Internet Engineering Task Force (IETF) D. Walton
Request for Comments: 7964 Cumulus Networks
Category: Standards Track A. Retana
ISSN: 2070-1721 E. Chen
Cisco Systems, Inc.
J. Scudder
Juniper Networks
September 2016
Internet Engineering Task Force (IETF) D. Walton
Request for Comments: 7964 Cumulus Networks
Category: Standards Track A. Retana
ISSN: 2070-1721 E. Chen
Cisco Systems, Inc.
J. Scudder
Juniper Networks
September 2016
Solutions for BGP Persistent Route Oscillation
BGP持久路由振荡的解决方案
Abstract
摘要
Routing information reduction by BGP Route Reflection or Confederation can result in persistent internal BGP route oscillations with certain routing setups and network topologies. This document specifies two sets of additional paths that can be used to eliminate these route oscillations in a network.
通过BGP路由反射或联合来减少路由信息会导致某些路由设置和网络拓扑的持续内部BGP路由振荡。本文件规定了两组附加路径，可用于消除网络中的这些路由振荡。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。有关互联网标准的更多信息，请参见RFC 7841第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7964.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问http://www.rfc-editor.org/info/rfc7964.
Copyright Notice
版权公告
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2016 IETF信托基金和确定为文件作者的人员。版权所有。
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(http://trustee.ietf.org/license-info)自本文件出版之日起生效。请仔细阅读这些文件，因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本，并提供简化BSD许可证中所述的无担保。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Advertise All the Available Paths . . . . . . . . . . . . . . 3
4. Advertise the Group Best Paths . . . . . . . . . . . . . . . 3
5. Route Reflection and Confederation . . . . . . . . . . . . . 4
5.1. Route Reflection . . . . . . . . . . . . . . . . . . . . 5
5.2. Confederation . . . . . . . . . . . . . . . . . . . . . . 5
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Why the Group Best Paths Are Adequate . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Advertise All the Available Paths . . . . . . . . . . . . . . 3
4. Advertise the Group Best Paths . . . . . . . . . . . . . . . 3
5. Route Reflection and Confederation . . . . . . . . . . . . . 4
5.1. Route Reflection . . . . . . . . . . . . . . . . . . . . 5
5.2. Confederation . . . . . . . . . . . . . . . . . . . . . . 5
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Why the Group Best Paths Are Adequate . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
As documented in [RFC3345], routing information reduction by BGP Route Reflection [RFC4456] or BGP Confederation [RFC5065] can result in persistent Internal BGP (IBGP) route oscillations with certain routing setups and network topologies. Except for a couple of artificially engineered network topologies, the MULTI_EXIT_DISC (MED) attribute [RFC4271] has played a pivotal role in virtually all known persistent IBGP route oscillations. For the sake of brevity, we use the term "MED-induced route oscillation" hereafter to refer to a persistent IBGP route oscillation in which the MED plays a role.
如[RFC3345]中所述，通过BGP路由反射[RFC4456]或BGP联合[RFC5065]减少路由信息会导致某些路由设置和网络拓扑的持续内部BGP（IBGP）路由振荡。除了几个人工设计的网络拓扑之外，多出口盘（MED）属性[RFC4271]在几乎所有已知的持续IBGP路由振荡中都起着关键作用。为了简洁起见，我们在下文中使用术语“MED诱导的路由振荡”来指MED发挥作用的持续IBGP路由振荡。
In order to eliminate MED-induced route oscillations and to achieve consistent routing in a network, a route reflector or a confederation Autonomous System Border Router (ASBR) needs to advertise more than just the best path for an address prefix. Our goal is to identify the necessary set of paths for an address prefix that needs to be advertised by a route reflector or a confederation ASBR to prevent the condition.
