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RFC1586 - Guidelines for Running OSPF Over Frame Relay Networks

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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). RFC1294
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
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

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