Internet Engineering Task Force (IETF) J. Farkas, Ed.
Request for Comments: 7813 Ericsson
Category: Standards Track N. Bragg
ISSN: 2070-1721 Ciena
P. Unbehagen
Avaya
G. Parsons
Ericsson
P. Ashwood-Smith
Huawei Technologies
C. Bowers
Juniper Networks
June 2016
Internet Engineering Task Force (IETF) J. Farkas, Ed.
Request for Comments: 7813 Ericsson
Category: Standards Track N. Bragg
ISSN: 2070-1721 Ciena
P. Unbehagen
Avaya
G. Parsons
Ericsson
P. Ashwood-Smith
Huawei Technologies
C. Bowers
Juniper Networks
June 2016
IS-IS Path Control and Reservation
IS-IS路径控制和保留
Abstract
摘要
IEEE 802.1Qca Path Control and Reservation (PCR) specifies explicit path control via IS-IS in Layer 2 networks in order to move beyond the shortest path capabilities provided by IEEE 802.1aq Shortest Path Bridging (SPB). IS-IS PCR provides capabilities for the establishment and control of explicit forwarding trees in a Layer 2 network domain. This document specifies the sub-TLVs for IS-IS PCR.
IEEE 802.1Qca路径控制和保留（PCR）规定通过第2层网络中的IS-IS进行显式路径控制，以超越IEEE 802.1aq最短路径桥接（SPB）提供的最短路径能力。IS-IS PCR提供了在第2层网络域中建立和控制显式转发树的能力。本文件规定了IS-IS PCR的子TLV。
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/rfc7813.
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/rfc7813.
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
4. Explicit Trees . . . . . . . . . . . . . . . . . . . . . . . 6
5. Explicit ECT Algorithms . . . . . . . . . . . . . . . . . . . 9
6. IS-IS PCR Sub-TLVs . . . . . . . . . . . . . . . . . . . . . 11
6.1. Topology Sub-TLV . . . . . . . . . . . . . . . . . . . . 11
6.2. Hop Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. Bandwidth Constraint Sub-TLV . . . . . . . . . . . . . . 19
6.4. Bandwidth Assignment Sub-TLV . . . . . . . . . . . . . . 21
6.5. Timestamp Sub-TLV . . . . . . . . . . . . . . . . . . . . 23
7. MRT-FRR Application . . . . . . . . . . . . . . . . . . . . . 24
8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 31
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
4. Explicit Trees . . . . . . . . . . . . . . . . . . . . . . . 6
5. Explicit ECT Algorithms . . . . . . . . . . . . . . . . . . . 9
6. IS-IS PCR Sub-TLVs . . . . . . . . . . . . . . . . . . . . . 11
6.1. Topology Sub-TLV . . . . . . . . . . . . . . . . . . . . 11
6.2. Hop Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. Bandwidth Constraint Sub-TLV . . . . . . . . . . . . . . 19
6.4. Bandwidth Assignment Sub-TLV . . . . . . . . . . . . . . 21
6.5. Timestamp Sub-TLV . . . . . . . . . . . . . . . . . . . . 23
7. MRT-FRR Application . . . . . . . . . . . . . . . . . . . . . 24
8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 31
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
IEEE 802.1Qca Path Control and Reservation (PCR) [IEEE8021Qca] specifies extensions to IS-IS for the control of Explicit Trees (ETs). The PCR extensions are compatible with the Shortest Path Bridging (SPB) extensions to IS-IS specified by [RFC6329] and [IEEE8021aq] (already rolled into [IEEE8021Q]). Furthermore, IS-IS with PCR extensions relies on the SPB architecture and terminology, and some of the IS-IS SPB sub-TLVs are also leveraged. IS-IS PCR builds upon IS-IS and uses IS-IS in a similar way to SPB. IS-IS PCR only addresses point-to-point physical links, although IS-IS also supports shared media LANs.
IEEE 802.1Qca路径控制和保留（PCR）[IEEE8021Qca]指定了IS-IS的扩展，用于控制显式树（ETs）。PCR扩展与[RFC6329]和[IEEE8021aq]指定的IS-IS最短路径桥接（SPB）扩展兼容（已并入[IEEE8021Q]）。此外，带有PCR扩展的IS-IS依赖于SPB体系结构和术语，并且还利用了一些IS-IS SPB子TLV。IS-IS PCR建立在IS-IS的基础上，并以与SPB类似的方式使用IS-IS。IS-IS PCR只处理点对点物理链路，尽管IS-IS也支持共享媒体LAN。
This document specifies five IS-IS sub-TLVs for the control of explicit trees by IS-IS PCR in a Layer 2 network as specified by IEEE Std 802.1Qca. In addition to the sub-TLVs specified here, IS-IS PCR relies on the following IS-IS SPB sub-TLVs specified by [RFC6329]:
本文件规定了五个IS-IS子TLV，用于按照IEEE Std 802.1Qca的规定，在第2层网络中通过IS-IS PCR控制显式树。除了此处指定的子TLV外，IS-IS PCR还依赖于[RFC6329]指定的以下IS-IS SPB子TLV：
o SPB Link Metric sub-TLV
o SPB链路公制子TLV
o SPB Base VLAN-Identifiers sub-TLV
o SPB基本VLAN标识符子TLV
o SPB Instance sub-TLV
o SPB实例子TLV
o SPBV MAC address sub-TLV
o SPBV MAC地址子TLV
o SPBM Service Identifier and Unicast Address sub-TLV
o SPBM服务标识符和单播地址子TLV
These sub-TLVs are used to provide the link metric and the associations among bridges, Media Access Control (MAC) addresses, VLAN IDs (VIDs), and I-SIDs within an IS-IS domain. The use of these SPB sub-TLVs for PCR is specified by IEEE Std 802.1Qca. Note that IS-IS PCR does not require the implementation of the full IS-IS SPB protocol but only the support of these SPB sub-TLVs. A bridge can support both IS-IS SPB and IS-IS PCR at the same time; however, when it supports both, they are implemented by the same IS-IS entity on a per-instance basis.
这些子TLV用于提供链路度量以及IS-IS域中网桥、媒体访问控制（MAC）地址、VLAN ID（VID）和I-SID之间的关联。IEEE标准802.1Qca规定了PCR使用这些SPB子TLV。注意，IS-IS PCR不需要实施完整的IS-IS SPB协议，只需要支持这些SPB子TLV。桥可以同时支持IS-IS SPB和IS-IS PCR；但是，当它同时支持这两种情况时，它们是由同一IS-IS实体在每个实例的基础上实现的。
The sub-TLVs specified in this document can also be applied for Fast Reroute using Maximally Redundant Trees (MRT-FRR) [RFC7812] in a Layer 2 network. Maximally Redundant Trees (MRTs) are computed as specified in [RFC7811]. If MRT computation is split such that the Generalized Almost Directed Acyclic Graph (GADAG) is computed centrally, then these sub-TLVs can be used to distribute the GADAG, which is identical for each network node throughout a network domain.
本文件中规定的子TLV也可用于第2层网络中使用最大冗余树（MRT-FRR）[RFC7812]的快速重路由。按照[RFC7811]中的规定计算最大冗余树（MRT）。如果MRT计算被拆分，从而集中计算广义几乎有向无环图（GADAG），那么这些子TLV可用于分布GADAG，GADAG对于整个网络域中的每个网络节点都是相同的。
PCR uses IS-IS, the SPB sub-TLVs listed above, and the new sub-TLVs defined in this document. IS-IS PCR has no impact on IETF protocols.
PCR使用IS-IS、上述SPB子TLV以及本文件中定义的新子TLV。IS-IS PCR对IETF协议没有影响。
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。
This document uses the terminology defined in [RFC7812]. Only the abbreviations are resolved here for the MRT terms; please refer to [RFC7812] for the complete definition.
