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Sangli 4 Expiration Date: September 2007 Cisco Systems 6 Avoid BGP Best Path Transitions from One External to Another 8 draft-ietf-idr-avoid-transition-05.txt 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 Abstract 35 In this document we propose an extension to the BGP route selection 36 rules that would avoid unnecessary best path transitions between 37 external paths under certain conditions. The proposed extension would 38 help the overall network stability, and more importantly, would 39 eliminate certain BGP route oscillations in which more than one 40 external path from one BGP speaker contributes to the churn. 42 1. Introduction 44 The last two steps of the BGP route selection (Sect. 9.1.2.2, [BGP]) 45 involve comparing the BGP identifiers and the peering addresses. The 46 BGP identifier (treated either as an IP address, or just an integer 47 [BGP-ID]) for a BGP speaker is allocated by the AS to which the 48 speaker belongs. As a result, for a local BGP speaker, the BGP 49 identifier of a route received from an external peer is just an 50 random number. When routes under consideration are from external 51 peers, the result from the last two steps of the route selection is 52 therefore "random" as far as the local BGP speaker is concerned. 54 It is based on this observation that we propose an extension to the 55 BGP route selection rules that would avoid unnecessary best path 56 transitions between external paths under certain conditions. The 57 proposed extension would help the overall network stability, and more 58 importantly, would eliminate certain BGP route oscillations in which 59 more than one external path from one BGP speaker contributes to the 60 churn. 62 2. Specification of Requirements 64 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 65 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 66 document are to be interpreted as described in RFC 2119 [RFC2119]. 68 3. The Algorithm 70 Consider the case in which the existing best path A is from an 71 external peer, and another external path B is then selected as the 72 new best path by the route selection algorithm described in [BGP]. 73 When comparing all the paths in route selection, if neither Path A 74 nor Path B is eliminated by the route selection algorithm prior to 75 Step f) - BGP identifier comparison (Sect. 9.1.2.2 [BGP]), we propose 76 that the existing best path (Path A) be kept as the best path (thus 77 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 BGP speakers). As the peering addresses for 85 the parallel sessions are typically allocated by one AS (possibly 86 with route selection considerations), the algorithm (if applied) 87 could impact the existing routing setup. Furthermore, by not applying 88 the 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 4. The Benefits 94 The proposed extension to the BGP route selection rules avoids 95 unnecessary best path transitions between external paths under 96 certain conditions. Clearly the extension would help reduce routing 97 and forwarding changes in a network, thus help the overall network 98 stability. 100 More importantly, as shown in the following example, the proposed 101 extension can be used to eliminate certain BGP route oscillations in 102 which more than one external path from one BGP speaker contributes to 103 the churn. Note however, that there are permanent BGP route 104 oscillation scenarios [RFC3345] that the mechanism described in this 105 document does not 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 the route reflection [BGP-RR] and the 140 MULTI_EXIT_DISC (MED) attribute, the best path on R1 keeps churning 141 between (a) and (c), and the best path on R3 keeps churning between 142 (a) and (b). 144 With the proposed algorithm R3 would not switch the best path from 145 (a) to (b) even after R1 withdraws (c) toward its clients, and that 146 is enough to stop the route oscillation. 148 Although this type of route oscillations can also be eliminated by 149 other route reflection enhancements being developed, the proposed 150 algorithm is extremely simple and can be implemented and deployed 151 immediately without introducing any backward compatibility issues. 153 5. Remarks 155 The proposed algorithm is backward-compatible, and can be deployed on 156 a per-BGP-speaker basis. The deployment of the algorithm is highly 157 recommended on a BGP speaker with multiple external BGP peers 158 (especially the ones connecting to an inter-exchange point). 160 Compared to the existing behavior, the proposed algorithm may 161 introduce some "non-determinism" in the BGP route selection - 162 although one can argue that the BGP Identifier comparison in the 163 existing route selection has already introduced some "randomness" as 164 described in the introduction section. Such "non-determinism" has 165 not been shown to be detrimental in practice, and can be completely 166 eliminated by using the existing mechanisms (such as setting 167 LOCAL_PREF or MED) if so desired. 169 6. IANA Considerations 171 This extension does not require any action by IANA. 173 7. Security Considerations 175 This extension does not introduce any security issues. 177 8. Acknowledgments 179 The idea presented was inspired by a route oscillation case observed 180 on the BBN/Genuity backbone in 1998. The algorithm was also 181 implemented and deployed at that time. 183 The authors would like to thank Yakov Rekhter and Ravi Chandra for 184 their comments on the initial idea. 186 9. Normative References 188 [BGP] Rekhter, Y., Li, T., and Hares, S., "A Border Gateway Protocol 189 4 (BGP-4)", RFC 4271, January 2006. 191 [BGP-RR] T. Bates, R. Chandra, and E. Chen, "BGP Route Reflection - 192 An Alternative to Full Mesh IBGP", RFC 4456, April 2006. 194 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 195 Requirement Levels", BCP 14, RFC 2119, March 1997. 197 10. Non-normative References 199 [BGP-ID] E. Chen and J. Yuan, "AS-wide Unique BGP Identifier for 200 BGP-4", Work in Progress, draft-ietf-idr-bgp-identifier-08.txt, 201 November 2006. 203 [RFC3345] D. McPherson, V, Gill, D. Walton, and A. Retana, "Border 204 Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 205 3345, August 2002. 207 11. Author Information 209 Enke Chen 210 Cisco Systems, Inc. 211 170 W. Tasman Dr. 212 San Jose, CA 95134 214 Email: enkechen@cisco.com 216 Srihari R. Sangli 217 Cisco Systems, Inc. 218 170 W. Tasman Dr. 219 San Jose, CA 95134 221 Email: rsrihari@cisco.com 223 12. Intellectual Property Considerations 225 The IETF takes no position regarding the validity or scope of any 226 Intellectual Property Rights or other rights that might be claimed to 227 pertain to the implementation or use of the technology described in 228 this document or the extent to which any license under such rights 229 might or might not be available; nor does it represent that it has 230 made any independent effort to identify any such rights. Information 231 on the procedures with respect to rights in RFC documents can be 232 found in BCP 78 and BCP 79. 234 Copies of IPR disclosures made to the IETF Secretariat and any 235 assurances of licenses to be made available, or the result of an 236 attempt made to obtain a general license or permission for the use of 237 such proprietary rights by implementers or users of this 238 specification can be obtained from the IETF on-line IPR repository at 239 http://www.ietf.org/ipr. 241 The IETF invites any interested party to bring to its attention any 242 copyrights, patents or patent applications, or other proprietary 243 rights that may cover technology that may be required to implement 244 this standard. Please address the information to the IETF at ietf- 245 ipr@ietf.org. 247 13. Full Copyright Notice 249 Copyright (C) The IETF Trust (2007). 251 This document is subject to the rights, licenses and restrictions 252 contained in BCP 78, and except as set forth therein, the authors 253 retain all their rights. 255 This document and the information contained herein are provided on an 256 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 257 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 258 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 259 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 260 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 261 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.