Network Working Group A. Retana
Internet-Draft Hewlett-Packard Co.
Obsoletes: RFC3137 (if approved) L. Nguyen
Intended status: Informational R. White
Expires: January 1, 2012 A. Zinin
Cisco Systems, Inc.
D. McPherson
Verisign, Inc.
June 30, 2011
OSPF Stub Router Advertisement
draft-ietf-ospf-rfc3137bis-00
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 1, 2012.
Copyright Notice
Copyright (c) 2011 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. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . . 3
4. Compatibility issues . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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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.
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 the router's own IP
addresses, i.e., the router is announced as a 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. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. 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
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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 LSInfinity (16-bit value 0xFFFF, rather than 24-
bit value 0xFFFFFF used in summary and AS-external LSAs).
The solution above applies to both OSPFv2 [RFC2328] and OSPFv3
[RFC5340].
4. 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
for transit (which is the desired behavior), while the routers of the
second type will still use this path.
5. Security Considerations
The technique described in this document does not introduce any new
security issues into the OSPF protocol.
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 also like to thank Shishio Tsuchiya, Gunter Van de Velde and
Tomohiro Yamagata for reminding us of the need to document the OSPFv3
behavior.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[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.
Authors' Addresses
Alvaro Retana
Hewlett-Packard Co.
2610 Wycliff Road
Raleigh, NC 27607
USA
Email: alvaro.retana@hp.com
Liem Nguyen
Cisco Systems, Inc.
3750 Cisco Way
San Jose, CA 95134
USA
Email: lhnguyen@cisco.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Email: russwh@cisco.com
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Alex Zinin
Cisco Systems, Inc.
Capital Tower, 168 Robinson Rd.
Singapore, Singapore 068912
Singapore
Email: azinin@cisco.com
Danny McPherson
Verisign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166
USA
Email: dmcpherson@verisign.com
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