本文件使用[RFC7812]中定义的术语。此处仅解析捷运术语的缩写；完整定义请参考[RFC7812]。
ADAG: Almost Directed Acyclic Graph [RFC7812]
ADAG：几乎有向无环图[RFC7812]
B-VID: Backbone VID [IEEE8021Q]
B-VID：主干视频[IEEE8021Q]
Base VID: The VID used to identify a VLAN in management operations. [IEEE8021Q]
基本VID：用于在管理操作中标识VLAN的VID。[IEEE8021Q]
BLCE: Bridge Local Computation Engine - A computation engine in a bridge that performs path and routing computations. The BLCE implements e.g., SPF, CSPF, or the Maximally Redundant Trees algorithm. [IEEE8021Qca]
桥接器本地计算引擎-桥接器中执行路径和路由计算的计算引擎。BLCE实现例如SPF、CSPF或最大冗余树算法。[IEEE8021Qca]
Constrained tree: A tree meeting a certain constraint, e.g., providing minimally available bandwidth. [IEEE8021Qca]
约束树：满足特定约束的树，例如，提供最小可用带宽。[IEEE8021Qca]
CSPF: Constrained Shortest Path First
约束最短路径优先
DAG: Directed Acyclic Graph [RFC7812]
DAG：有向无环图[RFC7812]
DEI: Drop Eligible Indicator [IEEE8021Q]
DEI：删除合格指标[IEEE8021Q]
ECT Algorithm: Equal-Cost Tree algorithm - The algorithm and mechanism that is used for the control of the active topology, i.e., forwarding trees. It can be one of the shortest path algorithms specified by [IEEE8021Q]. It can be also one of the explicit path-control algorithms specified by [IEEE8021Qca]. Each ECT Algorithm has a 32-bit unique ID.
ECT算法：等成本树算法-用于控制活动拓扑的算法和机制，即转发树。它可以是[IEEE8021Q]指定的最短路径算法之一。它也可以是[IEEE8021Qca]指定的显式路径控制算法之一。每个ECT算法都有一个32位的唯一ID。
ET: Explicit Tree - An explicitly defined tree, which is specified by its Edge Bridges and the paths among the Edge Bridges. If only the Edge Bridges are specified but the paths are not, then it is a loose explicit tree. If the paths are also specified, then it is a strict explicit tree. [IEEE8021Qca]
ET：显式树-显式定义的树，由其边桥和边桥之间的路径指定。如果只指定了边桥，但没有指定路径，则它是一个松散的显式树。如果还指定了路径，则它是一个严格的显式树。[IEEE8021Qca]
ETDB: Explicit Tree Database - A database storing explicit trees. [IEEE8021Qca]
ETDB：显式树数据库-存储显式树的数据库。[IEEE8021Qca]
FDB: Filtering Database [IEEE8021Q]
FDB:筛选数据库[IEEE8021Q]
GADAG: Generalized ADAG [RFC7812]
GADAG：广义ADAG[RFC7812]
Hop: A hop is specified by two nodes. A strict hop has no intermediate nodes, whereas a loose hop can have one or more intermediate nodes. IS-IS PCR specifies an explicit tree by an ordered list of hops starting at the root, each successive hop being defined by the next element of the list. [IEEE8021Qca]
跃点：跃点由两个节点指定。严格跃点没有中间节点，而松散跃点可以有一个或多个中间节点。IS-IS PCR通过从根开始的有序跃点列表指定显式树，每个连续跃点由列表的下一个元素定义。[IEEE8021Qca]
I-SID: Backbone Service Instance Identifier - A 24-bit ID. [IEEE8021Q]
I-SID：主干服务实例标识符-一个24位ID。[IEEE8021Q]
LSDB: Link State Database
链路状态数据库
MRT: Maximally Redundant Trees [RFC7812]
MRT：最大冗余树[RFC7812]
MRT-Blue: MRT-Blue is used to describe one of the two MRTs. [RFC7812]
MRT蓝色：MRT蓝色用于描述两个MRT中的一个。[RFC7812]
MRT-Red: MRT-Red is used to describe one of the two MRTs. [RFC7812]
捷运红：捷运红用于描述两条捷运中的一条。[RFC7812]
MRT Root: The common root of the two MRTs: MRT-Blue and MRT-Red.
MRT根：两个MRT的公共根：MRT蓝色和MRT红色。
MSRP: Multiple Stream Registration Protocol, standardized as IEEE Std 802.1Qat, already rolled into [IEEE8021Q].
MSRP：多流注册协议，标准化为IEEE Std 802.1Qat，已经在[IEEE8021Q]中推出。
PCA: Path Control Agent - The agent that is part of the IS-IS domain and thus can perform IS-IS operations on behalf of a PCE, e.g., maintain the LSDB and send LSPs. [IEEE8021Qca]
PCA：路径控制代理-作为is-is域一部分的代理，因此可以代表PCE执行is-is操作，例如，维护LSDB和发送LSP。[IEEE8021Qca]
PCE: Path Computation Element - An entity that is capable of computing a path through a network based on a representation of the topology of the network (obtained by undefined means external to the PCE). [RFC4655]
PCE：路径计算元素-能够根据网络拓扑的表示（通过PCE外部的未定义方式获得）计算通过网络的路径的实体。[RFC4655]
PCP: Priority Code Point, which identifies a traffic class. [IEEE8021Q]
PCP：优先级代码点，用于标识流量类别。[IEEE8021Q]
PTP: Precision Time Protocol specified by [IEEE1588].
PTP:IEEE1588规定的精确时间协议。
SPB: Shortest Path Bridging
最短路径桥接
SPBM: SPB MAC - The SPB mode where a MAC or its shorthand (SPSourceID: Shortest Path Source ID) is used to identify an SPT. [IEEE8021Q]
SPBM:SPB MAC-使用MAC或其缩写（SPSourceID：最短路径源ID）标识SPT的SPB模式。[IEEE8021Q]
SPBV: SPB VID - The SPB mode where a unique VID is assigned to each SPT Root bridge and is used to identify an SPT. [IEEE8021Q]
SPBV:SPB VID-SPB模式，其中为每个SPT根网桥分配唯一的VID，并用于识别SPT。[IEEE8021Q]
SPF: Shortest Path First
最短路径优先
SPT: Shortest Path Tree [IEEE8021Q]
SPT：最短路径树[IEEE8021Q]
SRLG: Shared Risk Link Group - A set of links that share a resource whose failure affects each link. [RFC5307]
SRLG：共享风险链接组-共享资源的一组链接，其故障会影响每个链接。[RFC5307]
TAI: Temps Atomique International - International Atomic Time [IEEE1588]
TAI:临时国际原子能机构-国际原子时[IEEE1588]
TED: Traffic Engineering Database - A database storing the traffic engineering information propagated by IS-IS. [RFC5305]
TED：交通工程数据库-存储IS-IS传播的交通工程信息的数据库。[RFC5305]
VID: VLAN ID [IEEE8021Q]
VID:VLAN ID[IEEE8021Q]
VLAN: Virtual Local Area Network [IEEE8021Q]
VLAN：虚拟局域网[IEEE8021Q]
Explicit trees may be determined in some fashion. For example, an explicit tree may be determined by a Path Computation Element (PCE) [RFC4655]. A PCE is an entity that is capable of computing a topology for forwarding based on a network topology, its corresponding attributes, and potential constraints. If a PCE is used, it MUST explicitly specify an explicit tree as described in Section 6.1. Either a single PCE or multiple PCEs determine explicit trees for a domain. Even if there are multiple PCEs in a domain, each explicit tree MUST only be determined by one PCE, which is referred to as the owner PCE of the tree. PCEs and IS-IS PCR can be used in combination with IS-IS SPB shortest path routing. The remainder of this section, and subsequent sections, are written assuming PCE use.
可以以某种方式确定显式树。例如，显式树可以由路径计算元素（PCE）[RFC4655]确定。PCE是能够基于网络拓扑、其相应属性和潜在约束计算用于转发的拓扑的实体。如果使用PCE，则必须按照第6.1节所述明确指定一个明确的树。单个PCE或多个PCE确定域的显式树。即使域中有多个PCE，每个显式树也只能由一个PCE确定，该PCE称为树的所有者PCE。PCE和IS-IS PCR可与IS-IS SPB最短路径路由结合使用。本节的其余部分以及后续章节均假定使用PCE编写。
The PCE interacts with the active topology control protocol, i.e., with IS-IS. The collaboration with IS-IS can be provided by a Path Control Agent (PCA) on behalf of a PCE. Either the PCE or the corresponding PCA is part of the IS-IS domain. If the PCE is not part of the IS-IS domain, then the PCE MUST be associated with a PCA that is part of the IS-IS domain. The PCE or its PCA MUST establish IS-IS adjacency in order to receive all the LSPs transmitted by the bridges in the domain. The PCE, either on its own or via its PCA, can control the establishment of explicit trees in that domain by injecting an LSP conveying an explicit tree and thus instruct IS-IS to set up the explicit tree determined by the PCE. If instructed to do so by a PCE, IS-IS MAY also record and communicate bandwidth
PCE与主动拓扑控制协议交互，即与IS-IS交互。与IS-IS的协作可由代表PCE的路径控制代理（PCA）提供。PCE或相应的PCA是is-is域的一部分。如果PCE不属于is-is域，则PCE必须与属于is-is域的PCA相关联。PCE或其PCA必须建立IS-IS邻接，以便接收网桥在域中传输的所有LSP。PCE本身或通过其PCA，可以通过注入传送显式树的LSP来控制该域中显式树的建立，从而指示IS-IS建立由PCE确定的显式树。如果PCE指示这样做，IS-IS也可以记录和通信带宽
assignments, which MUST NOT be applied if reservation protocol (e.g., Multiple Stream Registration Protocol (MSRP)) is used in the domain. Both MSRP and IS-IS MUST NOT be used to make bandwidth assignments in the same domain.
