Internet Engineering Task Force (IETF) N. Shen
Request for Comments: 8500 Cisco Systems
Category: Standards Track S. Amante
ISSN: 2070-1721 Apple Inc.
M. Abrahamsson
T-Systems Nordic
February 2019
Internet Engineering Task Force (IETF) N. Shen
Request for Comments: 8500 Cisco Systems
Category: Standards Track S. Amante
ISSN: 2070-1721 Apple Inc.
M. Abrahamsson
T-Systems Nordic
February 2019
IS-IS Routing with Reverse Metric
具有反向度量的IS-IS路由
Abstract
摘要
This document describes a mechanism to allow IS-IS routing to quickly and accurately shift traffic away from either a point-to-point or multi-access LAN interface during network maintenance or other operational events. This is accomplished by signaling adjacent IS-IS neighbors with a higher reverse metric, i.e., the metric towards the signaling IS-IS router.
本文档描述了一种机制，允许IS-IS路由在网络维护或其他操作事件期间快速准确地将流量从点到点或多址LAN接口转移出去。这是通过使用更高的反向度量（即，朝向信令is-is路由器的度量）来信令相邻is-is邻居来实现的。
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 https://www.rfc-editor.org/info/rfc8500.
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https://www.rfc-editor.org/info/rfc8500.
Copyright Notice
版权公告
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
版权（c）2019 IETF信托基金和被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 (https://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(https://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
1.1. Node and Link Isolation . . . . . . . . . . . . . . . . . 2
1.2. Distributed Forwarding Planes . . . . . . . . . . . . . . 3
1.3. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3
1.4. LDP IGP Synchronization . . . . . . . . . . . . . . . . . 3
1.5. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . 3
1.6. Specification of Requirements . . . . . . . . . . . . . . 4
2. IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . 4
3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 6
3.1. Processing Changes to Default Metric . . . . . . . . . . 6
3.2. Multi-Topology IS-IS Support on Point-to-Point Links . . 7
3.3. Multi-access LAN Procedures . . . . . . . . . . . . . . . 7
3.4. LDP/IGP Synchronization on LANs . . . . . . . . . . . . . 8
3.5. Operational Guidelines . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 13
Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Node and Link Isolation . . . . . . . . . . . . . . . . . 2
1.2. Distributed Forwarding Planes . . . . . . . . . . . . . . 3
1.3. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3
1.4. LDP IGP Synchronization . . . . . . . . . . . . . . . . . 3
1.5. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . 3
1.6. Specification of Requirements . . . . . . . . . . . . . . 4
2. IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . 4
3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 6
3.1. Processing Changes to Default Metric . . . . . . . . . . 6
3.2. Multi-Topology IS-IS Support on Point-to-Point Links . . 7
3.3. Multi-access LAN Procedures . . . . . . . . . . . . . . . 7
3.4. LDP/IGP Synchronization on LANs . . . . . . . . . . . . . 8
3.5. Operational Guidelines . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 13
Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
The IS-IS [ISO10589] routing protocol has been widely used in Internet Service Provider IP/MPLS networks. Operational experience with the protocol combined with ever increasing requirements for lossless operations have demonstrated some operational issues. This document describes the issues and a mechanism for mitigating them.
IS-IS[ISO10589]路由协议已广泛应用于互联网服务提供商IP/MPLS网络。该协议的运行经验以及不断增加的无损运行要求表明了一些运行问题。本文档描述了这些问题以及缓解这些问题的机制。
This document defines the IS-IS "Reverse Metric" mechanism that allows an IS-IS node to send a Reverse Metric TLV through the IS-IS Hello (IIH) PDU to the neighbor or pseudonode to adjust the routing metric on the inbound direction.
本文档定义了IS-IS“反向度量”机制，该机制允许IS-IS节点通过IS-IS Hello（IIH）PDU向邻居或伪节点发送反向度量TLV，以调整入站方向上的路由度量。
The IS-IS routing mechanism has the overload bit, which can be used by operators to perform disruptive maintenance on the router. But in many operational maintenance cases, it is not necessary to divert all the traffic away from this node. It is necessary to avoid only a single link during the maintenance. More detailed descriptions of the challenges can be found in Appendices A and B of this document.
IS-IS路由机制具有过载位，操作员可以使用该位对路由器执行中断性维护。但在许多运行维护情况下，不需要将所有流量从该节点转移出去。在维护过程中，必须避免仅使用单个链路。有关这些挑战的更详细说明，请参见本文件附录A和附录B。
In a distributed forwarding platform, different forwarding line cards may have interfaces and IS-IS connections to neighbor routers. If one of the line card's software resets, it may take some time for the forwarding entries to be fully populated on the line card, in particular if the router is a PE (Provider Edge) router in an ISP's MPLS VPN. An IS-IS adjacency may be established with a neighbor router long before the entire BGP VPN prefixes are downloaded to the forwarding table. It is important to signal to the adjacent IS-IS routers to raise metric values and not to use the corresponding IS-IS adjacency inbound to this router if possible. Temporarily signaling the 'Reverse Metric' over this link to discourage the traffic via the corresponding line card will help to reduce the traffic loss in the network. In the meantime, the remote PE routers will select a different set of PE routers for the BGP best path calculation or use a different link towards the same PE router on which a line card is resetting.