为了消除MED引起的路由振荡并实现网络中的一致路由，路由反射器或联邦自治系统边界路由器（ASBR）需要公布的不仅仅是地址前缀的最佳路径。我们的目标是为地址前缀确定一组必要的路径，该地址前缀需要由路由反射器或联合ASBR公布，以防止出现这种情况。
In this document, we describe two sets of paths for an address prefix that can be advertised by a BGP route reflector or confederation ASBR to eliminate MED-induced route oscillations in a network. The first set involves all the available paths, and would achieve the same routing consistency as the full IBGP mesh. The second set, which is a subset of the first one, involves the neighbor-AS-based Group Best Paths, and would be sufficient to eliminate MED-induced route oscillations (subject to certain commonly adopted topological constraints).
在本文中，我们描述了地址前缀的两组路径，它们可以由BGP路由反射器或联合ASBR公布，以消除网络中由MED引起的路由振荡。第一组涉及所有可用路径，并将实现与完整IBGP网格相同的路由一致性。第二个集合是第一个集合的子集，涉及基于邻居的组最佳路径，足以消除MED引起的路由振荡（受某些常用拓扑约束的约束）。
These paths can be advertised using the mechanism described in ADD-PATH [RFC7911] for advertising multiple paths. No other assumptions in functionality beyond the base BGP specification [RFC4271] are made.
这些路径可以使用ADD-PATH[RFC7911]中描述的机制来通告多个路径。除基本BGP规范[RFC4271]外，未对功能进行任何其他假设。
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 [RFC2119].
本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照[RFC2119]中所述进行解释。
Observe that in a network that maintains a full IBGP mesh, all the BGP speakers have consistent and equivalent routing information. Such a network is thus free of MED-induced route oscillations and other routing inconsistencies such as forwarding loops.
请注意，在维护完整IBGP网络的网络中，所有BGP扬声器都具有一致且等效的路由信息。因此，这样的网络没有MED引起的路由振荡和其他路由不一致，例如转发环路。
Therefore, one approach is to allow a route reflector or a confederation ASBR to advertise all the available paths for an address prefix. Clearly this approach would yield the same amount of routing information and achieve the same routing consistency as the full IBGP mesh in a network. In this document, "Available Paths" refers to the advertisement of all the available paths.
因此，一种方法是允许路由反射器或联合ASBR公布地址前缀的所有可用路径。显然，这种方法将产生与网络中完整IBGP网格相同数量的路由信息，并实现相同的路由一致性。在本文档中，“可用路径”是指所有可用路径的广告。
This approach can be implemented using the mechanism described in ADD-PATH [RFC7911] for advertising multiple paths for certain prefixes.
这种方法可以使用ADD-PATH[RFC7911]中描述的机制来实现，该机制用于为某些前缀发布多个路径。
For the sake of scalability, the advertisement of multiple paths should be limited to those prefixes that are affected by MED-induced route oscillation in a network carrying a large number of alternate paths. A detailed description of how these oscillations can occur can be found in [RFC3345]; the description of how a node would locally detect such conditions is outside the scope of this document.
为了可伸缩性，在承载大量备用路径的网络中，多条路径的通告应限于那些受MED诱导的路由振荡影响的前缀。[RFC3345]中详细描述了这些振荡是如何发生的；节点如何在本地检测此类情况的描述不在本文档的范围内。
The term "neighbor-AS" for a route refers to the neighboring autonomous system (AS) from which the route was received. The calculation of the neighbor-AS is specified in Section 9.1.2.2 of [RFC4271], and Section 5.3 of [RFC5065]. By definition, the MED is comparable only among routes with the same neighbor-AS. Thus, the route selection procedures specified in [RFC4271] would conceptually involve two steps: first, organize the paths for an address prefix into groups according to their respective neighbor-ASes, and
用于路由的术语“邻居AS”指从其接收路由的相邻自治系统（AS）。[RFC4271]第9.1.2.2节和[RFC5065]第5.3节中规定了相邻点的计算。根据定义，MED仅在具有相同邻居的路由之间具有可比性。因此，[RFC4271]中规定的路由选择过程在概念上包括两个步骤：首先，根据地址前缀的各自相邻ASE将路径组织成组，以及
calculate the most preferred one (termed "Group Best Path") for each of the groups; then, calculate the overall best path among all the Group Best Paths.
