Network Working Group E. Chen
Internet Draft S. Sangli
Expiration Date: September 2007 Cisco Systems
Avoid BGP Best Path Transitions from One External to Another
draft-ietf-idr-avoid-transition-05.txt
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Abstract
In this document we propose an extension to the BGP route selection
rules that would avoid unnecessary best path transitions between
external paths under certain conditions. The proposed extension would
help the overall network stability, and more importantly, would
eliminate certain BGP route oscillations in which more than one
external path from one BGP speaker contributes to the churn.
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1. Introduction
The last two steps of the BGP route selection (Sect. 9.1.2.2, [BGP])
involve comparing the BGP identifiers and the peering addresses. The
BGP identifier (treated either as an IP address, or just an integer
[BGP-ID]) for a BGP speaker is allocated by the AS to which the
speaker belongs. As a result, for a local BGP speaker, the BGP
identifier of a route received from an external peer is just an
random number. When routes under consideration are from external
peers, the result from the last two steps of the route selection is
therefore "random" as far as the local BGP speaker is concerned.
It is based on this observation that we propose an extension to the
BGP route selection rules that would avoid unnecessary best path
transitions between external paths under certain conditions. The
proposed extension would help the overall network stability, and more
importantly, would eliminate certain BGP route oscillations in which
more than one external path from one BGP speaker contributes to the
churn.
2. Specification of Requirements
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 RFC 2119 [RFC2119].
3. The Algorithm
Consider the case in which the existing best path A is from an
external peer, and another external path B is then selected as the
new best path by the route selection algorithm described in [BGP].
When comparing all the paths in route selection, if neither Path A
nor Path B is eliminated by the route selection algorithm prior to
Step f) - BGP identifier comparison (Sect. 9.1.2.2 [BGP]), we propose
that the existing best path (Path A) be kept as the best path (thus
avoiding switching the best path to Path B).
This algorithm SHOULD NOT be applied when either path is from a BGP
Confederation peer.
In addition, the algorithm SHOULD NOT be applied when both paths are
from peers with identical BGP identifier (i.e., there exist parallel
BGP sessions between two BGP speakers). As the peering addresses for
the parallel sessions are typically allocated by one AS (possibly
with route selection considerations), the algorithm (if applied)
could impact the existing routing setup. Furthermore, by not applying
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the algorithm, the allocation of peering addresses would remain as a
simple and effective tool in influencing route selection when
parallel BGP sessions exist.
4. The Benefits
The proposed extension to the BGP route selection rules avoids
unnecessary best path transitions between external paths under
certain conditions. Clearly the extension would help reduce routing
and forwarding changes in a network, thus help the overall network
stability.
More importantly, as shown in the following example, the proposed
extension can be used to eliminate certain BGP route oscillations in
which more than one external path from one BGP speaker contributes to
the churn. Note however, that there are permanent BGP route
oscillation scenarios [RFC3345] that the mechanism described in this
document does not eliminate.
Consider the example in Fig. 1 where
o R1, R2, R3 and R4 belong to one AS
o R1 is a route reflector with R3 as its client.
o R2 is a route reflector with R4 as its client.
o The IGP metrics are as listed.
o External paths (a), (b) and (c) are as described in Fig. 2.
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+----+ 40 +----+
| R1 |--------------| R2 |
+----+ +----+
| |
| |
| 10 | 10
| |
| |
+----+ +----+
| R3 | | R4 |
+----+ +----+
/ \ |
/ \ |
(a) (b) (c)
Figure 1
Path AS MED Identifier
a 1 0 2
b 2 20 1
c 2 10 5
Figure 2
Due to the interaction of the route reflection [BGP-RR] and the
MULTI_EXIT_DISC (MED) attribute, the best path on R1 keeps churning
between (a) and (c), and the best path on R3 keeps churning between
(a) and (b).
With the proposed algorithm R3 would not switch the best path from
(a) to (b) even after R1 withdraws (c) toward its clients, and that
is enough to stop the route oscillation.
Although this type of route oscillations can also be eliminated by
other route reflection enhancements being developed, the proposed
algorithm is extremely simple and can be implemented and deployed
immediately without introducing any backward compatibility issues.
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5. Remarks
The proposed algorithm is backward-compatible, and can be deployed on
a per-BGP-speaker basis. The deployment of the algorithm is highly
recommended on a BGP speaker with multiple external BGP peers
(especially the ones connecting to an inter-exchange point).
Compared to the existing behavior, the proposed algorithm may
introduce some "non-determinism" in the BGP route selection -
although one can argue that the BGP Identifier comparison in the
existing route selection has already introduced some "randomness" as
described in the introduction section. Such "non-determinism" has
not been shown to be detrimental in practice, and can be completely
eliminated by using the existing mechanisms (such as setting
LOCAL_PREF or MED) if so desired.
6. IANA Considerations
This extension does not require any action by IANA.
7. Security Considerations
This extension does not introduce any security issues.
8. Acknowledgments
The idea presented was inspired by a route oscillation case observed
on the BBN/Genuity backbone in 1998. The algorithm was also
implemented and deployed at that time.
The authors would like to thank Yakov Rekhter and Ravi Chandra for
their comments on the initial idea.
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9. Normative References
[BGP] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway Protocol
4 (BGP-4)", RFC 4271, January 2006.
[BGP-RR] T. Bates, R. Chandra, and E. Chen, "BGP Route Reflection -
An Alternative to Full Mesh IBGP", RFC 4456, April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10. Non-normative References
[BGP-ID] E. Chen and J. Yuan, "AS-wide Unique BGP Identifier for
BGP-4", Work in Progress, draft-ietf-idr-bgp-identifier-08.txt,
November 2006.
[RFC3345] D. McPherson, V, Gill, D. Walton, and A. Retana, "Border
Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC
3345, August 2002.
11. Author Information
Enke Chen
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Email: enkechen@cisco.com
Srihari R. Sangli
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Email: rsrihari@cisco.com
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