如果域中使用了保留协议（例如，多流注册协议（MSRP）），则不得应用分配。MSRP和IS-IS不得用于在同一域中进行带宽分配。
The operation details of the PCE are not specified by this document or by IEEE Std 802.1Qca. If the PCE is part of the IS-IS domain, then the PCE uses IS-IS PDUs to communicate with the IS-IS domain and the PCE has a live IS-IS LSDB (i.e., the PCE implements the PCA functions too). A PCE can instead communicate with the IS-IS domain via a PCA, e.g., to retrieve the LSDB or instruct the creation of an explicit tree. However, the means of communication between the PCE and the PCA is not specified by this document or by IEEE Std 802.1Qca.
本文件或IEEE标准802.1Qca未规定PCE的操作细节。如果PCE是is-is域的一部分，则PCE使用is-is PDU与is-is域通信，并且PCE具有实时is-is LSDB（即，PCE也实现PCA功能）。PCE可以通过PCA与IS-IS域通信，例如检索LSDB或指示创建显式树。然而，本文件或IEEE Std 802.1Qca未规定PCE和PCA之间的通信方式。
An Explicit Tree (ET) is an undirected loop-free topology, whose use is under the control of the owner PCE by means of associating VIDs and MAC addresses with it. An ET MUST NOT contain cycles. As it is undirected, an ET contains no assumptions about the direction of any flows that use it; it can be used in either direction as specified by the VIDs and MAC addresses associated with it. It is the responsibility of the PCE to ensure reverse-path congruency and multicast-unicast congruency if that is required.
显式树（ET）是一种无向环路拓扑，其使用由所有者PCE通过将VID和MAC地址与其关联来控制。ET不得包含循环。由于它是无向的，ET不包含关于使用它的任何流的方向的假设；它可以在与之相关的VIDs和MAC地址指定的任何方向上使用。PCE有责任确保反向路径一致性和多播单播一致性（如果需要）。
An explicit tree is either strict or loose. A strict explicit tree specifies all bridges and paths it comprises. A loose tree only specifies the bridges as a list of hops that have a special role in the tree, e.g., an Edge Bridge, and no path or path segment is specified between the bridges, which are therefore loose hops even if Edge Bridges are adjacent neighbors. The special role of a hop can be: Edge Bridge, root, leaf, a bridge to be avoided, or a transit hop in case of a tree with a single leaf. The path for a loose hop is determined by the Bridge Local Computation Engine (BLCE) of the bridges. The shortest path is used for a loose hop unless specified otherwise by the descriptor (Section 6.1) of the tree or by the corresponding ECT Algorithm (Section 5).
显式树要么严格，要么松散。严格的显式树指定它包含的所有桥和路径。松散树仅将网桥指定为在树中具有特殊角色的跃点列表，例如边网桥，并且网桥之间未指定任何路径或路径段，因此即使边网桥是相邻的，它们也是松散跃点。跃点的特殊作用可以是：边桥、根、叶、要避免的桥，或者在树只有一片叶子的情况下是过渡跃点。松散跳跃的路径由网桥的网桥本地计算引擎（BLCE）确定。除非树的描述符（第6.1节）或相应的ECT算法（第5节）另有规定，否则最短路径用于松散跳跃。
A loose explicit tree is constrained if the tree descriptor includes one or more constraints, e.g., the administrative group that the links of the tree have to belong to. The BLCE of the bridges then applies the Constrained Shortest Path First (CSPF) algorithm, which is Shortest Path First (SPF) on the topology that only contains the links meeting the constraint(s).
如果树描述符包含一个或多个约束（例如，树的链接必须属于的管理组），则松散显式树受到约束。然后，桥梁的BLCE应用约束最短路径优先（CSPF）算法，即仅包含满足约束的链路的拓扑上的最短路径优先（SPF）。
An explicit tree is specified by a Topology sub-TLV (Section 6.1). The Topology sub-TLV associates one or more VIDs with an explicit tree. The Topology sub-TLV includes two or more Hop sub-TLVs (Section 6.2), and a hop is specified by an IS-IS System ID. A Hop
显式树由拓扑子TLV指定（第6.1节）。拓扑子TLV将一个或多个VID与显式树关联。拓扑子TLV包括两个或多个跃点子TLV（第6.2节），跃点由is-is系统ID指定。跃点
sub-TLV MAY include a delay constraint for a loose hop. A Topology sub-TLV MAY also include further sub-TLVs to constrain loose hops. The bridges involved in an explicit tree store the corresponding Topology sub-TLVs in their Explicit Tree Database (ETDB).
子TLV可包括用于松跳的延迟约束。拓扑子TLV还可以包括约束松散跳数的进一步子TLV。显式树中涉及的网桥将相应的拓扑子TLV存储在其显式树数据库（ETDB）中。
Explicit trees are propagated and set up by IS-IS PCR in a domain. The PCE or its PCA assembles the Topology sub-TLVs (Section 6.1), and adds it into an LSP, which is flooded throughout the domain. The Topology sub-TLV is flooded by the same techniques used for the SPB LSPs. The bridges then MUST process the Topology sub-TLV upon reception. If the Topology sub-TLV specifies one or more loose trees, then the path for the loose hops is determined by the BLCE of the bridges. The bridges then install the appropriate FDB entries for frame forwarding along the tree described by the Topology sub-TLV or the trees computed based on the Topology sub-TLV. Dynamic Filtering Entries are maintained by IS-IS for the [VID, MAC address] two-tuples associated with an ET.
显式树通过IS-IS PCR在域中传播和建立。PCE或其PCA组装拓扑子TLV（第6.1节），并将其添加到LSP中，LSP在整个域中被淹没。拓扑子TLV采用与SPB LSP相同的技术。然后，网桥必须在接收时处理拓扑子TLV。如果拓扑子TLV指定了一个或多个松散树，则松散跃点的路径由桥的BLCE确定。然后，网桥沿拓扑子TLV描述的树或基于拓扑子TLV计算的树安装用于帧转发的适当FDB条目。IS-IS为与ET相关联的[VID，MAC地址]两个元组维护动态过滤条目。
Due to the LSP aging of IS-IS, the Topology sub-TLVs (Section 6.1) have to be refreshed similar to other IS-IS TLVs in order to keep the integrity of the LSDB. The corresponding Dynamic Filtering Entries are also refreshed in the FDB when a Topology sub-TLV is refreshed. Refreshing Topology sub-TLVs is the task of the entity being part of the IS-IS domain, i.e., either the PCE or the PCA.
由于IS-IS的LSP老化，必须像其他IS-IS TLV一样刷新拓扑子TLV（第6.1节），以保持LSDB的完整性。刷新拓扑子TLV时，FDB中也会刷新相应的动态筛选条目。刷新拓扑子TLV是作为is-is域一部分的实体的任务，即PCE或PCA。
The owner PCE can withdraw an explicit tree by sending an updated LSP that does not include the Topology sub-TLV (Section 6.1). If a Topology sub-TLV is removed from an LSP (or has been changed) so that (previous) Topology sub-TLV is no longer present (or has been changed) in the LSDB, then that (previous) Topology sub-TLV is implicitly withdrawn. IS-IS PCR then removes (or updates) the explicit tree.
