draft-ietf-idr-avoid-transition-05.txt   rfc5004.txt 
Network Working Group E. Chen Network Working Group E. Chen
Internet Draft S. Sangli Request for Comments: 5004 S. Sangli
Expiration Date: September 2007 Cisco Systems Category: Standards Track Cisco Systems
September 2007
Avoid BGP Best Path Transitions from One External to Another Avoid BGP Best Path Transitions from One External to Another
draft-ietf-idr-avoid-transition-05.txt Status of This Memo
Status of this Memo
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Abstract Abstract
In this document we propose an extension to the BGP route selection In this document, we propose an extension to the BGP route selection
rules that would avoid unnecessary best path transitions between rules that would avoid unnecessary best path transitions between
external paths under certain conditions. The proposed extension would external paths under certain conditions. The proposed extension
help the overall network stability, and more importantly, would would help the overall network stability, and more importantly, would
eliminate certain BGP route oscillations in which more than one eliminate certain BGP route oscillations in which more than one
external path from one BGP speaker contributes to the churn. external path from one BGP speaker contributes to the churn.
1. Introduction 1. Introduction
The last two steps of the BGP route selection (Sect. 9.1.2.2, [BGP]) The last two steps of the BGP route selection (Section 9.1.2.2,
involve comparing the BGP identifiers and the peering addresses. The [BGP]) involve comparing the BGP identifiers and the peering
BGP identifier (treated either as an IP address, or just an integer addresses. The BGP identifier (treated either as an IP address or
[BGP-ID]) for a BGP speaker is allocated by the AS to which the just an integer [BGP-ID]) for a BGP speaker is allocated by the
speaker belongs. As a result, for a local BGP speaker, the BGP Autonomous System (AS) to which the speaker belongs. As a result,
identifier of a route received from an external peer is just an for a local BGP speaker, the BGP identifier of a route received from
random number. When routes under consideration are from external an external peer is just a random number. When routes under
peers, the result from the last two steps of the route selection is consideration are from external peers, the result from the last two
therefore "random" as far as the local BGP speaker is concerned. 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 It is based on this observation that we propose an extension to the
BGP route selection rules that would avoid unnecessary best path BGP route selection rules that would avoid unnecessary best-path
transitions between external paths under certain conditions. The transitions between external paths under certain conditions. The
proposed extension would help the overall network stability, and more proposed extension would help the overall network stability, and more
importantly, would eliminate certain BGP route oscillations in which importantly, would eliminate certain BGP route oscillations in which
more than one external path from one BGP speaker contributes to the more than one external path from one BGP speaker contributes to the
churn. churn.
2. Specification of Requirements 2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. The Algorithm 3. The Algorithm
Consider the case in which the existing best path A is from an 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 external peer, and another external path B is then selected as the
new best path by the route selection algorithm described in [BGP]. new best path by the route selection algorithm described in [BGP].
When comparing all the paths in route selection, if neither Path A When comparing all the paths in route selection, if neither Path A
nor Path B is eliminated by the route selection algorithm prior to 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 Step f) -- BGP identifier comparison (Section 9.1.2.2, [BGP]) -- we
that the existing best path (Path A) be kept as the best path (thus propose that the existing best path (Path A) be kept as the best path
avoiding switching the best path to Path B). (thus avoiding switching the best path to Path B).
This algorithm SHOULD NOT be applied when either path is from a BGP This algorithm SHOULD NOT be applied when either path is from a BGP
Confederation peer. Confederation peer.
In addition, the algorithm SHOULD NOT be applied when both paths are In addition, the algorithm SHOULD NOT be applied when both paths are
from peers with identical BGP identifier (i.e., there exist parallel from peers with an identical BGP identifier (i.e., there exist
BGP sessions between two BGP speakers). As the peering addresses for parallel BGP sessions between two BGP speakers). As the peering
the parallel sessions are typically allocated by one AS (possibly addresses for the parallel sessions are typically allocated by one AS
with route selection considerations), the algorithm (if applied) (possibly with route selection considerations), the algorithm (if
could impact the existing routing setup. Furthermore, by not applying applied) could impact the existing routing setup. Furthermore, by
the algorithm, the allocation of peering addresses would remain as a not applying the algorithm, the allocation of peering addresses would
simple and effective tool in influencing route selection when remain as a simple and effective tool in influencing route selection
parallel BGP sessions exist. when parallel BGP sessions exist.
4. The Benefits 4. The Benefits
The proposed extension to the BGP route selection rules avoids The proposed extension to the BGP route selection rules avoids
unnecessary best path transitions between external paths under unnecessary best-path transitions between external paths under
certain conditions. Clearly the extension would help reduce routing certain conditions. Clearly, the extension would help reduce routing
and forwarding changes in a network, thus help the overall network and forwarding changes in a network, thus helping the overall network
stability. stability.
More importantly, as shown in the following example, the proposed More importantly, as shown in the following example, the proposed
extension can be used to eliminate certain BGP route oscillations in extension can be used to eliminate certain BGP route oscillations in
which more than one external path from one BGP speaker contributes to which more than one external path from one BGP speaker contributes to
the churn. Note however, that there are permanent BGP route the churn. Note however, that there are permanent BGP route
oscillation scenarios [RFC3345] that the mechanism described in this oscillation scenarios [RFC3345] that the mechanism described in this
document does not eliminate. document does not eliminate.
