Network Working Group Enke Chen
Internet Draft Redback Networks
Expiration Date: December 2002
BGP Route Oscillation Reduction with Confederation
draft-chen-confed-oscillation-reduce-01.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
This document proposes a simple revision to Confederation that allows
a BGP speaker in a Confederation to send a route advertisement
(instead of a route withdraw) in certain cases. The route
advertisement helps narrow the gap between Confederation and IBGP
full-mesh in terms of routing information. It has been shown that the
proposed mechanism helps achieve stable route selection and eliminate
a number of persistent route oscillations involving Confederation.
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3. Introduction
As documented in [1], the routing information reduction by BGP
Confederation [2] can result in persistent route oscillations with
certain routing setup and network topologies.
This document proposes a simple revision to Confederation that allows
a BGP speaker in a Confederation to send a route advertisement
(instead of a route withdraw) in certain cases. The route
advertisement helps narrow the gap between Confederation and IBGP
full-mesh in terms of routing information. It has been shown that the
proposed mechanism helps achieve stable route selection and eliminate
a number of persistent route oscillations involving Confederation.
The proposed mechanisms work within the current BGP protocol [4] that
limits route advertisement to only one path per prefix. In addition,
only the Confederation Sub-AS Border Routers need to be upgraded. One
can upgrade one router at a time when required, and then immediately
benefit from the route oscillation reduction or elimination.
4. Modification to Confederation
Currently a BGP speaker in a Confederation [3] follows the basic BGP
principle that only the best path is advertised to a BGP peer.
Therefore when the overall best path for a speaker is from a peer in
a neighboring AS of the same Confederation, none of the paths from
peers in the same AS would be advertised by the speaker to a peer in
a neighboring AS of the same Confederation, and be considered in
route selection. Similarly when the overall best path for a speaker
is from a peer in the same AS, none of paths from peers in a
neighboring AS of the same Confederation would be advertised by the
speaker to a peer in the same AS, and be considered in route
selection.
In order to increase the routing information advertised in a
Confederation, the following modification is proposed:
In addition to calculating the overall best path among all the
received paths, a BGP speaker in a Confederation may calculate
a best path ("I-BEST") among all the paths received from peers
within the same AS. It may also calculate a best path ("E-BEST")
among all the paths received from peers in neighboring ASs of
the same Confederation.
When the E-BEST for a speaker exists, and the I-BEST is the
overall best path for the speaker, the speaker may advertise the
E-BEST to a peer in the same AS of the Confederation. When the
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E-BEST becomes non-existence, or when it should no longer be
advertised, a replacement path or route withdraw must be sent to
a peer in the same AS if an E-BEST was advertised to the peer
previously.
Consider the case in which a BGP speaker in a Confederation
maintains BGP sessions with remote speakers in neighboring ASs
of the Confederation, and all the remote speakers maintain direct
BGP sessions among them. In this case the speaker does not need to
advertise routes learned from one such a remote peer to another.
When the I-BEST for the speaker exists, and the E-BEST is the
overall best path for the speaker, the speaker may advertise
the I-BEST to peers in neighboring ASs of the Confederation.
When the I-BEST becomes non-existence, or when it should no
longer be advertised, a replacement path or route withdraw
must be sent to a peer in a neighboring AS of the Confederation
if an I-BEST was advertised to the peer previously.
It is noted that the advertisement of I-BEST (or E-BEST) is not
useful and should not be sent when the overall best path wins
over the I-BEST (or E-BEST) prior to (and including) the step
of MED comparison in the route selection procedure [3, Sect. 9.1].
This modification allows additional routing information to be
advertised, and be available in route selection.
5. Remarks
The proposed mechanism alleviates to some degree, but does not fully
resolve the concern of routing information reduction by
Confederation. It is possible that the proposed mechanism may not be
adequate for certain persistent route oscillation cases in which the
advertisement of multiple paths for a prefix (as proposed in [4]) may
be required for their resolution.
It should be noted that compared to the existing mechanism of
Confederation, the proposed revision may cause memory usage in a
network to increase due to the advertisement of additional routing
information.
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6. Intellectual Property Considerations
Redback Networks, Inc. may seek patent protection on some of the
technology described in this Internet Draft. If technology in this
document is adopted as a standard, Redback Networks agrees to
license, on reasonable and non-discriminatory terms, any patent
rights it obtains covering such technology to the extent necessary to
comply with the standard.
7. Security Considerations
This document introduces no new security concerns to BGP or other
specifications referenced in this document.
8. Acknowledgments
The author would like to thank Naiming Shen and Jenny Yuan for their
review and comments.
9. References
[1] McPherson, D., Gill, V., Walton, D., Retana, A., "BGP Persistent
Route Oscillation Condition", Work in Progress (draft-ietf-idr-
route-oscillation-01), February 2002.
[2] Traina, P., McPherson, D., Scudder, J.. "Autonomous System Con-
federations for BGP", RFC 3065, February 2001.
[3] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
Work in Progress (draft-ietf-idr-bgp4-17), January 2002
[4] Walton, D., Cook, D., Retana, A., Scudder, J., "Advertisement of
Multiple Paths in BGP", Work in Progress (draft-walton-bgp-add-
paths-00), May 2002.
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10. Author Information
Enke Chen
Redback Networks, Inc.
350 Holger Way
San Jose, CA 95134
Email: enke@redback.com
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