Network Working Group A. Retana
Internet-Draft Hewlett-Packard Co.
Obsoletes: RFC3137 (if approved) L. Nguyen
Intended status: Informational A. Zinin
Expires: December 31, 2012 Cisco Systems, Inc.
R. White
D. McPherson
Verisign, Inc.
June 29, 2012
OSPF Stub Router Advertisement
draft-ietf-ospf-rfc3137bis-01
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 December 31, 2012.
Copyright Notice
Copyright (c) 2012 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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publication of this document. Please review these documents
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Table of Contents
1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . . 3
4. Deployment Considerations . . . . . . . . . . . . . . . . . . . 4
4.1. Other Solutions . . . . . . . . . . . . . . . . . . . . . . 4
5. Maximum Link Metric . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. Changes between the -00 and -01 versions. . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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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 MaxLinkMetric.
The solution above applies to both OSPFv2 [RFC2328] and OSPFv3
[RFC5340].
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.
4.1. Other Solutions
This document describes a technique that has been implemented and
deployed in a wide variety of networks. OSPFv3 [RFC5340] introduced
additional options to provide similar, if not better, control of the
forwarding topology; the R-bit and the V6-bit provide a more granular
indication of whether a router is active and/or whether it should be
used specifically for IPv6 traffic, respectively.
It is left to network operators to decide which technique to use in
their network.
5. Maximum Link Metric
Section 3 refers to the cost of all non-stub links as MaxLinkMetric,
which is a new fixed architectural value introduced in this document.
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MaxLinkMetric
The metric value indicating that the link described by an LSA
should not be used as transit. Used in router-LSAs (see
Section 3). It is defined to be the 16-bit binary value of all
ones: 0xffff.
6. Security Considerations
The technique described in this document does not introduce any new
security issues into the OSPF protocol.
7. 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,
Tomohiro Yamagata, Faraz Shamim and Acee Lindem who provided
significant input for the latest version of this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. 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.
Appendix A. Changes between the -00 and -01 versions.
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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.
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
Alex Zinin
Cisco Systems, Inc.
Capital Tower, 168 Robinson Rd.
Singapore, Singapore 068912
Singapore
Email: azinin@cisco.com
Russ White
Verisign, Inc.
12061 Bluemont Way
Reston, VA 20190
USA
Email: riwhite@verisign.com
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Danny McPherson
Verisign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166
USA
Email: dmcpherson@verisign.com
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