Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


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

deSouza & Rodrigues [Page 1] 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

deSouza & Rodrigues [Page 2] 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

deSouza & Rodrigues [Page 3] 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

deSouza & Rodrigues [Page 4] 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.

deSouza & Rodrigues [Page 5] 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
 Manoel A. Rodrigues
 Room 1K-608
 AT&T Bell Laboratories
 101 Crawfords Corner Road
 Holmdel, NJ 07733
 Phone: (908) 949-4655

deSouza & Rodrigues [Page 6]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1586.txt · Last modified: 1994/03/23 01:05 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki