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Sangli 3 Expiration Date: May 2005 Cisco Systems, Inc. 5 Avoid BGP Best Path Transitions from One External to Another 7 draft-ietf-idr-avoid-transition-01.txt 9 1. Status of this Memo 11 By submitting this Internet-Draft, I certify that any applicable 12 patent or other IPR claims of which I am aware have been disclosed, 13 or will be disclosed, and any of which I become aware will be 14 disclosed, in accordance with RFC 3668. 16 This document is an Internet-Draft and is in full conformance with 17 all provisions of Section 10 of RFC2026. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as ������work in progress.��ւ�� 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 2. Abstract 37 In this document we propose a revision to the BGP route selection 38 rules that would avoid unnecessary best path transitions between 39 external paths under certain conditions. The proposed revision would 40 help the overall network stability, and more importantly, would 41 eliminate certain BGP route oscillations in which more than one 42 external paths from one router contribute to the churn. 44 3. Introduction 46 The last two steps of the BGP route selection (Sect. 9.1.2.2, [1]) 47 involve comparing the BGP identifiers and the peering addresses. The 48 BGP identifier, (treated either as an IP address, or just an integer 49 [2]) for a BGP speaker is allocated by the AS to which the speaker 50 belongs. As a result, for a local BGP speaker, the BGP identifier of 51 a route received from an external peer is just an random number. When 52 routes under consideration are from external peers, the result from 53 the last two steps of the route selection is therefore "random" as 54 far as the local BGP speaker is concerned. 56 It is based on this observation that we propose a revision to the BGP 57 route selection rules that would avoid unnecessary best path 58 transitions between external paths under certain conditions. The 59 proposed revision would help the overall network stability, and more 60 importantly, would eliminate certain BGP route oscillations in which 61 more than one external paths from one router contribute to the churn. 63 4. Specification of Requirements 65 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 66 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 67 document are to be interpreted as described in RFC 2119 [5]. 69 5. The Algorithm 71 Consider the case in which the existing best path A is from an 72 external peer, and another external path B is then selected as the 73 new best path by the route selection algorithm described in [1]. 74 When neither Path A nor Path B is eliminated by the route selection 75 algorithm prior to Step f) - BGP identifier comparison (Sect. 9.1.2.2 76 [1]), we propose that the existing best path (Path A) be kept as the 77 best path (thus avoiding switching the best path to Path B). 79 This algorithm SHOULD NOT be applied when either path is from a BGP 80 Confederation peer. 82 In addition, the algorithm SHOULD NOT be applied when both paths are 83 from peers with identical BGP identifier (i.e., there exist parallel 84 BGP sessions between two routers). As the peering addresses for the 85 parallel sessions are typically allocated by one AS (possibly with 86 route selection considerations), the algorithm (if applied) could 87 impact the existing routing setup. Furthermore, by not applying the 88 algorithm, the allocation of peering addresses would remain as a 89 simple and effective tool in influencing route selection when 90 parallel BGP sessions exist. 92 6. The Benefits 94 The proposed revision to the BGP route selection rules avoids 95 unnecessary best path transitions between external paths under 96 certain conditions. Clearly the revision would help reduce routing 97 and forwarding changes in a network, thus help the overall network 98 stabilities. 100 More importantly, as shown in the following example, the proposed 101 revision can be used to eliminate certain BGP route oscillations in 102 which more than one external paths from one router contribute to the 103 churn. Note however, that there are permanent BGP route oscillation 104 scenarios [3] that the mechanism described in this document does not 105 eliminate. 107 Consider the example in Fig. 1 where 109 o R1, R2, R3 and R4 belong to one AS 110 o R1 is a route reflector with R3 as its client. 111 o R2 is a route reflector with R4 as its client. 112 o The IGP metrics are as listed. 113 o External paths (a), (b) and (c) are as described in Fig. 2. 