所有者PCE可以通过发送不包括拓扑子TLV的更新LSP来撤销显式树（第6.1节）。如果从LSP中删除（或已更改）拓扑子TLV，使得（先前）拓扑子TLV不再存在（或已更改）在LSDB中，则隐式撤回（先前）拓扑子TLV。IS-IS PCR然后删除（或更新）显式树。
There is no precedence order between explicit trees. Precedence order among bandwidth assignments recorded by IS-IS PCR is specified in Section 6.4.
显式树之间没有优先顺序。第6.4节规定了IS-IS PCR记录的带宽分配的优先顺序。
If it is not possible to install an explicit tree, e.g., constraint(s) cannot be met or the Topology sub-TLV is ill-formed, then no tree is installed, but a management report is generated.
如果无法安装显式树，例如无法满足约束或拓扑子TLV格式不正确，则不会安装树，但会生成管理报告。
The bridges MAY support the following IS-IS features for the computation of explicit trees. The Extended IS Reachability TLV (type 22) specified in [RFC5305] provides the following link attribute IS-IS sub-TLVs:
桥可以支持以下IS-IS功能，用于计算显式树。[RFC5305]中指定的扩展IS可达性TLV（类型22）提供以下链接属性IS-IS子TLV：
o Administrative Group (color) (sub-TLV type 3),
o 管理组（颜色）（子TLV类型3），
o Maximum Link Bandwidth (sub-TLV type 9),
o 最大链路带宽（子TLV类型9），
o Maximum Reservable Link Bandwidth (sub-TLV type 10),
o 最大可保留链路带宽（子TLV类型10），
o Unreserved Bandwidth (sub-TLV type 11),
o 无保留带宽（分TLV类型11），
o TE Default Metric (sub-TLV type 18).
o TE默认指标（子TLV类型18）。
When the Unreserved Bandwidth sub-TLV is used in a Layer 2 bridge network, the priority value encoded in the sub-TLV provides the PCP, i.e., identifies a traffic class (not a setup priority level).
当在第2层网桥网络中使用无保留带宽子TLV时，子TLV中编码的优先级值提供PCP，即，标识业务类别（而不是设置优先级）。
Further attributes are provided by the IS-IS TE Metric Extension link attribute sub-TLVs specified in [RFC7810]:
[RFC7810]中指定的IS-IS TE度量扩展链路属性子TLV提供了更多属性：
o Unidirectional Link Delay (sub-TLV type 33),
o 单向链路延迟（sub-TLV类型33），
o Min/Max Unidirectional Link Delay (sub-TLV type 34),
o 最小/最大单向链路延迟（子TLV类型34），
o Unidirectional Delay Variation (sub-TLV type 35),
o 单向延迟变化（35型次级TLV），
o Unidirectional Link Loss (sub-TLV type 36),
o 单向链路损耗（子TLV类型36），
o Unidirectional Residual Bandwidth (sub-TLV type 37),
o 单向剩余带宽（37型亚TLV），
o Unidirectional Available Bandwidth (sub-TLV type 38),
o 单向可用带宽（sub-TLV类型38），
o Unidirectional Utilized Bandwidth (sub-TLV type 39).
o 单向利用带宽（子TLV类型39）。
The Shared Risk Link Group (SRLG) information provided by the SRLG TLV (type 138) [RFC5307] MAY also be used. In order to indicate that the interface is unnumbered in this case, the corresponding flag takes value 0. The Link Local Identifier is an Extended Local Circuit Identifier and the Link Remote Identifier is a Neighbor Extended Local Circuit ID.
也可以使用SRLG TLV（138型）[RFC5307]提供的共享风险链接组（SRLG）信息。在这种情况下，为了指示接口未编号，相应的标志值为0。链路本地标识符是扩展本地电路标识符，链路远程标识符是邻居扩展本地电路ID。
The exact IS-IS control mode of operation MUST be selected for a VLAN by associating its Base VID with the appropriate ECT Algorithm in the SPB Base VLAN-Identifiers sub-TLV [RFC6329], in addition to allocating the Base VID to IS-IS control. There are five distinct ECT Algorithms for the five explicit path control modes. The operation details of the explicit ECT Algorithms and their configuration is specified by IEEE Std 802.1Qca; a high-level overview is given here. An ECT Algorithm value consists of the IEEE 802.1 OUI (Organizationally Unique Identifier) value 00-80-C2 concatenated with an index [RFC6329].
除了将基本VID分配给IS-IS控制之外，还必须通过将其基本VID与SPB基本VLAN标识符子TLV[RFC6329]中的适当ECT算法相关联，为VLAN选择准确的IS-IS控制操作模式。对于五种显式路径控制模式，有五种不同的ECT算法。显式ECT算法及其配置的操作细节由IEEE Std 802.1Qca规定；这里给出了一个高层次的概述。ECT算法值由IEEE 802.1 OUI（组织唯一标识符）值00-80-C2和索引[RFC6329]组成。
The Strict Tree (ST) ECT Algorithm MUST be used for a strict explicit tree. A strict ET is static, as no other entity can update it but the owner PCE. In case of a topology change, it is the task of the owner PCE to detect the topology change, e.g., based on the changes in the LSDB and to update the strict trees if needed. That is, the owner PCE computes the new tree, assembles its descriptor (Section 6.1), and then instructs IS-IS PCR to install it. The value for the ST ECT Algorithm is 00-80-C2-17.
严格树（ST）ECT算法必须用于严格的显式树。严格的ET是静态的，因为除了所有者PCE之外，没有其他实体可以更新它。在拓扑变化的情况下，所有者PCE的任务是检测拓扑变化，例如，基于LSDB中的变化，并在需要时更新严格树。也就是说，所有者PCE计算新树，组装其描述符（第6.1节），然后指示is-is PCR安装它。ST ECT算法的值为00-80-C2-17。
The Loose Tree (LT) ECT Algorithm MAY also be supported. It is used for a single loose explicit tree. The path for loose hops is determined by the BLCE of the bridges; therefore, the Topology sub-TLV (Section 6.1) specifying the tree MUST indicate which hop is the root of the tree. The loose hops are maintained by IS-IS, i.e., restored upon a topology change if a loop-free path is available. If the tree computed by the BLCE visits the same bridge twice (implying that a loop or hairpin has been created), then that loop or hairpin MUST be pruned from the tree even if it contains a hop specified by the Topology sub-TLV. It is a constraint if a bridge is not to be included, which can be specified by the Exclude flag of a Hop sub-TLV (Section 6.2) conveyed by the Topology sub-TLV specifying the tree. The range of values for the LT ECT Algorithms is 00-80-C2-21...00-80-C2-30.
也可支持松散树（LT）ECT算法。它用于单个松散的显式树。松散跳跃的路径由桥梁的BLCE决定；因此，指定树的拓扑子TLV（第6.1节）必须指明哪个跃点是树的根。松散跳数由IS-IS维护，即，如果无环路路径可用，则在拓扑更改时恢复。如果BLCE计算的树访问同一网桥两次（意味着已经创建了一个循环或发夹），则必须从树中修剪该循环或发夹，即使它包含拓扑子TLV指定的跃点。如果不包括网桥，这是一个约束，可以通过指定树的拓扑子TLV传递的跃点子TLV（第6.2节）的排除标志来指定。LT ECT算法的值范围为00-80-C2-21…00-80-C2-30。
The Loose Tree Set (LTS) ECT Algorithm MAY also be supported. It is used if connectivity among the Edge Bridges specified by the Topology sub-TLV (Section 6.1) is to be provided by a set of loose trees such that one tree is rooted at each Edge Bridge. The BLCE of the bridges compute the loose trees, which are maintained by IS-IS, i.e., restored upon a topology change. One constraint can be to avoid some bridges in these trees, which can be specified by the Exclude flag (item c.6. in Section 6.2). Further constraints can be specified by the Topology sub-TLV. The range of values for the LT ECT Algorithms is 00-80-C2-31...00-80-C2-40.
