Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 7775 Cisco Systems
Updates: 5308 S. Litkowski
Category: Standards Track Orange Business Service
ISSN: 2070-1721 S. Previdi
Cisco Systems
February 2016
Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 7775 Cisco Systems
Updates: 5308 S. Litkowski
Category: Standards Track Orange Business Service
ISSN: 2070-1721 S. Previdi
Cisco Systems
February 2016
IS-IS Route Preference for Extended IP and IPv6 Reachability
扩展IP和IPv6可达性的IS-IS路由首选项
Abstract
摘要
In existing specifications, the route preferences for IPv4/IPv6 Extended Reachability TLVs are not explicitly stated. There are also inconsistencies in the definition of how the up/down bit applies to route preference when the prefix advertisement appears in Level 2 Link State Protocol Data Units (LSPs). This document addresses these issues.
在现有规范中，未明确说明IPv4/IPv6扩展可达性TLV的路由首选项。当前缀广告出现在2级链路状态协议数据单元（LSP）中时，上行/下行位如何应用于路由首选项的定义也存在不一致。本文件涉及这些问题。
This document updates RFC 5308.
本文档更新了RFC 5308。
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 5741.
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 5741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/rfc7775.
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/rfc7775.
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。
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
本文件可能包含2008年11月10日之前发布或公开的IETF文件或IETF贡献中的材料。控制某些材料版权的人员可能未授予IETF信托允许在IETF标准流程之外修改此类材料的权利。在未从控制此类材料版权的人员处获得充分许可的情况下，不得在IETF标准流程之外修改本文件，也不得在IETF标准流程之外创建其衍生作品，除了将其格式化以RFC形式发布或将其翻译成英语以外的其他语言。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use of the Up/Down Bit in Level 2 LSPs . . . . . . . . . . . 3
3. Types of Routes in IS-IS Supported by Extended Reachability
TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Types of Routes Supported by TLVs 135 and 235 . . . . . . 4
3.2. Types of Routes Supported by TLVs 236 and 237 . . . . . . 6
3.3. Order of Preference for All Types of Routes Supported by
TLVs 135 and 235 . . . . . . . . . . . . . . . . . . . . 7
3.4. Order of Preference for All Types of Routes Supported by
TLVs 236 and 237 . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Example Interoperability Issue . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use of the Up/Down Bit in Level 2 LSPs . . . . . . . . . . . 3
3. Types of Routes in IS-IS Supported by Extended Reachability
TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Types of Routes Supported by TLVs 135 and 235 . . . . . . 4
3.2. Types of Routes Supported by TLVs 236 and 237 . . . . . . 6
3.3. Order of Preference for All Types of Routes Supported by
TLVs 135 and 235 . . . . . . . . . . . . . . . . . . . . 7
3.4. Order of Preference for All Types of Routes Supported by
TLVs 236 and 237 . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Example Interoperability Issue . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
[RFC5302] defines the route preference rules as they apply to TLVs 128 and 130. [RFC5305] introduced the IP Extended Reachability TLV 135 but did not explicitly adapt the route preference rules defined in [RFC5302] for the new TLV. [RFC5308] defines the IPv6 Reachability TLV 236 and does include an explicit statement regarding route preference -- but the statement introduces use of the up/down bit in advertisements that appear in Level 2 LSPs, which is inconsistent with statements made in [RFC5302] and [RFC5305]. This document defines explicit route preference rules for TLV 135, revises the route preference rules for TLV 236, and clarifies the usage of the up/down bit when it appears in TLVs in Level 2 LSPs. This document is a clarification (NOT a correction) of [RFC5302] and [RFC5305]; it is a correction of the route preference rules defined in [RFC5308] to be consistent with the rules for IPv4. It also makes explicit that the same rules apply to the Multi-Topology (MT) equivalent TLVs 235 and 237.
