Network Working Group A. Retana
Request for Comments: 3137 L. Nguyen
Category: Informational R. White
Cisco Systems
A. Zinin
Nexsi Systems
D. McPherson
Amber Networks
June 2001
OSPF Stub Router Advertisement
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This memo describes a backward-compatible technique that may be used
by OSPF (Open Shortest Path First) implementations to advertise
unavailability to forward transit traffic or to lower the preference
level for the paths through such a router. In some cases, it is
desirable not to route transit traffic via a specific OSPF router.
However, OSPF does not specify a standard way to accomplish this.
1. Motivation
In some situations, it may be advantageous to inform routers in a
network not to use a specific router as a transit point, but still
route to it. Possible situations include the following.
o The router is in a critical condition (for example, has very
high CPU load or does not have enough memory to store all LSAs
or build the routing table).
o Graceful introduction and removal of the router to/from the
network.
o Other (administrative or traffic engineering) reasons.
Retana, et al. Informational [Page 1]
RFC 3137 OSPF Stub Router Advertisement June 2001
Note that the proposed solution does not remove the router from the
topology view of the network (as could be done by just flushing that
router's router-LSA), but prevents other routers from using it for
transit routing, while still routing packets to router's own IP
addresses, i.e., the router is announced as stub.
It must be emphasized that the proposed solution provides real
benefits in networks designed with at least some level of redundancy
so that traffic can be routed around the stub router. Otherwise,
traffic destined for the networks reachable through such a stub
router will be still routed through it.
2. Proposed Solution
The solution described in this document solves two challenges
associated with the outlined problem. In the description below,
router X is the router announcing itself as a stub.
1) Making other routers prefer routes around router X while
performing the Dijkstra calculation.
2) Allowing other routers to reach IP prefixes directly connected
to router X.
Note that it would be easy to address issue 1) alone by just flushing
router X's router-LSA from the domain. However, it does not solve
problem 2), since other routers will not be able to use links to
router X in Dijkstra (no back link), and because router X will not
have links to its neighbors.
To address both problems, router X announces its router-LSA to the
neighbors as follows.
o costs of all non-stub links (links of the types other than 3)
are set to LSInfinity (16-bit value 0xFFFF, rather than 24-bit
value 0xFFFFFF used in summary and AS-external LSAs).
o costs of stub links (type 3) are set to the interface output
cost.
This addresses issues 1) and 2).
Retana, et al. Informational [Page 2]
RFC 3137 OSPF Stub Router Advertisement June 2001
3. Compatibility issues
Some inconsistency may be seen when the network is constructed of the
routers that perform intra-area Dijkstra calculation as specified in
[RFC1247] (discarding link records in router-LSAs that have
LSInfinity cost value) and routers that perform it as specified in
[RFC1583] and higher (do not treat links with LSInfinity cost as
unreachable). Note that this inconsistency will not lead to routing
loops, because if there are some alternate paths in the network, both
types of routers will agree on using them rather than the path
through the stub router. If the path through the stub router is the
only one, the routers of the first type will not use the stub router
datatracker.ietf.org