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IETF RFC 6987
Last modified on Wednesday, September 11th, 2013
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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. White
D. McPherson
Verisign, Inc.
September 2013
OSPF Stub Router Advertisement
 Abstract
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
http://www.rfc-editor.org/info/RFC 6987.
Retana, et al. Informational PAGE 1 
RFC 6987 OSPF Stub Router Advertisement September 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. OSPFv3-Only Solution . . . . . . . . . . . . . . . . . . 3
3. Maximum Link Metric . . . . . . . . . . . . . . . . . . . . . 4
4. Deployment Considerations . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Changes from RFC 3137 . . . . . . . . . . . . . . . 6
1. 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
network.
o Other (administrative or traffic engineering) reasons.
Retana, et al. Informational PAGE 2
RFC 6987 OSPF Stub Router Advertisement September 2013
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.
2. Solutions
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
are
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 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 [RFC 2328] and OSPFv3
[RFC 5340].
2.1. OSPFv3-Only Solution
OSPFv3 [RFC 5340] 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.
Retana, et al. Informational PAGE 3
RFC 6987 OSPF Stub Router Advertisement September 2013
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.
MaxLinkMetric
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 [RFC 1247] (discarding link records in
router-LSAs that have a MaxLinkMetric cost value) and routers that
perform it as specified in [RFC 1583] 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
doesn't.
5. Security Considerations
The technique described in this document does not introduce any new
security issues into the OSPF protocol.
Retana, et al. Informational PAGE 4
RFC 6987 OSPF Stub Router Advertisement September 2013
6. Acknowledgements
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. References
7.1. Normative References
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC 5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
7.2. Informative References
[RFC 1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[RFC 1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[RFC 3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137,
June 2001.
Retana, et al. Informational PAGE 5
RFC 6987 OSPF Stub Router Advertisement September 2013
Appendix A. Changes from RFC 3137
This document obsoletes [RFC 3137].
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 [RFC 5340] as a
solution to the problem addressed in the text.
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RFC 6987 OSPF Stub Router Advertisement September 2013
Authors' Addresses
Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
EMail: aretana@cisco.com
Liem Nguyen
Cisco Systems, Inc.
3750 Cisco Way
San Jose, CA 95134
USA
EMail: lhnguyen@cisco.com
Alex Zinin
Cinarra Systems
Menlo Park, CA
USA
EMail: alex.zinin@gmail.com
Russ White
1500 N. Greenville Avenue
Suite 1100
Richardson, TX 75081
USA
EMail: Russ.White@vce.com
Danny McPherson
Verisign, Inc.
12061 Bluemont Way
Reston, VA 20190
USA
EMail: dmcpherson@verisign.com
Retana, et al. Informational PAGE 7
RFC TOTAL SIZE: 10890 bytes
PUBLICATION DATE: Wednesday, September 11th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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