115 +----+ 40 +----+ 116 | R1 |--------------| R2 | 117 +----+ +----+ 118 | | 119 | | 120 | 10 | 10 121 | | 122 | | 123 +----+ +----+ 124 | R3 | | R4 | 125 +----+ +----+ 126 / \ | 127 / \ | 128 (a) (b) (c) 130 Figure 1 132 Path AS MED Identifier 133 a 1 0 2 134 b 2 20 1 135 c 2 10 5 137 Figure 2 139 Due to the interaction of route reflection [4] and MEDs, the best 140 path on R1 keeps churning between (a) and (c), and the best path on 141 R3 keeps churning between (a) and (b). 143 With the proposed algorithm R3 would not switch the best path from 144 (a) to (b) even after R1 withdraws (c) toward its clients, and that 145 is enough to stop the route oscillation. 147 Although this type of route oscillations can also be eliminated by 148 other route reflection enhancements being developed, the proposed 149 algorithm is very simple and can be deployed immediately. 151 7. Remarks 153 The proposed algorithm is backward-compatible, and can be deployed on 154 a per-node basis. The deployment of the algorithm is highly 155 recommended on a router with multiple external BGP peers (especially 156 the ones connecting to an inter-exchange point). 158 Compared to the existing behavior, the proposed algorithm may 159 introduce some "non-determinism" in the BGP route selection - 160 although one can argue that the BGP Identifier comparison in the 161 existing route selection has already introduced some "randomness" as 162 described in the introduction section. Such "non-determinism" or 163 "randomness" has not been shown to be detrimental in practice, and 164 can be completely eliminated by using the existing mechanisms (such 165 as setting LOCAL_PREF or MED) if so desired. 167 8. Security Considerations 169 This extension does not introduce any security issues. 171 9. Acknowledgments 173 The idea presented was inspired by a route oscillation case observed 174 on the BBN/Genuity backbone in 1998. The algorithm was also 175 implemented at that time. 177 The authors would like to thank Yakov Rekhter and Ravi Chandra for 178 their comments on the initial idea. 180 10. References 182 [1] Y. Rekhter, T. Li, and S. Hares, "A Border Gateway Protocol 4 183 (BGP-4)", draft-ietf-idr-bgp4-26.txt, October 2004. 185 [2] E. Chen and J. Yuan, "AS-wide Unique BGP Identifier for BGP-4", 186 , September 2004. 188 [3] D. McPherson, V, Gill, D. Walton, and A. Retana, "Border Gateway 189 Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, 190 August 2002. 192 [4] T. Bates, R. Chandra, and E. Chen, "BGP Route Reflection - An 193 Alternative to Full Mesh IBGP", RFC 2796, April 2000. 195 [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement 196 Levels", BCP 14, RFC 2119, March 1997. 198 11. Author Information 200 Enke Chen 201 Cisco Systems, Inc. 202 e-mail: enkechen@cisco.com 204 Srihari R. Sangli 205 Cisco Systems, Inc. 206 170 W. Tasman Dr. 207 San Jose, CA 95134 208 e-mail: rsrihari@cisco.com 210 12. Intellectual Property Considerations 212 The IETF takes no position regarding the validity or scope of any 213 Intellectual Property Rights or other rights that might be claimed to 214 pertain to the implementation or use of the technology described in 215 this document or the extent to which any license under such rights 216 might or might not be available; nor does it represent that it has 217 made any independent effort to identify any such rights. Information 218 on the procedures with respect to rights in RFC documents can be 219 found in BCP 78 and BCP 79. 221 Copies of IPR disclosures made to the IETF Secretariat and any 222 assurances of licenses to be made available, or the result of an 223 attempt made to obtain a general license or permission for the use of 224 such proprietary rights by implementers or users of this 225 specification can be obtained from the IETF on-line IPR repository at 226 http://www.ietf.org/ipr. 228 The IETF invites any interested party to bring to its attention any 229 copyrights, patents or patent applications, or other proprietary 230 rights that may cover technology that may be required to implement 231 this standard. Please address the information to the IETF at ietf- 232 ipr@ietf.org. 234 13. Full Copyright Notice 236 Copyright (C) The Internet Society (2004). This document is subject 237 to the rights, licenses and restrictions contained in BCP 78, and 238 except as set forth therein, the authors retain all their rights. 240 This document and the information contained herein are provided on an 241 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 242 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 243 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 244 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 245 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 246 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.