Network Working Group E. Chen
Request for Comments: 5004 S. Sangli
Category: Standards Track Cisco Systems
September 2007
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
避免从一个外部到另一个外部的BGP最佳路径转换
Status of This Memo
关于下段备忘
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.
本文件规定了互联网社区的互联网标准跟踪协议，并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态，请参考当前版本的“互联网官方协议标准”（STD 1）。本备忘录的分发不受限制。
Abstract
摘要
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.
在本文中，我们建议对BGP路由选择规则进行扩展，以避免在某些条件下外部路径之间不必要的最佳路径转换。提议的扩展将有助于整体网络的稳定性，更重要的是，将消除某些BGP路由振荡，其中来自一个BGP扬声器的多条外部路径会造成搅动。
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.
BGP路由选择的最后两个步骤（第9.1.2.2节[BGP]）涉及比较BGP标识符和对等地址。BGP说话人的BGP标识符（被视为IP地址或整数[BGP-ID]）由说话人所属的自治系统（as）分配。因此，对于本地BGP说话者，从外部对等方接收的路由的BGP标识符只是一个随机数。当考虑中的路由来自外部对等方时，路由选择的最后两个步骤的结果因此对本地BGP说话人而言是“随机”的。
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.
正是基于这一观察，我们提出了对BGP路由选择规则的扩展，以避免在某些条件下外部路径之间不必要的最佳路径转换。提议的扩展将有助于整体网络的稳定性，更重要的是，将消除某些BGP路由振荡，其中来自一个BGP扬声器的多条外部路径会造成搅动。
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].
本文件中的关键词“必须”、“不得”、“要求”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照RFC 2119[RFC2119]中所述进行解释。
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).
考虑现有的最佳路径A是来自外部对等体的情况，然后通过[BGP]中描述的路由选择算法将另一个外部路径B选择为新的最佳路径。当比较路由选择中的所有路径时，如果在步骤f之前路由选择算法未消除路径A和路径B）--BGP标识符比较（第9.1.2.2节[BGP]）--我们建议将现有最佳路径（路径A）保留为最佳路径（从而避免将最佳路径切换为路径B）。
This algorithm SHOULD NOT be applied when either path is from a BGP Confederation peer.
当任一路径来自BGP联盟对等方时，不应应用此算法。
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.
此外，当两条路径都来自具有相同BGP标识符的对等方（即，两个BGP扬声器之间存在并行BGP会话）时，不应应用该算法。由于并行会话的对等地址通常由一个As分配（可能考虑到路由选择），因此该算法（如果应用）可能会影响现有的路由设置。此外，当存在并行BGP会话时，通过不应用该算法，对等地址的分配仍然是影响路由选择的一个简单而有效的工具。
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.
建议的BGP路由选择规则扩展避免了在某些条件下外部路径之间不必要的最佳路径转换。显然，扩展将有助于减少网络中的路由和转发更改，从而有助于整体网络的稳定性。
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.
更重要的是，如以下示例所示，所提议的扩展可用于消除某些BGP路由振荡，其中来自一个BGP扬声器的多个外部路径导致了抖动。但是，请注意，本文档中描述的机制并未消除永久性BGP路由振荡场景[RFC3345]。
Consider the example in Figure 1 where
考虑图1中的示例
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.
o R1、R2、R3和R4属于一个AS。R1是一个路由反射器，R3是它的客户端。R2是以R4为客户端的路由反射器。o IGP指标如下所列。o外部路径（a）、（b）和（c）如图2所示。
+----+ 40 +----+
| R1 |--------------| R2 |
+----+ +----+
| |
| |
| 10 | 10
| |
| |
+----+ +----+
| R3 | | R4 |
+----+ +----+
/ \ |
/ \ |
(a) (b) (c)
+----+ 40 +----+
| R1 |--------------| R2 |
+----+ +----+
| |
| |
| 10 | 10
| |
| |
+----+ +----+
| R3 | | R4 |
+----+ +----+
/ \ |
/ \ |
(a) (b) (c)
Figure 1
图1
Path AS MED Identifier a 1 0 2 b 2 20 1 c 2 10 5
路径作为MED标识符a 1 0 2 b 2 20 1 c 2 10 5
Figure 2
图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).
由于路由反射[BGP-RR]和MULTI_EXIT_DISC（MED）属性的交互作用，R1上的最佳路径在（a）和（c）之间不断波动，R3上的最佳路径在（a）和（b）之间不断波动。
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.
在所提出的算法中，即使在R1向其客户端退出（c）之后，R3也不会将最佳路径从（a）切换到（b），这足以停止路由振荡。
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.
虽然这种类型的路由振荡也可以通过正在开发的其他路由反射增强来消除，但所提出的算法非常简单，可以立即实现和部署，而不会引入任何向后兼容性问题。
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).
所提出的算法是向后兼容的，并且可以部署在每个BGP说话人的基础上。强烈建议在具有多个外部BGP对等点（尤其是连接到内部交换点的对等点）的BGP扬声器上部署该算法。
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.
与现有行为相比，所提出的算法可能会在BGP路由选择中引入一些“非确定性”——尽管有人可能认为，现有路由选择中的BGP标识符比较已经引入了一些“随机性”，如引言部分所述。这种“非决定论”在实践中没有被证明是有害的，如果需要的话，可以通过使用现有机制（如设置局部预处理或MED）完全消除。
This extension does not introduce any security issues.
此扩展不会引入任何安全问题。
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.
提出的想法受到1998年BBN/Genuity网络中观察到的路由振荡案例的启发。该算法也在当时实施和部署。
The authors would like to thank Yakov Rekhter and Ravi Chandra for their comments on the initial idea.
作者要感谢雅科夫·雷克特和拉维·钱德拉对最初想法的评论。
[BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
[BGP]Rekhter，Y.，Ed.，Li，T.，Ed.，和S.Hares，Ed.，“边境网关协议4（BGP-4）”，RFC 42712006年1月。
[BGP-RR] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, April 2006.
[BGP-RR]Bates，T.，Chen，E.，和R.Chandra，“BGP路由反射：全网格内部BGP（IBGP）的替代方案”，RFC 4456，2006年4月。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[BGP-ID] Chen, E. and J. Yuan, "AS-wide Unique BGP Identifier for BGP-4", Work in Progress, November 2006.
[BGP-ID]Chen，E.和J.Yuan，“作为BGP-4的宽唯一BGP标识符”，正在进行的工作，2006年11月。
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, "Border Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, August 2002.
[RFC3345]McPherson，D.，Gill，V.，Walton，D.，和A.Retana，“边界网关协议（BGP）持续路由振荡条件”，RFC 33452002年8月。
Author Information
作者信息
Enke Chen Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134
Enke Chen思科系统公司170 W.Tasman Dr.San Jose，CA 95134
EMail: enkechen@cisco.com
EMail: enkechen@cisco.com
Srihari R. Sangli Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134
Srihari R.Sangli Cisco Systems，Inc.170 W.Tasman Dr.San Jose，CA 95134
EMail: rsrihari@cisco.com
EMail: rsrihari@cisco.com
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