Consider the example in Fig. 1 where Consider the example in Figure 1 where
o R1, R2, R3 and R4 belong to one AS o R1, R2, R3, and R4 belong to one AS.
o R1 is a route reflector with R3 as its client. o R1 is a route reflector with R3 as its client.
o R2 is a route reflector with R4 as its client. o R2 is a route reflector with R4 as its client.
o The IGP metrics are as listed. o The IGP metrics are as listed.
o External paths (a), (b) and (c) are as described in Fig. 2. o External paths (a), (b), and (c) are as described in Figure 2.
+----+ 40 +----+ +----+ 40 +----+
| R1 |--------------| R2 | | R1 |--------------| R2 |
+----+ +----+ +----+ +----+
| | | |
| | | |
| 10 | 10 | 10 | 10
| | | |
| | | |
+----+ +----+ +----+ +----+
skipping to change at page 4, line 34 skipping to change at page 3, line 42
b 2 20 1 b 2 20 1
c 2 10 5 c 2 10 5
Figure 2 Figure 2
Due to the interaction of the route reflection [BGP-RR] and the Due to the interaction of the route reflection [BGP-RR] and the
MULTI_EXIT_DISC (MED) attribute, the best path on R1 keeps churning 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 between (a) and (c), and the best path on R3 keeps churning between
(a) and (b). (a) and (b).
With the proposed algorithm R3 would not switch the best path from 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 (a) to (b) even after R1 withdraws (c) toward its clients, and that
is enough to stop the route oscillation. is enough to stop the route oscillation.
Although this type of route oscillations can also be eliminated by Although this type of route oscillation can also be eliminated by
other route reflection enhancements being developed, the proposed other route reflection enhancements being developed, the proposed
algorithm is extremely simple and can be implemented and deployed algorithm is extremely simple and can be implemented and deployed
immediately without introducing any backward compatibility issues. immediately without introducing any backward compatibility issues.
5. Remarks 5. Remarks
The proposed algorithm is backward-compatible, and can be deployed on The proposed algorithm is backward-compatible, and can be deployed on
a per-BGP-speaker basis. The deployment of the algorithm is highly a per-BGP-speaker basis. The deployment of the algorithm is highly
recommended on a BGP speaker with multiple external BGP peers recommended on a BGP speaker with multiple external BGP peers
(especially the ones connecting to an inter-exchange point). (especially the ones connecting to an inter-exchange point).
Compared to the existing behavior, the proposed algorithm may Compared to the existing behavior, the proposed algorithm may
introduce some "non-determinism" in the BGP route selection - introduce some "non-determinism" in the BGP route selection --
although one can argue that the BGP Identifier comparison in the although one can argue that the BGP Identifier comparison in the
existing route selection has already introduced some "randomness" as existing route selection has already introduced some "randomness" as
described in the introduction section. Such "non-determinism" has described in the introduction section. Such "non-determinism" has
not been shown to be detrimental in practice, and can be completely not been shown to be detrimental in practice and can be completely
eliminated by using the existing mechanisms (such as setting eliminated by using the existing mechanisms (such as setting
LOCAL_PREF or MED) if so desired. LOCAL_PREF or MED) if so desired.
6. IANA Considerations 6. Security Considerations
This extension does not require any action by IANA.
7. Security Considerations
This extension does not introduce any security issues. This extension does not introduce any security issues.
8. Acknowledgments 7. Acknowledgments
The idea presented was inspired by a route oscillation case observed The idea presented was inspired by a route oscillation case observed
on the BBN/Genuity backbone in 1998. The algorithm was also in the BBN/Genuity network in 1998. The algorithm was also
implemented and deployed at that time. implemented and deployed at that time.
The authors would like to thank Yakov Rekhter and Ravi Chandra for The authors would like to thank Yakov Rekhter and Ravi Chandra for
their comments on the initial idea. their comments on the initial idea.
9. Normative References 8. Normative References
[BGP] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway Protocol [BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
4 (BGP-4)", RFC 4271, January 2006. Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
[BGP-RR] T. Bates, R. Chandra, and E. Chen, "BGP Route Reflection - [BGP-RR] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection:
An Alternative to Full Mesh IBGP", RFC 4456, April 2006. An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456,
April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
10. Non-normative References 9. Informative References
[BGP-ID] E. Chen and J. Yuan, "AS-wide Unique BGP Identifier for [BGP-ID] Chen, E. and J. Yuan, "AS-wide Unique BGP Identifier for
BGP-4", Work in Progress, draft-ietf-idr-bgp-identifier-08.txt, BGP-4", Work in Progress, November 2006.
November 2006.
[RFC3345] D. McPherson, V, Gill, D. Walton, and A. Retana, "Border [RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, "Border
Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC Gateway Protocol (BGP) Persistent Route Oscillation
3345, August 2002. Condition", RFC 3345, August 2002.
11. Author Information Author Information
Enke Chen Enke Chen
Cisco Systems, Inc. Cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
Email: enkechen@cisco.com EMail: enkechen@cisco.com
Srihari R. Sangli Srihari R. Sangli
Cisco Systems, Inc. Cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
Email: rsrihari@cisco.com EMail: rsrihari@cisco.com
12. Intellectual Property Considerations Full Copyright Statement
Copyright (C) The IETF Trust (2007).
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contained in BCP 78, and except as set forth therein, the authors
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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