也可支持松散树集（LTS）ECT算法。如果拓扑子TLV（第6.1节）规定的边缘网桥之间的连接由一组松散的树提供，则使用该方法，这样每个边缘网桥上都有一棵树。桥梁的BLCE计算松散的树，这些树由IS-IS维护，即在拓扑变化时恢复。一个约束是避免这些树中的某些桥，这可以通过排除标志（第6.2节中的项目c.6）指定。拓扑子TLV可以指定其他约束。LT ECT算法的值范围为00-80-C2-31…00-80-C2-40。
The LT and LTS ECT Algorithms use the shortest paths after pruning the topology according to the constraint(s), if any. The shortest path tie-breaking specified by Section 12 of [RFC6329] is applied (see also subclauses 28.5 - 28.8 of [IEEE8021aq]), that's why range of values are associated with the LT and LTS ECT Algorithms. In case of the LT ECT Algorithm, the indexes are 0x21...0x30, and ECT-MASK{index-0x20} is applied to retrieve the ECT-MASK of Section 12 of [RFC6329]. In case of the LTS ECT Algorithm, the indexes are 0x31...0x40, and ECT-MASK{index-0x30} is applied to retrieve the ECT-MASK for shortest path tie-breaking.
LT和LTS ECT算法根据约束（如果有）修剪拓扑后使用最短路径。应用了[RFC6329]第12节规定的最短路径连接中断（另见[IEEE8021aq]第28.5-28.8款），这就是为什么值范围与LT和LTS ECT算法相关。对于LT ECT算法，索引为0x21…0x30，ECT-MASK{index-0x20}用于检索[RFC6329]第12节的ECT-MASK。对于LTS ECT算法，索引为0x31…0x40，ECT-MASK{index-0x30}用于检索ECT-MASK以进行最短路径连接中断。
The MRT ECT Algorithm MAY also be supported. It is used for the establishment and maintenance of MRTs in a distributed fashion. The MRT Lowpoint algorithm specified by [RFC7811] MUST be used for the computation of MRTs. The MRT Lowpoint algorithm first computes the GADAG and then produces two MRTs for each MRT Root: MRT-Blue and MRT-Red. If the level of redundancy provided by each bridge being an MRT Root is not required, then the MRT Roots can be specified by a Topology sub-TLV (Section 6.1). Both the GADAG and the MRT computation steps are performed distributed, i.e., by each bridge. The value for the MRT ECT Algorithm is 00-80-C2-18.
也可支持MRT ECT算法。它用于以分布式方式建立和维护MRT。计算MRT时必须使用[RFC7811]指定的MRT低点算法。MRT低点算法首先计算GADAG，然后为每个MRT根生成两个MRT：MRT蓝色和MRT红色。如果不需要每个网桥作为MRT根提供的冗余级别，则可以通过拓扑子TLV指定MRT根（第6.1节）。GADAG和MRT计算步骤都是分布式执行的，即通过每个桥梁执行。MRT ECT算法的值为00-80-C2-18。
The MRT GADAG (MRTG) ECT Algorithm MAY also be supported. It splits the computation into two. As the GADAG is identical for each MRT within a domain, it is computed by a single entity, which is the GADAG Computer. The GADAG is then described in a Topology sub-TLV (Section 6.1), which is flooded in the domain. The bridges then compute the MRTs for the MRT Roots based on the GADAG received. Section 7 provides more details on the description of the GADAG. The value for the MRTG ECT Algorithm is 00-80-C2-19.
也可支持MRT GADAG（MRTG）ECT算法。它将计算分成两部分。由于GADAG对于域内的每个MRT是相同的，因此它由单个实体（即GADAG计算机）计算。然后在拓扑子TLV（第6.1节）中描述GADAG，该拓扑子TLV在域中被淹没。然后，桥梁根据收到的GADAG计算MRT根的MRT。第7节提供了有关GADAG描述的更多详细信息。MRTG ECT算法的值为00-80-C2-19。
MRTs are loose trees as bridges are involved in their computation and restoration. Thus, both the MRT and the MRTG ECT Algorithms provide a set of loose trees: two MRTs for each MRT Root.
MRT是松散的树木，因为桥梁参与其计算和修复。因此，MRT和MRTG ECT算法都提供了一组松散的树：每个MRT根有两个MRT。
The SPB Link Metric sub-TLV [RFC6329] specifies the metric of each link for IS-IS PCR if the LT, the LTS, the MRT, or the MRTG ECT Algorithm is used. If the SPB Link Metric values advertised by different ends of an adjacency are different, then the maximum value MUST be used.
如果使用LT、LTS、MRT或MRTG ECT算法，则SPB链路度量子TLV[RFC6329]指定IS-IS PCR的每条链路的度量。如果相邻的不同端所公布的SPB链路度量值不同，则必须使用最大值。
The following sub-TLVs are specified for IS-IS PCR. The Topology sub-TLV MUST be carried in an MT-Capability TLV, the rest of the sub-TLVs are conveyed by the Topology sub-TLV.
IS-IS PCR指定了以下子TLV。拓扑子TLV必须在MT能力TLV中承载，其余子TLV由拓扑子TLV传输。
An explicit tree MUST be described by the variable-length Topology sub-TLV. A Generalized Almost Directed Acyclic Graph (GADAG) MAY be described by a Topology sub-TLV as explained in Section 7 in detail. The Topology sub-TLV MUST be carried in an MT-Capability TLV (type 144) [RFC6329] in a Link State PDU. A Topology sub-TLV specifying an explicit tree conveys one or more Base VIDs, two or more Hop sub-TLVs (Section 6.2). A Topology sub-TLV describing a loose tree MAY also convey further sub-TLVs to specify constraints. Figure 1 shows the format of the Topology sub-TLV.
显式树必须由可变长度拓扑子TLV描述。广义几乎有向无环图（GADAG）可由拓扑子TLV描述，如第7节中详细解释的。拓扑子TLV必须在链路状态PDU的MT能力TLV（144型）[RFC6329]中携带。指定显式树的拓扑子TLV传送一个或多个基本VID、两个或多个跃点子TLV（第6.2节）。描述松散树的拓扑子TLV还可以传递进一步的子TLV以指定约束。图1显示了拓扑子TLV的格式。
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| Num Base VIDs | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Base VID 1 (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Base VID n (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLV 1 (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLV m (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| Num Base VIDs | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Base VID 1 (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Res | Base VID n (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLV 1 (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLV m (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Topology Sub-TLV
图1：拓扑子TLV
The parameters of explicit trees are encoded by the Topology sub-TLV as follows:
显式树的参数由拓扑子TLV编码，如下所示：
a. Type (8 bits): The type of the sub-TLV, its value is 21.
a. 类型（8位）：子TLV的类型，其值为21。
b. Length (8 bits): The total number of bytes contained in the Value field.
b. 长度（8位）：值字段中包含的字节总数。
c. Number of Base VIDs (8 bits): The number of Base VIDs carried in the Topology sub-TLV. Its minimum value is 1 if the Topology sub-TLV specifies one or more explicit trees. Its value can be 0 if the Topology sub-TLV specifies a GADAG.
c. 基本视频数（8位）：拓扑子TLV中承载的基本视频数。如果拓扑子TLV指定一个或多个显式树，则其最小值为1。如果拓扑子TLV指定GADAG，则其值可以为0。
d. Reserved (Res) (4 bits): The reserved bits MUST be set to 0 on transmission and the value MUST be ignored on reception.
d. 保留（Res）（4位）：传输时必须将保留位设置为0，接收时必须忽略该值。
e. Base VID (12 bits): The Base VID parameter provides the Base VID of the VLAN that is associated with the explicit tree. Multiple Base VIDs can be associated with the same explicit tree. In addition to the Base VID, some of the explicit ECT Algorithms (Section 5) require further VIDs that are associated with the VLAN via the SPB Instance sub-TLV [RFC6329]. A Topology sub-TLV specifying a GADAG can have zero Base VID parameters. In this
e. Base VID（12位）：Base VID参数提供与显式树关联的VLAN的Base VID。多个基本Vid可以与同一显式树相关联。除了基本VID外，一些显式ECT算法（第5节）还需要通过SPB实例子TLV[RFC6329]与VLAN关联的其他VID。指定GADAG的拓扑子TLV可以具有零基本VID参数。在这个
case, the given GADAG MUST be applied for each VLAN associated with the MRTG ECT Algorithm (Section 5).
在这种情况下，给定的GADAG必须应用于与MRTG ECT算法相关的每个VLAN（第5节）。
f. sub-TLVs: The rest conveys further sub-TLVs that specify the hops of the topology and can also specify constraints as described in the following.
f. 子TLV：其余部分传递进一步的子TLV，这些子TLV指定拓扑的跃点，还可以指定约束，如下所述。
A topology is specified by a list of Hop sub-TLVs (Section 6.2), and a hop is specified by an IS-IS System ID. An ill-formed Topology sub-TLV (e.g., specifying an invalid or inconsistent tree) is ignored; no tree is installed, but a management report is generated.