计算每个组的最佳路径（称为“组最佳路径”）；然后，计算所有组最佳路径中的总体最佳路径。
As a practice that is generally recommended (in [RFC4456] and [RFC5065]) and widely adopted, a route reflection cluster or a confederation sub-AS should be designed such that BGP routes from within the cluster (or confederation sub-AS) are preferred over routes from other clusters (or confederation sub-AS) when the decision is based on the IGP cost to the BGP NEXT_HOP. This is typically done by setting IGP metrics for links within a cluster (or confederation sub-AS) to be much smaller than the IGP metrics for the links between the clusters (or confederation sub-AS). This practice helps achieve consistent routing within a route reflection cluster or a confederation sub-AS.
作为通常推荐（在[RFC4456]和[RFC5065]中）并被广泛采用的实践，路由反射集群或联盟子As的设计应确保来自集群（或联盟子As）内的BGP路由优先于来自其他集群（或联盟子As）的路由当决策基于BGP下一跳的IGP成本时。这通常是通过将集群（或联盟子AS）内链路的IGP度量设置为远小于集群（或联盟子AS）之间链路的IGP度量来实现的。这种做法有助于在路由反射集群或联盟子AS中实现一致的路由。
When the aforementioned practice for devising a route reflection cluster or confederation sub-AS is followed in a network, we claim that the advertisement of all the Group Best Paths by a route reflector or a confederation ASBR is sufficient to eliminate MED-induced route oscillations in the network. This claim is validated in Appendix A.
当在网络中遵循上述设计路由反射集群或联盟子AS的实践时，我们声称路由反射器或联盟ASBR发布的所有组最佳路径足以消除网络中MED诱导的路由振荡。该索赔在附录A中得到验证。
Note that a Group Best Path for an address prefix can be identified by the combination of the address prefix and the neighbor-AS. Thus, this approach can be implemented using the mechanism described in ADD-PATH [RFC7911] for advertising multiple paths, and in this case, the neighbor-AS of a path may be used as the path identifier of the path.
注意，地址前缀的组最佳路径可以通过地址前缀和邻居AS的组合来识别。因此，可以使用ADD-PATH[RFC7911]中描述的用于广告多条路径的机制来实现该方法，并且在这种情况下，路径的邻居AS可以用作该路径的路径标识符。
It should be noted that the approach of advertising the Group Best Paths requires certain topological constraints to be satisfied in order to eliminate MED-induced route oscillation. Specific topological considerations are described in [RFC3345].
应该注意的是，为了消除MED引起的路由振荡，公布组最佳路径的方法需要满足某些拓扑约束。[RFC3345]中描述了具体的拓扑注意事项。
To allow a route reflector or a confederation ASBR to advertise either the Available Paths or Group Best Paths using the mechanism described in ADD-PATH [RFC7911], the following revisions are proposed for BGP Route Reflection and BGP Confederation.
为了允许路由反射器或联盟ASBR使用ADD-PATH[RFC7911]中描述的机制公布可用路径或分组最佳路径，建议对BGP路由反射和BGP联盟进行以下修订。
For a particular <Address Family Identifier (AFI), Subsequent Address Family (SAFI)>, a route reflector MUST include the <AFI, SAFI> with the "Send/Receive" field set to 2 (send multiple paths) or 3 (send/receive multiple paths) in the ADD-PATH Capability [RFC7911] advertised to an IBGP peer. When the ADD-PATH Capability is also received from the IBGP peer with the "Send/Receive" field set to 1 (receive multiple paths) or 3 (send/receive multiple paths) for the same <AFI, SAFI>, then the following procedures apply:
对于特定的<Address Family Identifier（AFI），后续地址系列（SAFI）>，路由反射器必须包括<AFI，SAFI>，并将添加路径功能[RFC7911]中的“发送/接收”字段设置为2（发送多条路径）或3（发送/接收多条路径）。当从IBGP对等方接收添加路径功能时，对于相同的<AFI，SAFI>“发送/接收”字段设置为1（接收多条路径）或3（发送/接收多条路径），则以下过程适用：
If the peer is a route reflection client, the route reflector MUST advertise to the peer the Group Best Paths (or the Available Paths) received from its non-client IBGP peers. The route reflector MAY also advertise to the peer the Group Best Paths (or the Available Paths) received from its clients.
