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
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

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

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

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.

6. References

[1] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect
over Frame Relay", RFC1294, Wellfleet Communications, Inc., BBN
Communications, January 1992.

[2] Moy, J., "OSPF Version 2", RFC1583, 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, RFC1213, Hughes LAN Systems, Inc., Performance Systems
International, March 1991.

[4] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol",
RFC1293, Wellfleet Communications, Inc., January 1992.

[5] Baker, F., and R. Coltun, "OSPF Version 2 Management Information
Base", RFC1253, 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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