拓扑由跃点子TLV列表指定（第6.2节），跃点由is-is系统ID指定。忽略格式错误的拓扑子TLV（例如，指定无效或不一致的树）；未安装树，但会生成管理报告。
The Topology sub-TLV specifies a strict tree by decomposing the tree to branches. Each branch is a point-to-point path specified by an ordered list of hops where the end of each branch is a leaf. Each element of a branch is the direct link between adjacent neighbor bridges whose Hop sub-TLV is next to each other in the Topology sub-TLV. The first hop of the Topology sub-TLV is the root; hence, the first branch originates from the root. The rest of the branches fork from another branch. The first hop of a branch is a bridge that is already part of a former branch, and the last hop is a leaf bridge. Therefore, the hop after a leaf hop is the beginning of a new branch, if any. A hop of a branch is created if and only if the bridge specified for that hop is directly connected to the preceding bridge of the same branch. The first branch MUST begin with the root; after that, the order of the branches does not matter within the Topology sub-TLV. Figure 2 shows an example strict tree and its description.
拓扑子TLV通过将树分解为分支来指定严格的树。每个分支都是由有序的跃点列表指定的点对点路径，其中每个分支的末端都是一片叶子。分支的每个元素都是相邻网桥之间的直接链路，其跃点子TLV在拓扑子TLV中彼此相邻。拓扑子TLV的第一跳是根；因此，第一个分支起源于根。其余的树枝从另一根树枝分叉。分支的第一个跃点是已经是前一个分支的一部分的桥，最后一个跃点是叶桥。因此，叶跃点之后的跃点是新分支（如果有的话）的开始。当且仅当为分支指定的网桥直接连接到同一分支的前一网桥时，才会创建分支的跃点。第一个分支必须从根开始；之后，分支的顺序在拓扑子TLV中不再重要。图2显示了一个示例strict树及其描述。
+-----------+
| A |
+-----------+
| I |
+-----------+
| H |
[B]---[A]---[I] +-----------+
| | | G |
| | +-----------+
| | | E |
[C]---[F] [H] +-----------+
| | | A |
| | +-----------+
| | | B |
[D] [E]---[G] +-----------+
| C |
+-----------+
| D |
+-----------+
| C |
+-----------+
| F |
+-----------+
+-----------+
| A |
+-----------+
| I |
+-----------+
| H |
[B]---[A]---[I] +-----------+
| | | G |
| | +-----------+
| | | E |
[C]---[F] [H] +-----------+
| | | A |
| | +-----------+
| | | B |
[D] [E]---[G] +-----------+
| C |
+-----------+
| D |
+-----------+
| C |
+-----------+
| F |
+-----------+
Figure 2: A Strict Tree and Its Description; Root = Node A
Figure 2: A Strict Tree and Its Description; Root = Node A
The Topology sub-TLV of a loose tree does not provide any path or path segment other than the hops that are to participate. The root MUST be the first hop. The leaves of a single loose tree MUST also be specified. Hop sub-TLVs can be included in a Topology sub-TLV to specify bridges that have to be avoided. If the Topology sub-TLV only specifies a single leaf, then one or more transit hops can be specified by the Topology sub-TLV to direct the path along a sequence of bridges, specified by the order of hops. If bridges whose respective Hop sub-TLVs are adjacent to each other in the Topology sub-TLV are not topology neighbors, then it is a loose hop. If a Topology sub-TLV conveys one or more loose hops, then that sub-TLV defines a loose explicit tree and each hop is considered to be a loose hop. The path of a loose hop MUST be pruned from the tree if the path would create a loop or hairpin.
松散树的拓扑子TLV不提供除要参与的跃点以外的任何路径或路径段。根必须是第一个跃点。还必须指定单个松散树木的叶子。跃点子TLV可以包含在拓扑子TLV中，以指定必须避免的网桥。如果拓扑子TLV仅指定一个叶，则拓扑子TLV可以指定一个或多个传输跃点，以沿着由跃点顺序指定的桥序列引导路径。如果其各自的跃点子TLV在拓扑子TLV中彼此相邻的网桥不是拓扑邻居，则它是松散跃点。如果拓扑子TLV传递一个或多个松散跃点，则该子TLV定义一个松散显式树，并且每个跃点都被视为松散跃点。如果路径将创建一个循环或发夹，则必须从树上修剪松散跳跃的路径。
If the Base VIDs of the Topology sub-TLV are associated with the LTS ECT Algorithm or the MRT ECT Algorithm, then the Hop sub-TLVs conveyed by the Topology sub-TLV belong to Edge Bridges or bridges to be excluded. The BLCEs compute the loose trees, e.g., MRTs, such that they span the Edge Bridges and are rooted at an Edge Bridge.
如果拓扑子TLV的基本VID与LTS ECT算法或MRT ECT算法相关联，则拓扑子TLV传送的跃点子TLV属于要排除的边缘网桥或网桥。BLCE计算松散的树木，例如MRT，使其跨越边缘桥梁，并扎根于边缘桥梁。
The Topology sub-TLV specifies a GADAG if the Base VIDs conveyed by the Topology sub-TLV are associated with the MRTG ECT Algorithm. Section 7 provides the details on the description of a GADAG by a Topology sub-TLV.
如果拓扑子TLV传输的基本视频与MRTG ECT算法关联，则拓扑子TLV指定GADAG。第7节详细介绍了拓扑子TLV对GADAG的描述。
Each Edge Bridge of an explicit tree MUST always be specified in the Topology sub-TLV by the inclusion of the Hop sub-TLVs corresponding to the Edge Bridges. The Edge Bridges of a tree are identified by setting the Edge Bridge flag (item c.3. in Section 6.2) in the appropriate Hop sub-TLVs.
显式树的每个边网桥必须始终通过包含对应于边网桥的跃点子TLV在拓扑子TLV中指定。通过在适当的跃点子TLV中设置边缘网桥标志（第6.2节中的项目c.3），识别树的边缘网桥。
If the explicit tree is loose, then the Topology sub-TLV MAY convey further sub-TLVs to specify constraints, e.g., an Administrative Group sub-TLV [RFC5305] or a Bandwidth Constraint (Section 6.3). If it is not possible to meet the constraint(s) specified by the Topology sub-TLV, then no tree is installed but a management report is generated.
如果显式树是松散的，则拓扑子TLV可传送更多子TLV以指定约束，例如，管理组子TLV[RFC5305]或带宽约束（第6.3节）。如果不可能满足拓扑子TLV指定的约束，则不会安装树，但会生成管理报告。
IS-IS PCR MAY be used for recording bandwidth assignment. In that case, the Topology sub-TLV conveys Bandwidth Assignment sub-TLV (Section 6.4), and it MAY also convey Timestamp sub-TLV (Section 6.5). If assignment of the bandwidth indicated by the Bandwidth Assignment sub-TLV of the Topology sub-TLV would result in overbooking any link of the explicit tree, then bandwidth assignment MUST NOT be performed and a management report is generated. If the Topology sub-TLV specifies a new valid explicit tree, then the tree is installed without bandwidth assignment.
IS-IS PCR可用于记录带宽分配。在这种情况下，拓扑子TLV传送带宽分配子TLV（第6.4节），并且它还可以传送时间戳子TLV（第6.5节）。如果拓扑子TLV的带宽分配子TLV指示的带宽分配将导致显式树的任何链路超售，则不得执行带宽分配并生成管理报告。如果拓扑子TLV指定了新的有效显式树，则安装该树时不会分配带宽。
The Hop sub-TLV MUST be used to specify a hop of a topology. Each Hop sub-TLV conveys an IS-IS System ID, which specifies a hop. A Hop sub-TLV is conveyed by a Topology sub-TLV (Section 6.1). A strict explicit tree is decomposed to branches where each branch is a point-to-point path specified by an ordered list of Hop sub-TLVs as specified in Section 6.1. A hop of a branch is created if and only if the bridge specified for that hop is directly connected to the preceding bridge in the path. That is, a point-to-point LAN is identified by the two bridges it interconnects; and the LAN is part of the strict tree if and only if the Hop sub-TLVs of the two bridges are next to each other in the Topology sub-TLV. A Hop sub-TLV can convey a Circuit ID in order to distinguish multiple links between adjacent neighbor bridges. A Hop sub-TLV also specifies the role of a bridge, e.g., if it is the root or an Edge Bridge. The Topology sub-TLV of a loose tree only comprises the Hop sub-TLV of the bridges that have a special role in the tree. The Hop sub-TLV MAY also specify a delay budget for a loose hop.