如果对等方是路由反射客户端，路由反射方必须向对等方公布从其非客户端IBGP对等方接收的组最佳路径（或可用路径）。路由反射器还可以向对等方通告从其客户端接收的组最佳路径（或可用路径）。
If the peer is a non-client, the route reflector MUST advertise to the peer the Group Best Paths (or the Available Paths) received from its clients.
如果对等方是非客户端，路由反射器必须向对等方公布从其客户端接收的组最佳路径（或可用路径）。
For a particular <AFI, SAFI>, a confederation ASBR MUST include the <AFI, SAFI> with the "Send/Receive" field set to 2 (send multiple paths) or 3 (send/receive multiple paths) in the ADD-PATH Capability [RFC7911] advertised to an IBGP peer, and to a confederation external peer. When the ADD-PATH Capability is also received from the IBGP peer or the confederation-external peer with the "Send/Receive" field set to 1 (receive multiple paths) or 3 (send/receive multiple paths) for the same <AFI, SAFI>, then the following procedures apply:
对于特定的<AFI，SAFI>，联盟ASBR必须包括<AFI，SAFI>，在向IBGP对等方和联盟外部对等方通告的添加路径功能[RFC7911]中，“发送/接收”字段设置为2（发送多条路径）或3（发送/接收多条路径）。当从IBGP对等方或联邦外部对等方接收添加路径功能时，对于相同的<AFI，SAFI>，将“发送/接收”字段设置为1（接收多条路径）或3（发送/接收多条路径），则以下过程适用：
If the peer is internal, the confederation ASBR MUST advertise to the peer the Group Best Paths (or the Available Paths) received from its confederation-external peers.
如果对等方是内部的，联盟ASBR必须向对等方公布从其联盟外部对等方接收的组最佳路径（或可用路径）。
If the peer is confederation-external, the confederation ASBR MUST advertise to the peer the Group Best Paths (or the Available Paths) received from its IBGP peers.
如果对等方是联盟外部的，联盟ASBR必须向对等方公布从其IBGP对等方接收的组最佳路径（或可用路径）。
Some route oscillations, once detected, can be eliminated by simple configuration workarounds. As carrying additional paths impacts the memory usage and routing convergence in a network, it is recommended that the impact be evaluated and the approach of using a configuration workaround be considered in deciding whether to deploy the proposed mechanism in a network. In addition, the advertisement of multiple paths should be limited to those prefixes that are affected by MED-induced route oscillation.
一旦检测到某些路由振荡，就可以通过简单的配置解决方案来消除。由于承载额外路径会影响网络中的内存使用和路由收敛，建议在决定是否在网络中部署建议的机制时，评估影响并考虑使用配置解决方案的方法。此外，多路径的播发应限于受MED诱导的路由振荡影响的前缀。
While the route reflectors or confederation ASBRs in a network need to advertise the Group Best Paths or Available Paths, the vast majority of the BGP speakers in the network only need to receive the Group Best Paths or Available Paths, which would involve only minor software changes.
虽然网络中的路由反射器或联盟ASBR需要公布组最佳路径或可用路径，但网络中的绝大多数BGP扬声器只需要接收组最佳路径或可用路径，这将只涉及较小的软件更改。
It should be emphasized that, in order to eliminate MED-induced route oscillations in a network using the approach of advertising the Group Best Paths, the recommended practice for devising a route reflection cluster or confederation sub-AS with respect to the IGP metrics ([RFC4456] [RFC5065]) should be followed.
应该强调的是，为了使用公布组最佳路径的方法消除网络中MED引起的路由振荡，应遵循关于IGP度量（[RFC4456][RFC5065]）设计路由反射集群或联盟子AS的推荐实践。
It is expected that the approach of advertising the Group Best Paths would be adequate to achieve consistent routing for the vast majority of the networks. For a network that has a large number of alternate paths, the approach should be a good choice as the number of paths advertised by a reflector or a confederation ASBR is bounded by the number of the neighbor-ASes for a particular address prefix. The additional states for an address prefix would also be per neighbor-AS rather than per path. The number of neighbor-ASes for a particular address prefix is typically small because of the limited number of upstream providers for a customer and the nature of advertising only customer routes at the inter-exchange points.
预计宣传组最佳路径的方法足以实现绝大多数网络的一致路由。对于具有大量备用路径的网络，该方法应该是一个不错的选择，因为反射器或联合ASBR公布的路径数量受特定地址前缀的邻居ASE数量的限制。地址前缀的附加状态也将是每个邻居，而不是每个路径。特定地址前缀的邻居ase的数量通常很小，这是因为用于客户的上游提供者的数量有限，并且在交换点间仅广告客户路由的性质。
The approach of advertising the Group Best Paths, however, may still be inadequate for certain networks to avoid other routing inconsistencies such as forwarding loops. The required topological constraints could also be operationally challenging. In these cases the approach of advertising the Available Paths may be used, but should be limited to those prefixes that are affected by MED-induced route oscillation in a network carrying a large number of alternate paths.
然而，对于某些网络来说，公布组最佳路径的方法可能仍然不足以避免其他路由不一致，例如转发循环。所需的拓扑约束在操作上也可能具有挑战性。在这些情况下，可以使用公布可用路径的方法，但应限于在承载大量备用路径的网络中受MED诱导的路由振荡影响的前缀。
This extension to BGP does not change the underlying security issues inherent in the existing BGP [RFC4271].
对BGP的扩展不会改变现有BGP中固有的基本安全问题[RFC4271]。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，DOI 10.17487/RFC2119，1997年3月<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, <http://www.rfc-editor.org/info/rfc4271>.
[RFC4271]Rekhter，Y.，Ed.，Li，T.，Ed.，和S.Hares，Ed.，“边境网关协议4（BGP-4）”，RFC 4271，DOI 10.17487/RFC4271，2006年1月<http://www.rfc-editor.org/info/rfc4271>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, <http://www.rfc-editor.org/info/rfc4456>.
[RFC4456]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，DOI 10.17487/RFC4456，2006年4月<http://www.rfc-editor.org/info/rfc4456>.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 5065, DOI 10.17487/RFC5065, August 2007, <http://www.rfc-editor.org/info/rfc5065>.
[RFC5065]Traina，P.，McPherson，D.，和J.Scudder，“BGP自治系统联合会”，RFC 5065，DOI 10.17487/RFC5065，2007年8月<http://www.rfc-editor.org/info/rfc5065>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder, "Advertisement of Multiple Paths in BGP", RFC 7911, DOI 10.17487/RFC7911, July 2016, <http://www.rfc-editor.org/info/rfc7911>.
[RFC7911]Walton，D.，Retana，A.，Chen，E.，和J.Scudder，“BGP中多路径的广告”，RFC 7911，DOI 10.17487/RFC7911，2016年7月<http://www.rfc-editor.org/info/rfc7911>.
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, "Border Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, DOI 10.17487/RFC3345, August 2002, <http://www.rfc-editor.org/info/rfc3345>.
[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 3345，DOI 10.17487/RFC3345，2002年8月<http://www.rfc-editor.org/info/rfc3345>.
It is assumed that the following common practice is followed. A route reflection cluster or a confederation sub-AS should be designed such that the IGP metrics for links within a cluster (or confederation sub-AS) are much smaller than the IGP metrics for the links between the clusters (or confederation sub-AS). This practice helps achieve consistent routing within a route reflection cluster or a confederation sub-AS.