[RFC5302]定义适用于TLV 128和130的路线偏好规则。[RFC5305]引入了IP扩展可达性TLV 135，但没有明确调整[RFC5302]中为新TLV定义的路由首选规则。[RFC5308]定义了IPv6可达性TLV 236，并包含了关于路由偏好的明确声明——但该声明引入了在2级LSP中出现的广告中使用上/下位，这与[RFC5302]和[RFC5305]中的声明不一致。本文件定义了TLV 135的显式路由首选规则，修改了TLV 236的路由首选规则，并澄清了当上行/下行位出现在2级LSP的TLV中时的用法。本文件是对[RFC5302]和[RFC5305]的澄清（非更正）；这是对[RFC5308]中定义的路由首选项规则的修正，以与IPv4的规则一致。它还明确指出，相同的规则适用于多拓扑（MT）等效TLV235和237。
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。
The up/down bit was introduced in support of leaking prefixes downwards in the IS-IS level hierarchy. Routes that are leaked downwards have the bit set to 1. Such prefixes MUST NOT be leaked upwards in the hierarchy. So long as we confine ourselves to a single IS-IS instance and the current number of supported levels (two), it is impossible to have a prefix advertised in a Level 2 LSP and have the up/down bit set to 1. However, because [RFC5302] anticipated a future extension to IS-IS that might support additional levels, it allowed for the possibility that the up/down bit might be set in a Level 2 LSP and supported easy migration in the event such an extension was introduced. Section 3.3 of [RFC5302] states:
为了支持IS-IS级别层次结构中前缀向下泄漏，引入了up/down位。向下泄漏的路由的位设置为1。此类前缀不得在层次结构中向上泄漏。只要我们仅限于一个IS-IS实例和当前支持的级别数（两个），就不可能在级别2 LSP中公布前缀并将上/下位设置为1。然而，由于[RFC5302]预计IS-IS的未来扩展可能支持额外的级别，因此允许在2级LSP中设置上/下位，并在引入此类扩展时支持轻松迁移。[RFC5302]第3.3节规定：
...it is RECOMMENDED that implementations ignore the up/down bit in L2 LSPs, and accept the prefixes in L2 LSPs regardless of whether the up/down bit is set.
…建议实现忽略L2 LSP中的向上/向下位，并接受L2 LSP中的前缀，无论是否设置了向上/向下位。
[RFC5305] addressed an additional case wherein an implementation included support for multiple virtual routers running IS-IS in different areas. In such a case, it is possible to redistribute prefixes between two IS-IS instances in the same manner that prefixes are redistributed from other protocols into IS-IS. This introduced
[RFC5305]解决了另一种情况，其中实现包括支持在不同区域运行IS-IS的多个虚拟路由器。在这种情况下，可以在两个is-is实例之间重新分配前缀，其方式与将前缀从其他协议重新分配到is-is中的方式相同。这介绍
the possibility that a prefix could be redistributed from Level 1 to Level 1 (as well as between Level 2 and Level 2), and in the event the redistributed route was leaked from Level 1 to Level 2, two different routers in different areas would be advertising the same prefix into the Level 2 sub-domain. To prevent this, Section 4.1 of [RFC5305] specifies:
前缀可能从级别1重新分配到级别1（以及在级别2和级别2之间），并且如果重新分配的路由从级别1泄漏到级别2，则不同区域的两个不同路由器将向级别2子域播发相同的前缀。为防止出现这种情况，[RFC5305]第4.1节规定：
If a prefix is advertised from one area to another at the same level, then the up/down bit SHALL be set to 1.
如果前缀在同一级别从一个区域播发到另一个区域，则向上/向下位应设置为1。
However, the statement in [RFC5302] that the up/down bit is ignored in Level 2 LSPs is not altered by [RFC5305].
但是，[RFC5302]中关于在2级LSP中忽略向上/向下位的语句没有被[RFC5305]改变。
The conclusion then is that there is no "L2 inter-area route"; indeed, no such route type is defined by [RFC5302]. However, [RFC5308] ignored this fact and introduced such a route type in Section 5 when it specified a preference for "Level 2 down prefix". This is an error that this document corrects. As changing the use of the up/down bit in TLVs 236 and 237 may introduce interoperability issues, implementors may wish to support transition mechanisms from the behavior described in [RFC5308] to the behavior described in this document.
由此得出的结论是，不存在“L2区域间路由”；实际上，[RFC5302]没有定义这样的路由类型。然而，[RFC5308]忽略了这一事实，并在第5节中引入了这样一种路由类型，因为它指定了“2级下行前缀”的首选项。这是本文档更正的错误。由于更改TLV 236和237中向上/向下位的使用可能会引入互操作性问题，实施者可能希望支持从[RFC5308]中描述的行为到本文档中描述的行为的转换机制。
[RFC5302] is the authoritative reference for the types of routes supported by TLVs 128 and 130. However, a number of attributes supported by those TLVs are NOT supported by TLVs 135, 235, 236, and 237. Distinction between internal/external metrics is not supported. In the case of IPv4 TLVs (135 and 235), the distinction between internal and external route types is not supported. However, the Prefix Attribute Flags sub-TLV defined in [PFXATTR] reintroduces the distinction between internal and external route types. The definitions below include references to the relevant attribute bits from [PFXATTR].
[RFC5302]是TLV 128和130支持的路由类型的权威参考。但是，这些TLV支持的许多属性不受TLV 135、235、236和237的支持。不支持区分内部/外部指标。对于IPv4 TLV（135和235），不支持区分内部和外部路由类型。但是，[PFXATTR]中定义的前缀属性标志sub TLV重新引入内部和外部路由类型之间的区别。以下定义包括对[PFXATTR]中相关属性位的引用。
This section defines the types of route supported for IPv4 when using TLV 135 [RFC5305] and/or TLV 235 [RFC5120]. The text follows as closely as possible the original text from [RFC5302].
本节定义了使用TLV 135[RFC5305]和/或TLV 235[RFC5120]时IPv4支持的路由类型。文本尽可能紧跟[RFC5302]的原文。
L1 intra-area routes: These are advertised in L1 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 0. These IP prefixes are directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, both the X-Flag and the R-Flag are set to 0.
L1区域内路由：在L1 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为0。这些IP前缀直接连接到广告路由器。如果包含前缀属性标志子TLV，则X标志和R标志都设置为0。
L1 external routes: These are advertised in L1 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 0. These IP prefixes are learned from other protocols and are usually not directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, the X-Flag is set to 1, and the R-Flag is set to 0.
L1外部路线：在L1 LSP、TLV 135或TLV 235中公布。向上/向下位设置为0。这些IP前缀来自其他协议，通常不直接连接到广告路由器。如果包含前缀属性标志子TLV，则X标志设置为1，R标志设置为0。
L2 intra-area routes: These are advertised in L2 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 0. These IP prefixes are directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, both the X-Flag and the R-Flag are set to 0.
L2区域内路由：在L2 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为0。这些IP前缀直接连接到广告路由器。如果包含前缀属性标志子TLV，则X标志和R标志都设置为0。
L1->L2 inter-area routes: These are advertised in L2 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 0. These IP prefixes are learned via L1 routing and were derived during the L1 Shortest Path First (SPF) computation from prefixes advertised in L1 LSPs in TLV 135 or TLV 235. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 1.
L1->L2区域间路由：在L2 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为0。这些IP前缀通过L1路由学习，并在L1最短路径优先（SPF）计算期间从TLV 135或TLV 235中的L1 LSP中公布的前缀导出。如果包含前缀属性标志子TLV，则R标志设置为1。
L2->L2 inter-area routes: These are advertised in L2 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 1 but is ignored and treated as if it were set to 0. These IP prefixes are learned from another IS-IS instance usually operating in another area. If the Prefix Attribute Flags sub-TLV is included, the X-Flag is set to 1, and the R-Flag is set to 0.
L2->L2区域间路由：在L2 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为1，但会被忽略，并将其视为设置为0。这些IP前缀是从通常在另一个区域运行的另一个IS-IS实例中学习的。如果包含前缀属性标志子TLV，则X标志设置为1，R标志设置为0。
L2 external routes: These are advertised in L2 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 0. These IP prefixes are learned from other protocols and are usually not directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, the X-Flag is set to 1, and the R-Flag is set to 0.
L2外部路由：在L2 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为0。这些IP前缀来自其他协议，通常不直接连接到广告路由器。如果包含前缀属性标志子TLV，则X标志设置为1，R标志设置为0。
L2->L1 inter-area routes: These are advertised in L1 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 1. These IP prefixes are learned via L2 routing and were derived during the L2 SPF computation from prefixes advertised in TLV 135 or TLV 235. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 1.
L2->L1区域间路由：在L1 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为1。这些IP前缀通过L2路由学习，并在L2 SPF计算期间从TLV 135或TLV 235中公布的前缀导出。如果包含前缀属性标志子TLV，则R标志设置为1。
L1->L1 inter-area routes: These are advertised in L1 LSPs, in TLV 135 or TLV 235. The up/down bit is set to 1. These IP prefixes are learned from another IS-IS instance usually operating in another area. If the Prefix Attribute Flags sub-TLV is included, the X-Flag is set to 1, and the R-Flag is set to 0.
L1->L1区域间路由：在L1 LSP、TLV 135或TLV 235中公布这些路由。向上/向下位设置为1。这些IP前缀是从通常在另一个区域运行的另一个IS-IS实例中学习的。如果包含前缀属性标志子TLV，则X标志设置为1，R标志设置为0。
This section defines the types of route supported for IPv6 when using TLV 236 [RFC5308] and/or TLV 237 [RFC5120].
本节定义了使用TLV 236[RFC5308]和/或TLV 237[RFC5120]时IPv6支持的路由类型。
L1 intra-area routes: These are advertised in L1 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 0. The external bit is set to 0. These IPv6 prefixes are directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 0.
L1区域内路由：在L1 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为0。外部位设置为0。这些IPv6前缀直接连接到广告路由器。如果包含前缀属性标志子TLV，则R标志设置为0。
L1 external routes: These are advertised in L1 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 0. The external bit is set to 1. These IPv6 prefixes are learned from other protocols and are usually not directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 0.
L1外部路线：在L1 LSP、TLV 236或TLV 237中公布。向上/向下位设置为0。外部位设置为1。这些IPv6前缀来自其他协议，通常不直接连接到广告路由器。如果包含前缀属性标志子TLV，则R标志设置为0。
L2 intra-area routes: These are advertised in L2 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 0. The external bit is set to 0. These IPv6 prefixes are directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 0.
L2区域内路由：在L2 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为0。外部位设置为0。这些IPv6前缀直接连接到广告路由器。如果包含前缀属性标志子TLV，则R标志设置为0。
L1->L2 inter-area routes: These are advertised in L2 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 0. The external bit is set to 0. These IPv6 prefixes are learned via L1 routing and were derived during the L1 Shortest Path First (SPF) computation from prefixes advertised in L1 LSPs in TLV 236 or TLV 237. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 1.
L1->L2区域间路由：在L2 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为0。外部位设置为0。这些IPv6前缀通过L1路由学习，并在L1最短路径优先（SPF）计算期间从TLV 236或TLV 237中的L1 LSP中公布的前缀导出。如果包含前缀属性标志子TLV，则R标志设置为1。
L2 external routes: These are advertised in L2 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 0. The external bit is set to 1. These IPv6 prefixes are learned from other protocols and are usually not directly connected to the advertising router. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 0.
二级外部路由：在二级LSP、TLV 236或TLV 237中公布。向上/向下位设置为0。外部位设置为1。这些IPv6前缀来自其他协议，通常不直接连接到广告路由器。如果包含前缀属性标志子TLV，则R标志设置为0。
L1->L2 external routes: These are advertised in L2 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 0. The external bit is set to 1. These IPv6 prefixes are learned via L1 routing and were derived during the L1 Shortest Path First (SPF) computation from L1 external routes advertised in L1 LSPs in TLV 236 or TLV 237. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 1.
L1->L2外部路由：在L2 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为0。外部位设置为1。这些IPv6前缀通过L1路由学习，并在L1最短路径优先（SPF）计算期间从TLV 236或TLV 237中的L1 LSP中公布的L1外部路由导出。如果包含前缀属性标志子TLV，则R标志设置为1。
L2->L2 inter-area routes: These are advertised in L2 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 1 but is ignored and treated as if it were set to 0. The external bit is set to 1. These IP prefixes are learned from another IS-IS instance usually operating in another area. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 0.
L2->L2区域间路由：在L2 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为1，但会被忽略，并将其视为设置为0。外部位设置为1。这些IP前缀是从通常在另一个区域运行的另一个IS-IS实例中学习的。如果包含前缀属性标志子TLV，则R标志设置为0。
L2->L1 inter-area routes: These are advertised in L1 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 1. The external bit is set to 0. These IPv6 prefixes are learned via L2 routing and were derived during the L2 SPF computation from prefixes advertised in TLV 236 or TLV 237. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 1.
L2->L1区域间路由：这些路由在L1 LSP、TLV 236或TLV 237中公布。向上/向下位设置为1。外部位设置为0。这些IPv6前缀通过L2路由学习，并在L2 SPF计算期间从TLV 236或TLV 237中公布的前缀派生而来。如果包含前缀属性标志子TLV，则R标志设置为1。
L2->L1 external routes: These are advertised in L1 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 1. The external bit is set to 1. These IPv6 prefixes are learned via L2 routing and were derived during the L2 SPF computation from prefixes advertised in TLV 236 or TLV 237. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 1.
L2->L1外部路由：在L1 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为1。外部位设置为1。这些IPv6前缀通过L2路由学习，并在L2 SPF计算期间从TLV 236或TLV 237中公布的前缀派生而来。如果包含前缀属性标志子TLV，则R标志设置为1。
L1->L1 inter-area routes: These are advertised in L1 LSPs, in TLV 236 or TLV 237. The up/down bit is set to 1. The external bit is set to 1. These IP prefixes are learned from another IS-IS instance usually operating in another area. If the Prefix Attribute Flags sub-TLV is included, the R-Flag is set to 0.
L1->L1区域间路由：在L1 LSP、TLV 236或TLV 237中公布这些路由。向上/向下位设置为1。外部位设置为1。这些IP前缀是从通常在另一个区域运行的另一个IS-IS实例中学习的。如果包含前缀属性标志子TLV，则R标志设置为0。
3.3. Order of Preference for All Types of Routes Supported by TLVs 135 and 235
3.3. TLV 135和235支持的所有类型路线的优先顺序
This document defines the following route preferences for IPv4 routes advertised in TLVs 135 or 235. Note that all types of routes listed for a given preference are treated equally.
本文档为TLV 135或235中公布的IPv4路由定义了以下路由首选项。请注意，为给定的首选项列出的所有类型的路线都被同等对待。
1. L1 intra-area routes; L1 external routes
1. L1区域内路线；L1外部路线
2. L2 intra-area routes; L2 external routes; L1->L2 inter-area routes; L2-L2 inter-area routes
2. L2区域内路线；L2外部路线；L1->L2区域间路线；L2-L2区域间路由
3. L2->L1 inter-area routes; L1->L1 inter-area routes
3. L2->L1区域间路线；L1->L1区域间路线
3.4. Order of Preference for All Types of Routes Supported by TLVs 236 and 237
3.4. TLV 236和237支持的所有类型路线的优先顺序
This document defines the following route preferences for IPv6 routes advertised in TLVs 236 or 237. Note that all types of routes listed for a given preference are treated equally.
本文档为TLVs 236或237中公布的IPv6路由定义了以下路由首选项。请注意，为给定的首选项列出的所有类型的路线都被同等对待。
1. L1 intra-area routes; L1 external routes
1. L1区域内路线；L1外部路线
2. L2 intra-area routes; L2 external routes; L1->L2 inter-area routes; L1-L2 external routes; L2-L2 inter-area routes
2. L2区域内路线；L2外部路线；L1->L2区域间路线；L1-L2外部线路；L2-L2区域间路由
3. L2->L1 inter-area routes; L2->L1 external routes; L1->L1 inter-area routes
3. L2->L1区域间路线；L2->L1外部路线；L1->L1区域间路线
This document raises no new security considerations. Security considerations for the IS-IS protocol are covered in [ISO10589], [RFC5304], and [RFC5310].
本文件没有提出新的安全注意事项。IS-IS协议的安全注意事项见[ISO10589]、[RFC5304]和[RFC5310]。
[ISO10589] International Organization for Standardization, "Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)", ISO Standard 10589, 2002.
[ISO10589]国际标准化组织，“与提供无连接模式网络服务协议一起使用的中间系统到中间系统域内路由信息交换协议（ISO 8473）”，ISO标准10589，2002年。
[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>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, DOI 10.17487/RFC5120, February 2008, <http://www.rfc-editor.org/info/rfc5120>.
[RFC5120]Przygienda，T.，Shen，N.，和N.Sheth，“M-ISIS：中间系统到中间系统（IS-ISs）的多拓扑（MT）路由”，RFC 5120，DOI 10.17487/RFC5120，2008年2月<http://www.rfc-editor.org/info/rfc5120>.
[RFC5302] Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix Distribution with Two-Level IS-IS", RFC 5302, DOI 10.17487/RFC5302, October 2008, <http://www.rfc-editor.org/info/rfc5302>.
[RFC5302]Li，T.，Smit，H.，和T.Przygienda，“具有两级IS-IS的域范围前缀分布”，RFC 5302，DOI 10.17487/RFC5302，2008年10月<http://www.rfc-editor.org/info/rfc5302>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2008, <http://www.rfc-editor.org/info/rfc5304>.
[RFC5304]Li，T.和R.Atkinson，“IS-IS加密认证”，RFC 5304，DOI 10.17487/RFC5304，2008年10月<http://www.rfc-editor.org/info/rfc5304>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, <http://www.rfc-editor.org/info/rfc5305>.
[RFC5305]Li，T.和H.Smit，“交通工程的IS-IS扩展”，RFC 5305，DOI 10.17487/RFC5305，2008年10月<http://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, DOI 10.17487/RFC5308, October 2008, <http://www.rfc-editor.org/info/rfc5308>.
[RFC5308]Hopps，C.，“使用IS-IS路由IPv6”，RFC 5308，DOI 10.17487/RFC5308，2008年10月<http://www.rfc-editor.org/info/rfc5308>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., and M. Fanto, "IS-IS Generic Cryptographic Authentication", RFC 5310, DOI 10.17487/RFC5310, February 2009, <http://www.rfc-editor.org/info/rfc5310>.
[RFC5310]Bhatia，M.，Manral，V.，Li，T.，Atkinson，R.，White，R.，和M.Fanto，“IS-IS通用密码认证”，RFC 5310，DOI 10.17487/RFC5310，2009年2月<http://www.rfc-editor.org/info/rfc5310>.
[PFXATTR] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and U. Chunduri, "IS-IS Prefix Attributes for Extended IP and IPv6 Reachability", Work in Progress, draft-ietf-isis-prefix-attributes-04, January 2016.
[PFXATTR]Ginsberg，L.，Ed.，Decaene，B.，Previdi，S.，Xu，X.，和U.Chunduri，“扩展IP和IPv6可达性的IS-IS前缀属性”，正在进行中的工作，草案-ietf-isis-Prefix-ATTRICTS-042016年1月。
This example documents a real-world interoperability issue that occurs because implementations from different vendors have interpreted the use of the up/down bit in Level 2 LSPs inconsistently.
本例记录了一个现实世界中的互操作性问题，该问题是由于来自不同供应商的实现对2级LSP中向上/向下位的使用进行了不一致的解释。
L2 L2 L2 L2|L2 L2
10/8 - R0 ----- R1 ----- R2 ----- R3 ----- R4 ---- 10/8
|
Figure 1
L2 L2 L2 L2|L2 L2
10/8 - R0 ----- R1 ----- R2 ----- R3 ----- R4 ---- 10/8
|
Figure 1
In Figure 1, both R0 and R4 are advertising the prefix 10/8. Two IS-IS Level 2 instances are running on R3 to separate the network into two areas. R3 is performing route leaking and advertises prefixes from R4 to the other Level 2 process. The network is using extended metrics (TLV 135 defined in [RFC5305]). R0 advertises 10/8 with metric 2000, and R3 advertises 10/8 with metric 100. All links have a metric of 1. When advertising 10/8 in its Level 2 LSP, R3 sets the down bit as specified in [RFC5305].
在图1中，R0和R4都在宣传前缀10/8。两个IS-IS 2级实例正在R3上运行，以将网络分为两个区域。R3正在执行路由泄漏，并将前缀从R4播发到另一个2级进程。网络使用扩展度量（TLV 135在[RFC5305]中定义）。R0广告10/8采用公制2000，R3广告10/8采用公制100。所有链接的度量值均为1。当在其2级LSP中公布10/8时，R3按照[RFC5305]中的规定设置下行位。
R1, R2, and R3 are from three different vendors (R1->Vendor1, R2->Vendor2, R3->Vendor3). During interoperability testing, routing loops are observed in this scenario.
R1、R2和R3来自三个不同的供应商（R1->Vendor1、R2->Vendor2、R3->Vendor3）。在互操作性测试期间，在此场景中观察到路由循环。
o R2 has two possible paths to reach 10/8: Level 2 route with metric 2002 and up/down bit set to 0 (from R0) and Level 2 route with metric 101 and up/down bit set to 1 (from R3). R2 selects R1 as the next hop to 10/8 because it prefers the route that does NOT have the up/down bit set.
o R2有两种可能的路径达到10/8：2级路由，度量值为2002，上/下位设置为0（从R0开始），2级路由，度量值为101，上/下位设置为1（从R3开始）。R2选择R1作为10/8的下一个跃点，因为它更喜欢未设置向上/向下位的路由。
o R3 has two possible paths to reach 10/8: Level 2 route with metric 2003 and up/down bit set to 0 (from R0) and Level 2 route with metric 101 and up/down bit set to 0 (from R4). R3 selects R4 as the next hop due to lowest metric.
o R3有两种可能的路径达到10/8：第二级路由，度量2003和上/下位设置为0（从R0开始），第二级路由，度量101和上/下位设置为0（从R4开始）。由于最低度量，R3选择R4作为下一跳。
o R1 has two possible paths to reach 10/8: Level 2 route with metric 2001 and up/down bit set to 0 (from R0) and Level 2 route with metric 102 and up/down bit set to 1 (from R3). R1 selects R2 as the next hop due to lowest metric.
o R1有两种可能的路径可以达到10/8：第二级路由，度量值为2001，上/下位设置为0（从R0开始），第二级路由，度量值为102，上/下位设置为1（从R3开始）。由于度量值最低，R1选择R2作为下一个跃点。
When R1 or R2 try to send traffic to 10/8, packets loop due to inconsistent routing decisions between R1 and R2.
当R1或R2尝试向10/8发送流量时，由于R1和R2之间的路由决定不一致，数据包会循环。
Acknowledgements
致谢
The authors wish to thank Ahmed Bashandy for his insightful review.
作者希望感谢Ahmed Bashandy的深刻评论。
Authors' Addresses
作者地址
Les Ginsberg Cisco Systems 510 McCarthy Blvd. Milpitas, CA 95035 United States
莱斯金斯伯格思科系统公司，麦卡锡大道510号。美国加利福尼亚州米尔皮塔斯95035
Email: ginsberg@cisco.com
Email: ginsberg@cisco.com
Stephane Litkowski Orange Business Service
Stephane Litkowski橙色商务服务
Email: stephane.litkowski@orange.com
Email: stephane.litkowski@orange.com
Stefano Previdi Cisco Systems Via Del Serafico 200 Rome 0144 Italy
Stefano Previdi Cisco Systems Via Del Serafico 200罗马0144意大利
Email: sprevidi@cisco.com
Email: sprevidi@cisco.com