跃点子TLV必须用于指定拓扑的跃点。每个跃点子TLV传递一个IS-IS系统ID，该ID指定一个跃点。跃点子TLV由拓扑子TLV传输（第6.1节）。严格的显式树被分解为分支，其中每个分支都是由第6.1节中指定的跳跃子TLV的有序列表指定的点对点路径。当且仅当为分支指定的网桥直接连接到路径中的前一网桥时，才会创建分支的跃点。也就是说，点对点LAN由其互连的两个网桥标识；当且仅当两个网桥的跃点子TLV在拓扑子TLV中彼此相邻时，LAN才是严格树的一部分。跃点子TLV可以传送电路ID，以便区分相邻网桥之间的多个链路。跃点子TLV还指定网桥的角色，例如，它是根网桥还是边网桥。松散树的拓扑子TLV仅包括在树中具有特殊作用的网桥的跃点子TLV。跃点子TLV还可以为松散跃点指定延迟预算。
By default, the Edge Bridges both transmit and receive with respect to each VID associated with an explicit tree, except for an LTS (Section 5) associated with a learning VLAN, which uses a unidirectional VID per bridge. The Hop sub-TLV allows different configuration by means of the Transmit (T) and Receive (R) flags conveyed in the sub-TLV. The VID and its T/R flags are only present in the Hop sub-TLV if the behavior of the Edge Bridges differs from the default.
默认情况下，除了与学习VLAN相关联的LTS（第5节），边缘网桥针对与显式树相关联的每个VID进行传输和接收，该学习VLAN使用每个网桥的单向VID。跃点子TLV通过在子TLV中传送的发送（T）和接收（R）标志允许不同的配置。仅当边缘网桥的行为不同于默认值时，VID及其T/R标志才会出现在跃点子TLV中。
Figure 3 shows the format of the variable length Hop sub-TLV, which MUST be conveyed by a Topology sub-TLV (Section 6.1).
图3显示了可变长度跃点子TLV的格式，该格式必须由拓扑子TLV传输（第6.1节）。
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
|C|V|B|R|L|E|Res| (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| System ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| System ID | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Local Circuit ID (4 bytes if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num of VIDs | (1 byte if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|R|Res| VID 1 (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|R|Res| VID n (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay Constraint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay Constraint | (6 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
|C|V|B|R|L|E|Res| (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| System ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| System ID | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Local Circuit ID (4 bytes if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num of VIDs | (1 byte if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|R|Res| VID 1 (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|R|Res| VID n (12 bits) | (2 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay Constraint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay Constraint | (6 bytes if present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Hop Sub-TLV
图3:Hop子TLV
The parameters of a hop are encoded as follows:
跳的参数编码如下：
a. Type (8 bits): The type of the sub-TLV, its value is 22.
a. 类型（8位）：子TLV的类型，其值为22。
b. Length (8 bits): The total number of bytes contained in the Value field.
b. 长度（8位）：值字段中包含的字节总数。
c. Hop Flags (8 bits): The Hop sub-TLV conveys six one-bit flags. The Circuit and the VID flags influence the length of the Hop sub-TLV. Two bits are reserved for future use, transmitted as 0 and ignored on receipt.
c. 跃点标志（8位）：跃点子TLV传送六个一位标志。电路和VID标志影响跃点子TLV的长度。保留两位供将来使用，作为0传输，并在接收时忽略。
1. Circuit (C) flag (1 bit): The Circuit flag is a one-bit flag to indicate whether or not the Extended Local Circuit ID parameter is present. If the flag is set, then an Extended Local Circuit ID is also included in the Hop sub-TLV.
1. 电路（C）标志（1位）：电路标志为一位标志，用于指示是否存在扩展本地电路ID参数。如果设置了该标志，则扩展本地电路ID也包括在跃点子TLV中。
2. VID (V) flag (1 bit): The VID flag is a one-bit flag to indicate whether or not one or more VIDs are conveyed by the Hop sub-TLV. If the flag is set, then the Number of VIDs parameter is present and indicates how many VIDs are conveyed by the Hop sub-TLV. If the VID flag is reset, then neither the Number of VIDs parameter nor VIDs are present in the Hop sub-TLV.
2. VID（V）标志（1位）：VID标志是一个1位标志，用于指示跃点子TLV是否传送一个或多个VID。如果设置了该标志，则会出现“视频数量”参数，并指示跃点子TLV传输的视频数量。如果重置VID标志，则跃点子TLV中既不存在VIDs参数的数量，也不存在VIDs。
3. Edge Bridge (B) flag (1 bit): The Edge Bridge flag is a one-bit flag to indicate whether or not the given System is an Edge Bridge, i.e., transmitter and/or receiver. If the System is an Edge Bridge, then the Edge Bridge flag MUST be set. The Edge Bridge flag indicates that FDB entries have to be installed for the given hop as specified by the SPBV MAC address sub-TLV or SPBM Service Identifier and Unicast Address sub-TLV of the hop.
3. 边缘网桥（B）标志（1位）：边缘网桥标志是一位标志，用于指示给定系统是否为边缘网桥，即发射机和/或接收机。如果系统是边缘网桥，则必须设置边缘网桥标志。边缘网桥标志表示必须按照SPBV MAC地址子TLV或SPBM服务标识符以及该跃点的单播地址子TLV的指定，为给定跃点安装FDB条目。
4. Root (R) flag (1 bit): The Root flag is a one-bit flag to indicate whether or not the given System is a root of the explicit tree specified by the Topology sub-TLV. If the System is a root of a tree, then the Root flag MUST be set. If the Topology sub-TLV specifies a single tree, i.e., the Base VIDs conveyed by the Topology sub-TLV are associated with either the ST ECT Algorithm or the LT ECT Algorithm (Section 5), then the Root flag is only set for one of the Systems conveyed by the Topology sub-TLV. Furthermore, the first Hop sub-TLV of the Topology sub-TLV conveys the System that is the root of the tree. If the Topology sub-TLV specifies a Loose Tree Set, i.e., the Base VIDs conveyed by the Topology sub-TLV are associated with the LTS ECT Algorithm (Section 5), then the Root flag is set for each Edge Bridge as each of them roots a tree. If the Topology sub-TLV is used for MRT operations, i.e., the Base VIDs conveyed by the Topology sub-TLV are associated with either the MRT ECT Algorithm or the MRTG ECT algorithm (Section 5), then the Root flag is set for each MRT Root. If no MRT Root is specified by a Topology sub-TLV specifying a GADAG, then each SPT Root is an MRT Root as well.
4. 根（R）标志（1位）：根标志是一位标志，用于指示给定系统是否为拓扑子TLV指定的显式树的根。如果系统是树的根，则必须设置根标志。如果拓扑子TLV指定了一棵树，即拓扑子TLV传输的基本VID与ST ECT算法或LT ECT算法相关联（第5节），则根标志仅为拓扑子TLV传输的一个系统设置。此外，拓扑子TLV的第一跳子TLV传送作为树的根的系统。如果拓扑子TLV指定了一个松散的树集，即拓扑子TLV传输的基本VID与LTS ECT算法相关联（第5节），则为每个边桥设置根标志，因为每个边桥都是树的根。如果拓扑子TLV用于MRT操作，即拓扑子TLV传送的基本VID与MRT ECT算法或MRTG ECT算法（第5节）相关联，则为每个MRT根设置根标志。如果指定GADAG的拓扑子TLV未指定MRT根，则每个SPT根也是MRT根。
If the Base VIDs conveyed by the Topology sub-TLV are associated with the MRTG ECT algorithm (Section 5), then the Topology sub-TLV specifies a GADAG and the very first Hop sub-TLV specifies the GADAG Root. There is no flag for indicating the GADAG Root.
