Internet Engineering Task Force (IETF) A. Retana
Request for Comments: 6987 L. Nguyen
Obsoletes: 3137 Cisco Systems, Inc.
Category: Informational A. Zinin
ISSN: 2070-1721 Cinarra Systems
R. WhiteD. McPherson
September 2013 OSPF Stub Router Advertisement
This document describes a backward-compatible technique that may be
used by OSPF (Open Shortest Path First) implementations to advertise
a router's unavailability to forward transit traffic or to lower the
preference level for the paths through such a router.
This document obsoletes RFC 3137.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 22. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1. OSPFv3-Only Solution . . . . . . . . . . . . . . . . . . 33. Maximum Link Metric . . . . . . . . . . . . . . . . . . . . . 44. Deployment Considerations . . . . . . . . . . . . . . . . . . 45. Security Considerations . . . . . . . . . . . . . . . . . . . 46. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 57. References . . . . . . . . . . . . . . . . . . . . . . . . . 57.1. Normative References . . . . . . . . . . . . . . . . . . 57.2. Informative References . . . . . . . . . . . . . . . . . 5Appendix A. Changes from RFC 3137 . . . . . . . . . . . . . . . 61. Introduction
In some situations, it may be advantageous to inform routers in a
network not to use a specific router as a transit point but to still
route to it. Possible situations include the following:
o The router is in a critical condition (for example, has a very
high CPU load or does not have enough memory to store all Link
State Advertisements (LSAs) or build the routing table).
o Graceful introduction and removal of the router to/from the
o Other (administrative or traffic engineering) reasons.
Note that the solution introduced in this document 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 discourages other routers
from using it for transit routing, while still routing packets to the
router's own IP addresses, i.e., the router is announced as a stub.
It must be emphasized that the 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 and reachable through such a stub router
may still be routed through it.
The solution introduced 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. The challenges
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
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 with the cost of all non-stub links (links of the types
other than 3) being set to MaxLinkMetric (defined in Section 3).
The solution above applies to both OSPFv2 [RFC2328] and OSPFv3
2.1. OSPFv3-Only Solution
OSPFv3 [RFC5340] introduces additional options to provide similar
control of the forwarding topology; the R-bit provides an indication
of whether a router is active and should be used for transit traffic.
It is left to network operators to decide which technique to use in
their network. See Section 4 for more details.
3. Maximum Link Metric
Section 2 refers to the cost of all non-stub links as MaxLinkMetric,
which is a new fixed architectural value introduced in this document.
The metric value indicating that a router-LSA link (see Section 2)
should not be used for transit traffic. It is defined to be the
16-bit binary value of all ones: 0xffff.
4. Deployment Considerations
When using MaxLinkMetric, some inconsistency may be seen if the
network is constructed of routers that perform an intra-area Dijkstra
calculation as specified in [RFC1247] (discarding link records in
router-LSAs that have a MaxLinkMetric cost value) and routers that
perform it as specified in [RFC1583] and higher (do not treat links
with MaxLinkMetric 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 for transit (which is the
desired behavior), while the routers of the second type will still
use this path.
On the other hand, clearing the R-bit will consistently result in the
router not being used for transit.
The use of MaxLinkMetric or the R-bit in a network depends on the
objectives of the operator. One of the possible considerations for
selecting one or the other is in the desired behavior if the path
through the stub router is the only one available. Using
MaxLinkMetric allows for that path to be used while the R-bit
5. Security Considerations
The technique described in this document does not introduce any new
security issues into the OSPF protocol.
The authors of this document do not make any claims on the
originality of the ideas described. Among other people, we would
like to acknowledge Henk Smit for being part of one of the initial
discussions around this topic.
We would like to thank Shishio Tsuchiya, Gunter Van de Velde,
Tomohiro Yamagata, Faraz Shamim, and Acee Lindem who provided
significant input for the latest draft version of this document.
Dave Cridland and Tom Yu also provided valuable comments.
7.1. Normative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
7.2. Informative References
[RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137,
Appendix A. Changes from RFC 3137
This document obsoletes [RFC3137].
In addition to editorial updates, this document defines a new
architectural constant (MaxLinkMetric in Section 3) to eliminate any
confusion about the interpretation of LSInfinity. It also
incorporates and explains the use of the R-bit [RFC5340] as a
solution to the problem addressed in the text.
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