在分布式转发平台中，不同的转发线路卡可能具有与相邻路由器的接口和IS-IS连接。如果线路卡的某个软件重置，则可能需要一些时间才能在线路卡上完全填充转发条目，特别是如果路由器是ISP的MPLS VPN中的PE（提供商边缘）路由器。在整个BGP VPN前缀下载到转发表之前，可以与邻居路由器建立IS-IS邻接。重要的是向相邻is-is路由器发送信号以提高度量值，并且如果可能，不要使用该路由器入站的相应is-is邻接。通过此链路临时发送“反向度量”信号，以阻止通过相应线路卡的流量，这将有助于减少网络中的流量损失。同时，远程PE路由器将为BGP最佳路径计算选择一组不同的PE路由器，或使用不同的链路指向正在重置线路卡的同一PE路由器。
In the IS-IS Spine-Leaf extension [IS-IS-SL-EXT], the leaf nodes will perform equal-cost or unequal-cost load sharing towards all the spine nodes. In certain operational cases, for instance, when one of the backbone links on a spine node is congested, a spine node can push a higher metric towards the connected leaf nodes to reduce the transit traffic through the corresponding spine node or link.
在IS-IS脊椎叶扩展[IS-IS-SL-EXT]中，叶节点将对所有脊椎节点执行相同成本或不同成本负载共享。在某些操作情况下，例如，当脊椎节点上的一个主干链路拥塞时，脊椎节点可以向连接的叶节点推送更高的度量，以减少通过相应脊椎节点或链路的传输流量。
In [RFC5443], a mechanism is described to achieve LDP IGP synchronization by using the maximum link metric value on the interface. But in the case of a new IS-IS node joining the broadcast network (LAN), it is not optimal to change all the nodes on the LAN to the maximum link metric value, as described in [RFC6138]. In this case, the Reverse Metric can be used to discourage both outbound and inbound traffic without affecting the traffic of other IS-IS nodes on the LAN.
在[RFC5443]中，描述了通过使用接口上的最大链路度量值来实现LDP IGP同步的机制。但在新IS-IS节点加入广播网络（LAN）的情况下，将LAN上的所有节点更改为最大链路度量值并非最佳选择，如[RFC6138]所述。在这种情况下，反向度量可用于阻止出站和入站流量，而不会影响LAN上其他IS-IS节点的流量。
This document uses the routing protocol itself as the transport mechanism to allow one IS-IS router to advertise a "reverse metric" in an IS-IS Hello (IIH) PDU to an adjacent node on a point-to-point or multi-access LAN link. This would allow the provisioning to be performed only on a single node, setting a "reverse metric" on a link and having traffic bidirectionally shift away from that link gracefully to alternate viable paths.
本文档使用路由协议本身作为传输机制，允许一个IS-IS路由器将IS-IS Hello（IIH）PDU中的“反向度量”通告到点到点或多址LAN链路上的相邻节点。这将允许仅在单个节点上执行配置，在链路上设置“反向度量”，并使通信量从该链路优雅地双向转移到备用可行路径。
This Reverse Metric mechanism is used for both point-to-point and multi-access LAN links. Unlike the point-to-point links, the IS-IS protocol currently does not have a way to influence the traffic towards a particular node on LAN links. This mechanism provides IS-IS routing with the capability of altering traffic in both directions on either a point-to-point link or a multi-access link of an IS-IS node.
这种反向度量机制用于点到点和多访问LAN链路。与点到点链路不同，IS-IS协议目前无法影响LAN链路上特定节点的通信量。该机制为IS-IS路由提供了在IS-IS节点的点到点链路或多址链路上改变双向流量的能力。
The metric value in the Reverse Metric TLV and the Traffic Engineering metric in the sub-TLV being advertised are offsets or relative metrics to be added to the existing local link and Traffic Engineering metric values of the receiver; the accumulated metric value is bounded as described in Section 2.
反向度量TLV中的度量值和正在播发的子TLV中的业务工程度量是要添加到接收机的现有本地链路和业务工程度量值的偏移量或相对度量；累积度量值有界，如第2节所述。
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“建议”、“不建议”、“可”和“可选”在所有大写字母出现时（如图所示）应按照BCP 14[RFC2119][RFC8174]所述进行解释。
The Reverse Metric TLV is a new TLV to be used inside an IS-IS Hello PDU. This TLV is used to support the IS-IS Reverse Metric mechanism that allows a "reverse metric" to be sent to the IS-IS neighbor.
反向度量TLV是在is-is Hello PDU内使用的新TLV。此TLV用于支持is-is反向度量机制，该机制允许向is-is邻居发送“反向度量”。
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | Metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric (Continued) | sub-TLV Len |Optional sub-TLV
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | Metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric (Continued) | sub-TLV Len |Optional sub-TLV
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Reverse Metric TLV
图1：反向公制TLV
The Value part of the Reverse Metric TLV is composed of a 3 octet field containing an IS-IS Metric value, a 1 octet field of Flags, and a 1 octet Reverse Metric sub-TLV length field representing the length of a variable number of sub-TLVs. If the "sub-TLV Len" is non-zero, then the Value field MUST also contain one or more sub-TLVs.
反向度量TLV的值部分由包含is-is度量值的3个八位字段、标志的1个八位字段和表示可变数量子TLV长度的1个八位反向度量子TLV长度字段组成。如果“子TLV Len”为非零，则值字段还必须包含一个或多个子TLV。
The Reverse Metric TLV MAY be present in any IS-IS Hello PDU. A sender MUST only transmit a single Reverse Metric TLV in an IS-IS Hello PDU. If a received IS-IS Hello PDU contains more than one
任何IS-IS Hello PDU中都可能存在反向度量TLV。发送方只能在IS-IS Hello PDU中传输单个反向度量TLV。如果收到的IS-IS Hello PDU包含多个
Reverse Metric TLV, an implementation MUST ignore all the Reverse Metric TLVs.
反向度量TLV，实现必须忽略所有反向度量TLV。
TYPE: 16 LENGTH: variable (5 - 255 octets) VALUE:
类型：16长度：变量（5-255个八位字节）值：
Flags (1 octet) Metric (3 octets) sub-TLV length (1 octet) sub-TLV data (0 - 250 octets)
标志（1个八位字节）度量（3个八位字节）子TLV长度（1个八位字节）子TLV数据（0-250个八位字节）
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |U|W|
+-+-+-+-+-+-+-+-+
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |U|W|
+-+-+-+-+-+-+-+-+
Figure 2: Flags
图2：旗帜
The Metric field contains a 24-bit unsigned integer. This value is a metric offset that a neighbor SHOULD add to the existing configured Default Metric for the IS-IS link [ISO10589]. Refer to "Elements of Procedure" in Section 3 of this document for details on how an IS-IS router should process the Metric field in a Reverse Metric TLV.
公制字段包含一个24位无符号整数。该值是一个度量偏移量，邻居应将其添加到is-is链路的现有配置默认度量中[ISO10589]。有关IS-IS路由器应如何处理反向度量TLV中的度量字段的详细信息，请参阅本文件第3节中的“程序要素”。
The Metric field, in the Reverse Metric TLV, is a "reverse offset metric" that will either be in the range of 0 - 63 when a "narrow" IS-IS metric is used (IS Neighbors TLV / Pseudonode LSP) [RFC1195] or in the range of 0 - (2^24 - 2) when a "wide" Traffic Engineering metric value is used (Extended IS Reachability TLV) [RFC5305] [RFC5817]. As described below, when the U bit is set, the accumulated value of the wide metric is in the range of 0 - (2^24 - 1), with the (2^24 - 1) metric value as non-reachable in IS-IS routing. The IS-IS metric value of (2^24 - 2) serves as the link of last resort.
反向度量TLV中的度量字段是“反向偏移度量”，当使用“窄”is-is度量（与TLV/伪节点LSP相邻）[RFC1195]时，其将在0-63范围内，或者当使用“宽”流量工程度量值（扩展is可达性TLV）[RFC5305][RFC5817]时，其将在0-（2^24-2）范围内。如下所述，当设置U位时，宽度量的累积值在0-（2^24-1）的范围内，而（2^24-1）度量值在is-is路由中是不可到达的。（2^24-2）的IS-IS度量值作为最后手段的链接。
There are currently only two Flag bits defined.
目前只定义了两个标志位。
W bit (0x01): The "Whole LAN" bit is only used in the context of multi-access LANs. When a Reverse Metric TLV is transmitted from a node to the Designated Intermediate System (DIS), if the "Whole LAN" bit is set (1), then a DIS SHOULD add the received Metric value in the Reverse Metric TLV to each node's existing Default Metric in the Pseudonode LSP. If the "Whole LAN" bit is not set (0), then a DIS SHOULD add the received Metric value in the Reverse Metric TLV to the existing "default metric" in the Pseudonode LSP for the single node from whom the Reverse Metric TLV was received. Please refer to "Multi-access LAN Procedures", in Section 3.3, for additional
W位（0x01）：“整个LAN”位仅在多址LAN的上下文中使用。当反向度量TLV从节点传输到指定的中间系统（DIS）时，如果设置了“整个LAN”位（1），则DIS应将反向度量TLV中接收到的度量值添加到伪节点LSP中每个节点的现有默认度量。如果未设置“整个LAN”位（0），则DIS应将反向度量TLV中接收的度量值添加到伪节点LSP中接收反向度量TLV的单个节点的现有“默认度量”。有关更多信息，请参阅第3.3节中的“多址LAN程序”
details. The W bit MUST be clear when a Reverse Metric TLV is transmitted in an IIH PDU on a point-to-point link and MUST be ignored when received on a point-to-point link.
细节。当在点到点链路上的IIH PDU中传输反向度量TLV时，W位必须清除，并且在点到点链路上接收时必须忽略。
U bit (0x02): The "Unreachable" bit specifies that the metric calculated by the addition of the reverse metric to the "default metric" is limited to the maximum value of (2^24-1). This "U" bit applies to both the default metric in the Extended IS Reachability TLV and the Traffic Engineering Default Metric sub-TLV of the link. This is only relevant to the IS-IS "wide" metric mode.
U位（0x02）：“不可达”位指定通过将反向度量添加到“默认度量”计算的度量被限制为最大值（2^24-1）。此“U”位适用于扩展IS可达性TLV中的默认度量和链路的流量工程默认度量子TLV。这仅与is-is“宽”度量模式相关。
The Reserved bits of Flags field MUST be set to zero and MUST be ignored when received.
标志保留位字段必须设置为零，并且在接收时必须忽略。
The Reverse Metric TLV MAY include sub-TLVs when an IS-IS router wishes to signal additional information to its neighbor. In this document, the Reverse Metric Traffic Engineering Metric sub-TLV, with Type 18, is defined. This Traffic Engineering Metric contains a 24-bit unsigned integer. This sub-TLV is optional; if it appears more than once, then the entire Reverse Metric TLV MUST be ignored. Upon receiving this Traffic Engineering METRIC sub-TLV in a Reverse Metric TLV, a node SHOULD add the received Traffic Engineering Metric offset value to its existing configured Traffic Engineering Default Metric within its Extended IS Reachability TLV. The use of other sub-TLVs is outside the scope of this document. The "sub-TLV Len" value MUST be set to zero when an IS-IS router does not have Traffic Engineering sub-TLVs that it wishes to send to its IS-IS neighbor.
当IS-IS路由器希望向其邻居发送附加信息时，反向度量TLV可包括子TLV。在本文件中，定义了18型反向度量交通工程度量子TLV。此流量工程度量包含一个24位无符号整数。该子TLV是可选的；如果出现多次，则必须忽略整个反向度量TLV。在反向度量TLV中接收到此流量工程度量sub TLV时，节点应将接收到的流量工程度量偏移值添加到其扩展IS可达性TLV中现有配置的流量工程默认度量。其他子TLV的使用不在本文件范围内。当IS-IS路由器没有希望发送给其IS-IS邻居的流量工程子TLV时，“子TLV Len”值必须设置为零。
It is important to use the same IS-IS metric type on both ends of the link and in the entire IS-IS area or level. On the receiving side of the 'reverse-metric' TLV, the accumulated value of the configured metric and the reverse-metric needs to be limited to 63 in "narrow" metric mode and to (2^24 - 2) in "wide" metric mode. This applies to both the Default Metric of Extended IS Reachability TLV and the Traffic Engineering Default Metric sub-TLV in LSP or Pseudonode LSP for the "wide" metric mode case. If the "U" bit is present in the flags, the accumulated metric value is to be limited to (2^24 - 1) for both the normal link metric and Traffic Engineering metric in IS-IS "wide" metric mode.
在链路两端以及整个is-is区域或标高中使用相同的is-is度量类型非常重要。在“反向度量”TLV的接收侧，配置度量和反向度量的累积值需要在“窄”度量模式下限制为63，在“宽”度量模式下限制为（2^24-2）。这既适用于扩展IS可达性TLV的默认度量，也适用于“宽”度量模式情况下LSP或伪节点LSP中的流量工程默认度量sub TLV。如果标志中存在“U”位，则对于is-is“宽”度量模式中的正常链路度量和流量工程度量，累积度量值将限制为（2^24-1）。
If an IS-IS router is configured to originate a Traffic Engineering Default Metric sub-TLV for a link but receives a Reverse Metric TLV from its neighbor that does not contain a Traffic Engineering Default
如果IS-IS路由器配置为发起链路的流量工程默认度量子TLV，但从其邻居接收不包含流量工程默认值的反向度量TLV
Metric sub-TLV, then the IS-IS router MUST NOT change the value of its Traffic Engineering Default Metric sub-TLV for that link.
Metric sub TLV，则IS-IS路由器不得更改该链路的流量工程默认Metric sub TLV的值。
The Reverse Metric TLV is applicable to Multi-topology IS-IS (M-ISIS) [RFC5120]. On point-to-point links, if an IS-IS router is configured for M-ISIS, it MUST send only a single Reverse Metric TLV in IIH PDUs toward its neighbor(s) on the designated link. When an M-ISIS router receives a Reverse Metric TLV, it MUST add the received Metric value to its Default Metric of the link in all Extended IS Reachability TLVs for all topologies. If an M-ISIS router receives a Reverse Metric TLV with a Traffic Engineering Default Metric sub-TLV, then the M-ISIS router MUST add the received Traffic Engineering Default Metric value to each of its Default Metric sub-TLVs in all of its MT Intermediate Systems TLVs. If an M-ISIS router is configured to advertise Traffic Engineering Default Metric sub-TLVs for one or more topologies but does not receive a Traffic Engineering Default Metric sub-TLV in a Reverse Metric TLV, then the M-ISIS router MUST NOT change the value in each of the Traffic Engineering Default Metric sub-TLVs for all topologies.
反向度量TLV适用于多拓扑is-is（M-ISIS）[RFC5120]。在点到点链路上，如果IS-IS路由器配置为M-ISIS，则它必须仅向其指定链路上的邻居发送IIH PDU中的单个反向度量TLV。当M-ISIS路由器接收到反向度量TLV时，它必须将接收到的度量值添加到所有拓扑的所有扩展IS可达性TLV中链路的默认度量中。如果M-ISIS路由器接收到具有流量工程默认度量子TLV的反向度量TLV，则M-ISIS路由器必须将接收到的流量工程默认度量值添加到其所有MT中间系统TLV中的每个默认度量子TLV。如果M-ISIS路由器配置为通告一个或多个拓扑的流量工程默认度量子TLV，但在反向度量TLV中未接收到流量工程默认度量子TLV，则M-ISIS路由器不得更改所有拓扑的每个流量工程默认度量子TLV中的值。
On a Multi-access LAN, only the DIS SHOULD act upon information contained in a received Reverse Metric TLV. All non-DIS nodes MUST silently ignore a received Reverse Metric TLV. The decision process of the routers on the LAN MUST follow the procedure in Section 7.2.8.2 of [ISO10589], and use the "Two-way connectivity check" during the topology and route calculation.
在多址LAN上，只有DIS应对接收到的反向度量TLV中包含的信息进行操作。所有非DIS节点必须静默忽略接收到的反向度量TLV。LAN上路由器的决策过程必须遵循[ISO10589]第7.2.8.2节中的程序，并在拓扑和路由计算期间使用“双向连接检查”。
The Reverse Metric Traffic Engineering sub-TLV also applies to the DIS. If a DIS is configured to apply Traffic Engineering over a link and it receives Traffic Engineering Metric sub-TLV in a Reverse Metric TLV, it should update the Traffic Engineering Default Metric sub-TLV value of the corresponding Extended IS Reachability TLV or insert a new one if not present.
反向度量流量工程子TLV也适用于DIS。如果DIS配置为在链路上应用流量工程，并且在反向度量TLV中接收到流量工程度量sub-TLV，则其应更新相应扩展is可达性TLV的流量工程默认度量sub-TLV值，或者如果不存在，则插入新的度量sub-TLV值。
In the case of multi-access LANs, the "W" Flags bit is used to signal from a non-DIS to the DIS whether or not to change the metric and, optionally, Traffic Engineering parameters for all nodes in the Pseudonode LSP or solely the node on the LAN originating the Reverse Metric TLV.
在多址LAN的情况下，“W”标志位用于从非DIS向DIS发送信号，指示是否更改伪节点LSP中所有节点或仅LAN上发起反向度量TLV的节点的度量和可选流量工程参数。
A non-DIS node, e.g., Router B, attached to a multi-access LAN will send the DIS a Reverse Metric TLV with the W bit clear when Router B wishes the DIS to add the Metric value to the Default Metric contained in the Pseudonode LSP specific to just Router B. Other
当路由器B希望DIS将度量值添加到仅针对路由器B的伪节点LSP中包含的默认度量值时，连接到多址LAN的非DIS节点（例如路由器B）将向DIS发送W位清除的反向度量TLV。其他
non-DIS nodes, e.g., Routers C and D, may simultaneously send a Reverse Metric TLV with the W bit clear to request the DIS to add their own Metric value to their Default Metric contained in the Pseudonode LSP.
非DIS节点，例如路由器C和D，可以同时发送W位清除的反向度量TLV，以请求DIS将其自己的度量值添加到伪节点LSP中包含的其默认度量。
As long as at least one IS-IS node on the LAN sending the signal to DIS with the W bit set, the DIS would add the metric value in the Reverse Metric TLV to all neighbor adjacencies in the Pseudonode LSP, regardless if some of the nodes on the LAN advertise the Reverse Metric TLV without the W bit set. The DIS MUST use the reverse metric of the highest source MAC address Non-DIS advertising the Reverse Metric TLV with the W bit set.
只要LAN上至少有一个IS-IS节点使用W位集向DIS发送信号，DIS就会将反向度量TLV中的度量值添加到伪节点LSP中的所有邻居邻接中，而不管LAN上的一些节点是否在不使用W位集的情况下宣传反向度量TLV。DIS必须使用最高源MAC地址的反向度量，非DIS使用设置了W位的反向度量TLV。
Local provisioning on the DIS to adjust the Default Metric(s) is another way to insert Reverse Metric in the Pseudonode LSP towards an IS-IS node on a LAN. In the case where a Reverse Metric TLV is also used in the IS-IS Hello PDU of the node, the local provisioning MUST take precedence over received Reverse Metric TLVs. For instance, local policy on the DIS may be provisioned to ignore the W bit signaling on a LAN.
在DIS上进行本地配置以调整默认度量是向LAN上的is-is节点在伪节点LSP中插入反向度量的另一种方法。在节点的is-is Hello PDU中也使用反向度量TLV的情况下，本地配置必须优先于接收到的反向度量TLV。例如，可以设置DIS上的本地策略以忽略LAN上的W位信令。
Multi-topology IS-IS [RFC5120] specifies there is no change to construction of the Pseudonode LSP regardless of the Multi-topology (MT) capabilities of a multi-access LAN. If any MT capable node on the LAN advertises the Reverse Metric TLV to the DIS, the DIS should update, as appropriate, the Default Metric contained in the Pseudonode LSP. If the DIS updates the Default Metric and floods a new Pseudonode LSP, those default metric values will be applied to all topologies during Multi-topology Shortest Path First calculations.
多拓扑IS-IS[RFC5120]规定，无论多接入LAN的多拓扑（MT）能力如何，伪节点LSP的构造都不会发生变化。如果LAN上任何支持MT的节点向DIS播发反向度量TLV，则DIS应酌情更新伪节点LSP中包含的默认度量。如果DIS更新默认度量并泛洪新的伪节点LSP，则在多拓扑最短路径优先计算期间，这些默认度量值将应用于所有拓扑。
As described in [RFC6138], when a new IS-IS node joins a broadcast network, it is unnecessary and sometimes even harmful for all IS-IS nodes on the LAN to advertise the maximum link metric. [RFC6138] proposes a solution to have the new node not advertise its adjacency towards the pseudonode when it is not in a "cut-edge" position.
如[RFC6138]中所述，当一个新的IS-IS节点加入广播网络时，对LAN上的所有IS-IS节点来说，公布最大链路度量是不必要的，有时甚至是有害的。[RFC6138]提出了一种解决方案，使新节点在不处于“切割边缘”位置时，不会向伪节点公布其邻接。
With the introduction of Reverse Metric in this document, a simpler alternative solution to the above mentioned problem can be used. The Reverse Metric allows the new node on the LAN to advertise its inbound metric value to be the maximum, and this puts the link of this new node in the last resort position without impacting the other IS-IS nodes on the same LAN.
通过在本文档中引入反向度量，可以使用上述问题的更简单的替代解决方案。反向度量允许LAN上的新节点公布其入站度量值为最大值，这将使此新节点的链接处于最后位置，而不会影响同一LAN上的其他IS-IS节点。
Specifically, when IS-IS adjacencies are being established by the new node on the LAN, besides setting the maximum link metric value (2^24 - 2) on the interface of the LAN for LDP IGP synchronization as described in [RFC5443], it SHOULD advertise the maximum metric offset value in the Reverse Metric TLV in its IIH PDU sent on the LAN. It SHOULD continue this advertisement until it completes all the LDP label binding exchanges with all the neighbors over this LAN, either by receiving the LDP End-of-LIB [RFC5919] for all the sessions or by exceeding the provisioned timeout value for the node LDP/IGP synchronization.
具体而言，当LAN上的新节点建立IS-IS邻接时，除了在LAN接口上设置LDP IGP同步的最大链路度量值（2^24-2），如[RFC5443]所述，还应在LAN上发送的IIH PDU中公布反向度量TLV中的最大度量偏移值。它应该通过接收所有会话的LDP结束库[RFC5919]，或者通过超过为节点LDP/IGP同步设置的超时值，继续此播发，直到完成与该LAN上所有邻居的所有LDP标签绑定交换。
For the use case in Section 1.1, a router SHOULD limit the period of advertising a Reverse Metric TLV towards a neighbor only for the duration of a network maintenance window.
对于第1.1节中的用例，路由器应仅在网络维护窗口期间限制向邻居发布反向度量TLV的时间。
The use of a Reverse Metric does not alter IS-IS metric parameters stored in a router's persistent provisioning database.
反向度量的使用不会改变存储在路由器的持久配置数据库中的IS-IS度量参数。
If routers that receive a Reverse Metric TLV send a syslog message or SNMP trap, this will assist in rapidly identifying the node in the network that is advertising an IS-IS metric or Traffic Engineering parameters different from that which is configured locally on the device.
如果接收反向度量TLV的路由器发送syslog消息或SNMP陷阱，这将有助于快速识别网络中发布is-is度量或流量工程参数（与设备上本地配置的参数不同）的节点。
When the link Traffic Engineering metric is raised to (2^24 - 1) [RFC5817], either due to the Reverse Metric mechanism or by explicit user configuration, this SHOULD immediately trigger the CSPF (Constrained Shortest Path First) recalculation to move the Traffic Engineering traffic away from that link. It is RECOMMENDED also that the CSPF does the immediate CSPF recalculation when the Traffic Engineering metric is raised to (2^24 - 2) to be the last resort link.
当由于反向度量机制或显式用户配置，链路流量工程度量提升至（2^24-1）[RFC5817]时，这应立即触发CSPF（受限最短路径优先）重新计算，以将流量工程流量从该链路移出。当交通工程指标提升到（2^24-2）作为最后手段链路时，还建议CSPF立即重新计算CSPF。
It is advisable that implementations provide a configuration capability to disable any IS-IS metric changes by a Reverse Metric mechanism through neighbors' Hello PDUs.
建议实现提供配置功能，通过邻居的Hello PDU通过反向度量机制禁用任何is-is度量更改。
If an implementation enables this mechanism by default, it is RECOMMENDED that it be disabled by the operators when not explicitly using it.
如果实现在默认情况下启用此机制，则建议操作员在未明确使用该机制时禁用该机制。
Security concerns for IS-IS are addressed in [ISO10589], [RFC5304], [RFC5310], and with various deployment and operational security considerations in [RFC7645]. The enhancement in this document makes it possible for one IS-IS router to manipulate the IS-IS Default Metric and, optionally, Traffic Engineering parameters of adjacent IS-IS neighbors on point-to-point or LAN interfaces. Although IS-IS routers within a single Autonomous System nearly always are under the control of a single administrative authority, it is highly recommended that operators configure authentication of IS-IS PDUs to mitigate use of the Reverse Metric TLV as a potential attack vector.
IS-IS的安全问题在[ISO10589]、[RFC5304]、[RFC5310]以及[RFC7645]中的各种部署和操作安全考虑因素中得到了解决。本文档中的增强功能使一个IS-IS路由器能够操纵IS-IS默认指标，以及（可选）点到点或LAN接口上相邻IS-IS邻居的流量工程参数。虽然IS-IS是单个自治系统内的路由器，但几乎总是处于单个管理权限的控制之下，强烈建议运营商配置IS-IS PDU的认证，以减轻反向度量TLV作为潜在攻击向量的使用。
IANA has allocated IS-IS TLV Codepoint 16 for the Reverse Metric TLV. This new TLV has the following attributes: IIH = y, LSP = n, SNP = n, Purge = n.
IANA已为反向度量TLV分配IS-IS TLV代码点16。此新TLV具有以下属性：IIH=y、LSP=n、SNP=n、Purge=n。
This document also introduces a new registry for sub-TLVs of the Reverse Metric TLV. The registration policy is Expert Review as defined in [RFC8126]. This registry is part of the "IS-IS TLV Codepoints" registry. The name of the registry is "Sub-TLVs for TLV 16 (Reverse Metric TLV)". The defined values are:
本文档还介绍了反向度量TLV子TLV的新注册表。注册政策为[RFC8126]中定义的专家评审。此注册表是“is-is TLV代码点”注册表的一部分。注册表名称为“TLV 16的子TLV（反向度量TLV）”。定义的值为：
0: Reserved 1-17: Unassigned 18: Traffic Engineering Metric as specified in this document (Section 2) 19-255: Unassigned
0：保留1-17：未分配18：本文件（第2节）中规定的交通工程指标19-255：未分配
[ISO10589] ISO, "Information technology -- Telecommunications and information exchange between systems -- 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/IEC 10589:2002, Second Edition, November 2002.
[ISO10589]ISO，“信息技术——系统间的电信和信息交换——与提供无连接模式网络服务的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/IEC 10589:2002，第二版，2002年11月。
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, DOI 10.17487/RFC1195, December 1990, <https://www.rfc-editor.org/info/rfc1195>.
[RFC1195]Callon，R.“OSI IS-IS在TCP/IP和双环境中的路由使用”，RFC 1195，DOI 10.17487/RFC1195，1990年12月<https://www.rfc-editor.org/info/rfc1195>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://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<https://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, <https://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<https://www.rfc-editor.org/info/rfc5120>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, <https://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<https://www.rfc-editor.org/info/rfc5305>.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP Synchronization", RFC 5443, DOI 10.17487/RFC5443, March 2009, <https://www.rfc-editor.org/info/rfc5443>.
[RFC5443]Jork，M.，Atlas，A.，和L.Fang，“LDP IGP同步”，RFC 5443，DOI 10.17487/RFC5443，2009年3月<https://www.rfc-editor.org/info/rfc5443>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
[RFC8126]Cotton，M.，Leiba，B.，和T.Narten，“在RFC中编写IANA考虑事项部分的指南”，BCP 26，RFC 8126，DOI 10.17487/RFC8126，2017年6月<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8174]Leiba，B.，“RFC 2119关键词中大写与小写的歧义”，BCP 14，RFC 8174，DOI 10.17487/RFC8174，2017年5月<https://www.rfc-editor.org/info/rfc8174>.
[IS-IS-SL-EXT] Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS Routing for Spine-Leaf Topology", Work in Progress, draft-ietf-lsr-isis-spine-leaf-ext-00, December 2018.
[IS-IS-SL-EXT]Shen，N.，Ginsberg，L.，和S.Thyamagundalu，“棘叶拓扑的IS-IS路由”，正在进行的工作，草稿-ietf-lsr-isis-Spine-Leaf-EXT-00，2018年12月。
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2008, <https://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<https://www.rfc-editor.org/info/rfc5304>.
[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, <https://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<https://www.rfc-editor.org/info/rfc5310>.
[RFC5817] Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton, "Graceful Shutdown in MPLS and Generalized MPLS Traffic Engineering Networks", RFC 5817, DOI 10.17487/RFC5817, April 2010, <https://www.rfc-editor.org/info/rfc5817>.
[RFC5817]Ali，Z.，Vasseur，JP.，Zamfir，A.，和J.Newton，“MPLS和广义MPLS流量工程网络中的优雅关机”，RFC 5817，DOI 10.17487/RFC5817，2010年4月<https://www.rfc-editor.org/info/rfc5817>.
[RFC5919] Asati, R., Mohapatra, P., Chen, E., and B. Thomas, "Signaling LDP Label Advertisement Completion", RFC 5919, DOI 10.17487/RFC5919, August 2010, <https://www.rfc-editor.org/info/rfc5919>.
[RFC5919]Asati，R.，Mohapatra，P.，Chen，E.，和B.Thomas，“信号LDP标签广告完成”，RFC 5919，DOI 10.17487/RFC5919，2010年8月<https://www.rfc-editor.org/info/rfc5919>.
[RFC6138] Kini, S., Ed. and W. Lu, Ed., "LDP IGP Synchronization for Broadcast Networks", RFC 6138, DOI 10.17487/RFC6138, February 2011, <https://www.rfc-editor.org/info/rfc6138>.
[RFC6138]Kini，S.，Ed.和W.Lu，Ed.，“广播网络的LDP IGP同步”，RFC 6138，DOI 10.17487/RFC6138，2011年2月<https://www.rfc-editor.org/info/rfc6138>.
[RFC7645] Chunduri, U., Tian, A., and W. Lu, "The Keying and Authentication for Routing Protocol (KARP) IS-IS Security Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015, <https://www.rfc-editor.org/info/rfc7645>.
[RFC7645]Chunduri，U.，Tian，A.，和W.Lu，“路由协议的键控和认证（KARP）IS-IS安全分析”，RFC 7645，DOI 10.17487/RFC76452015年9月<https://www.rfc-editor.org/info/rfc7645>.
On rare occasions, it is necessary for an operator to perform disruptive network maintenance on an entire IS-IS router node, i.e., major software upgrades, power/cooling augments, etc. In these cases, an operator will set the IS-IS Overload Bit (OL bit) within the Link State Protocol Data Units (LSPs) of the IS-IS router about to undergo maintenance. The IS-IS router immediately floods its updated LSPs to all IS-IS routers in the IS-IS domain. Upon receipt of the updated LSPs, all IS-IS routers recalculate their Shortest Path First (SPF) tree excluding IS-IS routers whose LSPs have the OL bit set. This effectively removes the IS-IS router about to undergo maintenance from the topology, thus preventing it from receiving any transit traffic during the maintenance period.
在极少数情况下，运营商有必要对整个is-is路由器节点执行中断性网络维护，即主要软件升级、电源/冷却增强等。在这些情况下，运营商将在链路状态协议数据单元（LSP）内设置is-is过载位（OL位）IS-IS路由器即将进行维护。IS-IS路由器立即将其更新的LSP发送到IS-IS域中的所有IS-IS路由器。收到更新的LSP后，所有IS-IS路由器重新计算其最短路径优先（SPF）树，不包括其LSP设置了OL位的IS-IS路由器。这将有效地从拓扑中删除即将进行维护的IS-IS路由器，从而防止其在维护期间接收任何传输流量。
After the maintenance activity has completed, the operator resets the IS-IS Overload Bit within the LSPs of the original IS-IS router causing it to flood updated IS-IS LSPs throughout the IS-IS domain. All IS-IS routers recalculate their SPF tree and now include the original IS-IS router in their topology calculations, allowing it to be used for transit traffic again.
维护活动完成后，操作员在原始IS-IS路由器的LSP内重置IS-IS过载位，使其在整个IS-IS域内泛滥更新的IS-IS LSP。所有IS-IS路由器重新计算其SPF树，现在将原始IS-IS路由器包括在其拓扑计算中，允许其再次用于传输流量。
Isolating an entire IS-IS router from the topology can be especially disruptive due to the displacement of a large volume of traffic through an entire IS-IS router to other suboptimal paths (e.g., those with significantly larger delay). Thus, in the majority of network maintenance scenarios, where only a single link or LAN needs to be augmented to increase its physical capacity, or is experiencing an intermittent failure, it is much more common and desirable to gracefully remove just the targeted link or LAN from service temporarily, so that the least amount of user-data traffic is affected during the link-specific network maintenance.
由于大量流量通过整个IS-IS路由器转移到其他次优路径（例如延迟显著较大的路径），因此将整个IS-IS路由器与拓扑隔离可能会造成特别大的中断。因此，在大多数网络维护场景中，如果只需要增加单个链路或LAN以增加其物理容量，或者遇到间歇性故障，则更常见和更可取的做法是暂时删除目标链路或LAN，因此，在特定于链路的网络维护期间，受影响的用户数据流量最少。
Before network maintenance events are performed on individual physical links or LANs, operators substantially increase the IS-IS metric simultaneously on both devices attached to the same link or LAN. In doing so, the devices generate new Link State Protocol Data Units (LSPs) that are flooded throughout the network and cause all routers to gradually shift traffic onto alternate paths with very little or no disruption to in-flight communications by applications or end users. When performed successfully, this allows the operator to confidently perform disruptive augmentation, fault diagnosis, or repairs on a link without disturbing ongoing communications in the network.
在对单个物理链路或LAN执行网络维护事件之前，运营商在连接到同一链路或LAN的两个设备上同时大幅增加IS-IS度量。在这样做的过程中，设备生成新的链路状态协议数据单元（LSP），这些数据单元遍布整个网络，并使所有路由器逐渐将流量转移到备用路径上，而应用程序或最终用户的空中通信几乎没有中断。成功执行后，操作员可以自信地在链路上执行中断性增强、故障诊断或维修，而不会干扰网络中正在进行的通信。
There are a number of challenges with the above solution. First, it is quite common to have routers with several hundred interfaces and individual interfaces that move anywhere from several hundred gigabits/second to terabits/second of traffic. Thus, it is imperative that operators accurately identify the same point-to-point link on two separate devices in order to increase (and afterward decrease) the IS-IS metric appropriately. Second, the aforementioned solution is very time-consuming and even more error-prone to perform when it's necessary to temporarily remove a multi-access LAN from the network topology. Specifically, the operator needs to configure ALL devices that have interfaces attached to the multi-access LAN with an appropriately high IS-IS metric (and then decrease the IS-IS metric to its original value afterward). Finally, with respect to multi-access LANs, there is currently no method to bidirectionally isolate only a single node's interface on the LAN when performing more fine-grained diagnoses and repairs to the multi-access LAN.
上述解决方案存在许多挑战。首先，拥有数百个接口的路由器和单个接口的路由器非常常见，这些接口的传输速率从数百千兆位/秒到千兆位/秒不等。因此，运营商必须准确识别两个独立设备上的相同点对点链路，以便适当增加（并随后减少）is-is度量。第二，上述解决方案非常耗时，而且在需要临时从网络拓扑中删除多址LAN时更容易出错。具体而言，操作员需要使用适当高的IS-IS度量（然后将IS-IS度量降低到其原始值）配置所有连接到多址LAN的接口的设备。最后，关于多接入LAN，当前没有方法在对多接入LAN执行更细粒度的诊断和修复时，仅在LAN上双向隔离单个节点的接口。
In theory, use of a Network Management System (NMS) could improve the accuracy of identifying the appropriate subset of routers attached to either a point-to-point link or a multi-access LAN. It could also signal to those devices, using a network management protocol, to adjust the IS-IS metrics on the pertinent set of interfaces. The reality is that NMSs are, to a very large extent, not used within Service Provider's networks for a variety of reasons. In particular, NMSs do not interoperate very well across different vendors or even separate platform families within the same vendor.
理论上，使用网络管理系统（NMS）可以提高识别连接到点到点链路或多址LAN的路由器的适当子集的准确性。它还可以使用网络管理协议向这些设备发送信号，以调整相关接口集上的IS-IS度量。现实情况是，由于各种原因，NMS在很大程度上没有在服务提供商的网络中使用。特别是，NMS不能在不同供应商之间很好地互操作，甚至在同一供应商内的不同平台系列之间也不能很好地互操作。
Acknowledgments
致谢
The authors would like to thank Mike Shand, Dave Katz, Guan Deng, Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith, Uma Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu Shah, Wes George, Danny McPherson, Ed Crabbe, Russ White, Robert Raszuk, Tom Petch, Stewart Bryant, and Acee Lindem for their comments and contributions.
作者要感谢迈克·山德、戴夫·卡茨、关登、伊利亚·瓦拉什金、杰伊·陈、莱斯·金斯伯格、彼得·阿什伍德·史密斯、乌玛·春杜里、亚历山大·奥孔尼科夫、乔纳森·哈里森、戴夫·沃德、希曼苏·沙阿、韦斯·乔治、丹尼·麦克弗森、埃德·克拉布、罗斯·怀特、罗伯特·拉祖克、汤姆·佩奇、斯图尔特·布莱恩特、，以及Acee Lindem的评论和贡献。
Contributors
贡献者
Tony Li
李东尼
Email: tony.li@tony.li
Email: tony.li@tony.li
Authors' Addresses
作者地址
Naiming Shen Cisco Systems 560 McCarthy Blvd. Milpitas, CA 95035 United States of America
沈乃明思科系统公司麦卡锡大道560号。美国加利福尼亚州米尔皮塔斯95035
Email: naiming@cisco.com
Email: naiming@cisco.com
Shane Amante Apple Inc. One Apple Park Way Cupertino, CA 95014 United States of America
美国加利福尼亚州库珀蒂诺市苹果公园路一号Shane Amante苹果公司，邮编95014
Email: amante@apple.com
Email: amante@apple.com
Mikael Abrahamsson T-Systems Nordic Kistagangen 26 Stockholm Sweden
Mikael Abrahamsson T-Systems北欧基斯塔甘根26瑞典斯德哥尔摩
Email: Mikael.Abrahamsson@t-systems.se
Email: Mikael.Abrahamsson@t-systems.se