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.

Abstract

  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.

Acknowledgements

  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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  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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  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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            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
            pointTopoint).

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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  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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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: [email protected]


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

  Phone: (908) 949-4655
  EMail: [email protected]








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