     STEP 2: Configure the data-link layer over the interface,
             specifying frame relay as the encapsulation method.
             Parameters such as the DLCI encoding type and length,
             maximum frame size, management interface (Annex D, LMI),
             and address resolution procedure (manual, inverse ARP). If
             a management interface is not supported, FR VCs must be

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RFC 1586                 OSPF over Frame Relay                March 1994

             configured manually.

     STEP 3: Configure the IP network layer for the interface by
             specifying the number of logical interfaces and the IP
             address and subnet mask for each numbered logical
             interface. Specify the VCs (by DLCI) associated with each
             logical network interface if there is more than one.  If an
             address resolution protocol such as  Inverse ARP [4] is
             being used, it should suffice to specify a list of IP
             addresses for the FR interface and have Inverse ARP create
             the DLCI-IP address binding.

     STEP 4: Configure OSPF to run over each logical interface as
             appropriate, specifying the necessary interface parameters
             such as area ID, link metric, protocol timers and
             intervals, DR priority, and list of neighbors (for the DR).
             OSPF interfaces consisting of one VC can be treated as
             point-to-point links while multi-VC OSPF interfaces are
             treated as NBMA subnets. In its internal OSPF MIB [5], the
             router should create additional entries in the ospfIfTable
             each with the appropriate ospfIfType (nbma or

3.2  Unnumbered Point-to-Point Logical Interfaces

   OSPF uses the IP address to instance each numbered interface.
   However, since an unnumbered point-to-point link does not have an IP
   address, the ifIndex from the interface MIB is used instead [5].
   This is straightforward for a physical point-to-point network, since
   the ifIndex is assigned when the interface is configured.  Logical
   interfaces over FR however, do not have distinct and unique values
   for ifIndex. To allow OSPF to instance unnumbered logical point-to-
   point links, it is necessary to assign each such link a unique
   ifIndex in STEP 3 above. This could lead to some confusion in the
   interfaces table since a new ifTable entry would have to be created
   for each logical point-to-point link. This type of departure from the
   standard practice of creating interface table entries only for
   physical interfaces could be viewed as an unnecessary complication.

   Alternatively, it is possible to build a private MIB that contains
   data structures to instance unnumbered logical links. However, making
   recommendations for the structure and use of such a private MIB is
   beyond the scope of this work.  Even if unnumbered point-to-point
   logical links were implemented in this manner, it would still be
   necessary to allow a FR interface to be configured with multiple IP
   addresses when a router is connected to multiple NBMA subnets through
   a single physical interface.  Hence, while it is possible to define
   unnumbered logical point-to-point links in OSPF, we find this

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RFC 1586                 OSPF over Frame Relay                March 1994

   alternative less attractive than using numbered logical point-to-
   point links.

4.  Using OSPF over FR

   The ability to configure distinct logical interfaces over FR gives
   users a great deal of flexibility in designing FR networks for use
   with OSPF. Because routers can be partially interconnected over FR,
   it is possible to design networks more cost-effectively than before.
   The issues to consider are the price/cost structure for VCs (fixed,
   distance-sensitive, banded) and ports, performance guarantees
   provided, traffic distribution (local, long-haul), and protocol
   efficiency. We have mentioned that the NBMA model provides some
   economy in OSPF protocol processing and overhead and is recommended
   for small homogeneous networks. In general, users should configure
   their networks to contain several small "NBMA clusters," which are in
   turn interconnected by long-haul VCs. The best choices for the number
   of routers in each cluster and the size of the long-haul logical
   point-to-point links depends on the factors mentioned above. If it is
   necessary to architect a more "flat" network, the ability to assign
   different link metrics to different (groups of) VCs allows for
   greater efficiency in using FR resources since VCs with better
   performance characteristics (throughput, delay) could be assigned
   lower link metrics.

5.  Conclusion

   We have specified guidelines for OSPF implementors and users to bring
   about improvements in how the protocol runs over frame relay
   networks. These recommendations do not require any protocol changes
   and allow for simpler, more efficient and cost-effective network
   designs. We recommend that OSPF implementations be able to support
   logical interfaces, each consisting of one or more virtual circuits
   and used as numbered logical point-to-point links (one VC) or logical
   NBMA networks (more than one VC). The current NBMA model for frame
   relay should continue to be used for small homogeneous networks
   consisting of a few routers.

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RFC 1586                 OSPF over Frame Relay                March 1994

6.  References

   [1] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect
       over Frame Relay", RFC 1294, Wellfleet Communications, Inc., BBN
       Communications, January 1992.

   [2] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.

   [3] McCloghrie, K., and M. Rose, Editors, "Management Information
       Base for Network Management of TCP/IP-based Internets: MIB-II",
       STD 17, RFC 1213, Hughes LAN Systems, Inc., Performance Systems
       International, March 1991.

   [4] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol",
       RFC 1293, Wellfleet Communications, Inc., January 1992.

   [5] Baker, F.,  and R. Coltun, "OSPF Version 2 Management Information
       Base", RFC 1253, ACC, Computer Science Center, August 1991.

Security Considerations

   Security issues are not discussed in this memo.

7.  Authors' Addresses

   Osmund S. deSouza
   AT&T Bell Laboratories
   Room 1K-606
   101 Crawfords Corner Road
   Holmdel, NJ 07733

   Phone: (908) 949-1393
   EMail: osmund.desouza@att.com

   Manoel A. Rodrigues
   Room 1K-608
   AT&T Bell Laboratories
   101 Crawfords Corner Road
   Holmdel, NJ 07733

   Phone: (908) 949-4655
   EMail: manoel.rodrigues@att.com

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