Internet DRAFT - draft-ietf-ospf-rfc3137bis
draft-ietf-ospf-rfc3137bis
Network Working Group A. Retana
Internet-Draft L. Nguyen
Obsoletes: 3137 (if approved) Cisco Systems, Inc.
Intended status: Informational A. Zinin
Expires: October 25, 2013 Cinarra Systems
R. White
D. McPherson
Verisign, Inc.
April 23, 2013
OSPF Stub Router Advertisement
draft-ietf-ospf-rfc3137bis-04
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 [RFC3137].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on October 25, 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
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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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Changes from RFC3137 . . . . . . . . . . . . . . . . 5
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 5
B.1. Changes between the -00 and -01 versions. . . . . . . . . 6
B.2. Changes between the -01 and -02 versions. . . . . . . . . 6
B.3. Changes between the -02 and -03 versions. . . . . . . . . 6
B.4. Changes between the -03 and -04 versions . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 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 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 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.
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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 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.
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 costs of all non-stub links (links of the types
other than 3) set to MaxLinkMetric (defined in Section 3).
The solution above applies to both OSPFv2 [RFC2328] and OSPFv3
[RFC5340].
2.1. OSPFv3-only Solution
OSPFv3 [RFC5340] introduced 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.
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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 the link described by an LSA
should not be used as transit. Used in router-LSAs (see
Section 2). 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 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 as 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.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgements
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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 version of this document. Dave
Cridland and Tom Yu also provided valuable comments.
8. References
8.1. 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.
8.2. 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,
June 2001.
Appendix A. Changes from RFC3137
This document obsoletes [RFC3137].
In addition to editorial updates, this documents 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.
Appendix B. Change Log
This section should be removed by the RFC Editor before publication.
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B.1. Changes between the -00 and -01 versions.
o Defined a new architectural constant (MaxLinkMetric) to eliminate
any confusion about the interpretation of LSInfinity.
o Added a section to reference the R-bit and V6-bit in OSPFv3.
o Updated acks and contact information.
B.2. Changes between the -01 and -02 versions.
o Took out references to not having a standard solution and
incorporated the R-bit solution as part of the (renamed)
"Solutions" section.
o Various minor edits and reordered sections.
B.3. Changes between the -02 and -03 versions.
o Updated contact information.
o Renamed the 'Motivation' section to 'Introduction' becuase of an
error in idnits.
o Took out the rfc2119 references as none of the keywords are used
in the text.
o Added an 'IANA Considerations' section to indicate that there are
no actions required.
B.4. Changes between the -03 and -04 versions
o Clearly indicated in the text that this document obsoletes
RFC3137.
o Various minor edits.
o Clarified the use of the R-bit and included text in the
"Deployment Considerations" section about it.
o Updated acks.
o Ordered the References section to provide some Normative ones.
o Added an Appendix summarizing the changes form RFC3137.
Authors' Addresses
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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
Verisign, Inc.
12061 Bluemont Way
Reston, VA 20190
USA
Email: riwhite@verisign.com
Danny McPherson
Verisign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166
USA
Email: dmcpherson@verisign.com
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