假设遵循以下常见做法。路由反射集群或联盟子AS的设计应确保集群（或联盟子AS）内链路的IGP度量远小于集群（或联盟子AS）之间链路的IGP度量。这种做法有助于在路由反射集群或联盟子AS中实现一致的路由。
Observe that in a network that maintains full IBGP mesh only, the paths that survive the (Local_Pref, AS-PATH Length, Origin, and MED) comparisons [RFC4271] would contribute to route selection in the network.
请注意，在仅维护完整IBGP网格的网络中，在（本地_Pref、AS-PATH Length、Origin和MED）比较[RFC4271]中幸存的路径将有助于网络中的路由选择。
Consider a route reflection cluster that sources one or more paths that would survive the (Local_Pref, AS-PATH Length, Origin, and MED) comparisons among all the paths in the network. One of these surviving paths would be selected as the Group Best Path by the route reflector in the cluster. Due to the constraint on the IGP metrics as described previously, this path would remain as the Group Best Path and would be advertised to all other clusters even after a path is received from another cluster.
考虑一个路由反射群集，它可以在网络中的所有路径之间提供一个或多个路径来生存（LoalAlpRePF，AS-PATH长度，OFF，MED）比较。这些幸存路径中的一条将被集群中的路由反射器选择为组最佳路径。由于前面所述的IGP度量的约束，该路径将保持为组最佳路径，并且即使在从另一个集群接收到路径之后，也将被通告到所有其他集群。
On the other hand, when no path in a route reflection cluster would survive the (Local_Pref, AS-PATH Length, Origin, and MED) comparisons among all the paths in the network, the Group Best Path (when it exists) for a route reflector would be from another cluster. Clearly, the advertisement of the Group Best Path by the route reflector to the clients only depends on the paths received from other clusters.
另一方面，当路由反射簇中的任何路径都无法通过网络中所有路径之间的（本地_Pref、AS-path Length、Origin和MED）比较时，路由反射簇的组最佳路径（如果存在）将来自另一个簇。显然，路由反射器向客户端公布组最佳路径仅取决于从其他集群接收的路径。
Therefore, there is no MED-induced route oscillation in the network as the advertisement of a Group Best Path to a peer does not depend on the paths received from that peer.
因此，在网络中不存在MED诱导的路由振荡，因为到对等方的组最佳路径的通告不依赖于从该对等方接收的路径。
The claim for the confederation can be validated similarly.
联邦的主张同样可以得到证实。
Acknowledgements
致谢
We would like to thank David Cook and Naiming Shen for their contributions to the design and development of the solutions.
我们要感谢David Cook和Naiming Shen为解决方案的设计和开发做出的贡献。
Many thanks to Tony Przygienda, Sue Hares, Jon Mitchell, and Paul Kyzivat for their helpful suggestions.
非常感谢Tony Przygienda、Sue Hares、Jon Mitchell和Paul Kyzivat提供的有益建议。
Authors' Addresses
作者地址
Daniel Walton Cumulus Networks 140C S. Whisman Rd. Mountain View, CA 94041 United States of America
丹尼尔·沃尔顿积云网络美国加利福尼亚州山景城惠斯曼南路140C号，邮编94041
Email: dwalton@cumulusnetworks.com
Email: dwalton@cumulusnetworks.com
Alvaro Retana Cisco Systems, Inc. 7025 Kit Creek Rd. Research Triangle Park, NC 27709 United States of America
Alvaro Retana Cisco Systems，Inc.美国北卡罗来纳州三角研究公园Kit Creek路7025号，邮编：27709
Email: aretana@cisco.com
Email: aretana@cisco.com
Enke Chen Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134 United States of America
Enke Chen Cisco Systems，Inc.170 W.Tasman博士，加利福尼亚州圣何塞，美国95134
Email: enkechen@cisco.com
Email: enkechen@cisco.com
John Scudder Juniper Networks 1194 N. Mathilda Ave Sunnyvale, CA 94089 United States of America
John Scudder Juniper Networks 1194 N.Mathilda Ave Sunnyvale，加利福尼亚州，美利坚合众国，邮编94089
Email: jgs@juniper.net
Email: jgs@juniper.net