Network Working Group E. Chen
Request for Comments: 5004 S. Sangli
Category: Standards Track Cisco Systems
September 2007
Avoid BGP Best Path Transitions from One External to Another
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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.
1. Introduction
The last two steps of the BGP route selection (Section 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
Autonomous System (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 a 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.
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RFC 5004 Best BGP Route Selection September 2007
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 (Section 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 an 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 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 helping 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.
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RFC 5004 Best BGP Route Selection September 2007
Consider the example in Figure 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 Figure 2.
+----+ 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 oscillation 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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RFC 5004 Best BGP Route Selection September 2007
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. Security Considerations
This extension does not introduce any security issues.
7. Acknowledgments
The idea presented was inspired by a route oscillation case observed
in the BBN/Genuity network 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.
8. Normative References
[BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
[BGP-RR] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection:
An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456,
April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9. Informative References
[BGP-ID] Chen, E. and J. Yuan, "AS-wide Unique BGP Identifier for
BGP-4", Work in Progress, November 2006.
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RFC 5004 Best BGP Route Selection September 2007
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, "Border
Gateway Protocol (BGP) Persistent Route Oscillation
Condition", RFC 3345, August 2002.
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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RFC 5004 Best BGP Route Selection September 2007
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