如果拓扑子TLV传输的基本VID与MRTG ECT算法（第5节）相关联，则拓扑子TLV指定GADAG，第一跳子TLV指定GADAG根。没有用于指示GADAG根的标志。
5. Leaf (L) flag (1 bit): The Leaf flag is a one-bit flag to indicate whether or not the given System is a leaf of the explicit tree specified by the Topology sub-TLV. If the System is a leaf, then the Leaf flag MUST be set. The Leaf flag is only used to mark a leaf of a tree if the Topology sub-TLV specifies a single tree. The Leaf flag MUST be used to indicate the end of a topology block if the Topology sub-TLV specifies a GADAG, see Section 7.
5. 叶（L）标志（1位）：叶标志是一位标志，用于指示给定系统是否为拓扑子TLV指定的显式树的叶。如果系统是叶，则必须设置叶标志。如果拓扑子TLV指定了一棵树，则叶标志仅用于标记树的叶。如果拓扑子TLV指定GADAG，则必须使用叶标志指示拓扑块的结束，请参见第7节。
6. Exclude (E) flag (1 bit): The Exclude flag is a one-bit flag to indicate if the given System MUST be excluded from the topology. The Exclude flag and the Root flag cannot be set for a given hop at the same time.
6. 排除（E）标志（1位）：排除标志是一个1位标志，用于指示是否必须将给定系统从拓扑中排除。不能同时为给定跃点设置排除标志和根标志。
7. Reserved (Res) (2 bits): The reserved bits MUST be set to 0 on transmission, and the value MUST be ignored on reception.
7. 保留（Res）（2位）：传输时必须将保留位设置为0，接收时必须忽略该值。
d. System ID (48 bits): The six-byte IS-IS System Identifier of the bridge to which the Hop sub-TLV refers.
d. 系统ID（48位）：六字节的IS-IS系统标识符，是跃点子TLV所指网桥的系统标识符。
e. Extended Local Circuit ID (32 bits): The Extended Local Circuit ID [RFC5303] parameter is not necessarily present in the Hop sub-TLV. Its presence is indicated by the Circuit flag. Parallel links corresponding to different IS-IS adjacencies between a pair of neighbor bridges can be distinguished by means of the Extended Local Circuit ID. The Extended Local Circuit ID is conveyed by the Hop sub-TLV specifying the bridge nearer to the root of the tree, and identifies a circuit that attaches the given bridge to its neighbor cited by the next Hop sub-TLV of the Topology sub-TLV. The Extended Local Circuit ID can only be used in strict trees.
e. 扩展本地电路ID（32位）：扩展本地电路ID[RFC5303]参数不一定存在于跃点子TLV中。其存在由电路标志指示。可通过扩展本地电路ID来区分对应于一对相邻网桥之间不同IS-IS邻接的并行链路。扩展本地电路ID由指定更靠近树根的网桥的跃点子TLV传送，并标识将给定网桥连接到拓扑子TLV的下一跳子TLV引用的相邻网桥的电路。扩展的本地电路ID只能在严格的树中使用。
f. Number of VIDs (8 bits): The Number of VIDs parameter is not present if the Hop sub-TLV does not convey VIDs, which is indicated by the VID flag.
f. VID数量（8位）：如果跃点子TLV不传送VID，则VID数量参数不存在，这由VID标志指示。
g. VID and its T/R flags (14 bits): The VID and its T/R flags are only present in the Hop sub-TLV if the given bridge is an Edge Bridge and it behaves differently from the default with respect to that particular VID.
g. VID及其T/R标志（14位）：如果给定网桥是边缘网桥，且其行为与该特定VID的默认行为不同，则VID及其T/R标志仅存在于跃点子TLV中。
1. T flag (1 bit): This is the Transmit allowed flag for the VID following the flag.
1. T标志（1位）：这是标志后面的VID允许传输的标志。
2. R flag (1 bit): This is the Receive allowed flag for the VID following the flag.
2. R标志（1位）：这是标志后面的VID的允许接收标志。
3. Reserved (Res) (2 bits): The reserved bits MUST be set to 0 on transmission, and the value MUST be ignored on reception.
3. 保留（Res）（2位）：传输时必须将保留位设置为0，接收时必须忽略该值。
4. VID (12 bits): A VID.
4. VID（12位）：一个VID。
h. Delay Constraint (48 bits): A Hop sub-TLV MAY specify a delay constraint. The Length of the Hop sub-TLV indicates whether or not a delay constraint is present because the Length of a Hop sub-TLV conveying a delay constraint is six bytes greater than it would be without the delay constraint. The last six bytes then specify a delay constraint if they convey a Unidirectional Link Delay sub-TLV [RFC7810]. The delay constraint MAY be used in a Topology sub-TLV that specifies a single loose tree, i.e., the Base VIDs are associated with the LT ECT Algorithm (Section 5). If the delay constraint is applied, then the loose hop MUST fit in the delay budget specified by the Delay parameter of the Unidirectional Link Delay sub-TLV conveyed by the Hop sub-TLV. If the Topology sub-TLV specifies a single leaf, then the path between the preceding Hop sub-TLV and the current Hop sub-TLV MUST meet the delay budget. If the Topology sub-TLV specifies multiple leaves, then the path between the root and the current Hop sub-TLV MUST to meet the delay budget. If the tree is used as a reverse congruent tree, then the delay constraint applies in both directions. If the tree is used as a directed tree, then the delay constraint applies in the direction of the tree. If it is not possible to meet the delay constraint specified by the Topology sub-TLV, then no tree is installed but a management report is generated.
h. 延迟约束（48位）：跃点子TLV可以指定延迟约束。跃点子TLV的长度指示是否存在延迟约束，因为传递延迟约束的跃点子TLV的长度比没有延迟约束时的长度大六个字节。然后，如果最后六个字节传送单向链路延迟子TLV[RFC7810]，则指定延迟约束。延迟约束可用于指定单个松散树的拓扑子TLV中，即，基本VID与LT ECT算法相关联（第5节）。如果应用了延迟约束，则松散跃点必须适合由跃点子TLV传送的单向链路延迟子TLV的延迟参数指定的延迟预算。如果拓扑子TLV指定单个叶，则前一跳子TLV和当前跳子TLV之间的路径必须满足延迟预算。如果拓扑子TLV指定多个叶，则根和当前跃点子TLV之间的路径必须满足延迟预算。如果树用作反向同余树，则延迟约束将在两个方向上应用。如果树用作有向树，则延迟约束将应用于树的方向。如果不可能满足拓扑子TLV指定的延迟约束，则不会安装树，但会生成管理报告。
The Bandwidth Constraint sub-TLV MAY be included in a Topology sub-TLV (Section 6.1) in order to specify how much available bandwidth is to be provided by the constrained tree. Each loose hop MUST meet the bandwidth constraint. The bandwidth value of the constraint is a total value or it only refers to a single PCP as specified by the sub-TLV. Figure 4 shows the format of the Bandwidth Constraint sub-TLV.
带宽约束子TLV可包括在拓扑子TLV（第6.1节）中，以指定受约束树将提供多少可用带宽。每个松散跃点必须满足带宽限制。约束的带宽值是一个总值，或仅指子TLV指定的单个PCP。图4显示了带宽约束子TLV的格式。
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| PCP |D|P| Res | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Available Bandwidth (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| PCP |D|P| Res | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Available Bandwidth (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Bandwidth Constraint Sub-TLV
图4：带宽约束子TLV
The parameters of the bandwidth constraint are encoded as follows:
带宽约束的参数编码如下：
a. Type (8 bits): The type of the sub-TLV, its value is 23.
a. 类型（8位）：子TLV的类型，其值为23。
b. Length (8 bits): The total number of bytes contained in the Value field. The value of the Length field is 5 bytes.
b. 长度（8位）：值字段中包含的字节总数。长度字段的值为5字节。
c. PCP (4 bits): The Priority Code Point (PCP) parameter identifies the traffic class the Available Bandwidth parameter refers to, if any.
c. PCP（4位）：优先级代码点（PCP）参数标识可用带宽参数所指的流量类别（如果有）。
d. DEI (D) (1 bit): This is the Drop Eligible Indicator (DEI) parameter. If the DEI parameter is clear, then the bandwidth constraint refers to committed information rate. If the DEI parameter is set, then the bandwidth constraint refers to the peak information rate.
d. DEI（D）（1位）：这是下降合格指示器（DEI）参数。如果DEI参数是清除的，那么带宽约束是指提交的信息速率。如果设置了DEI参数，则带宽约束是指峰值信息速率。
e. PCP (P) flag (1 bit): If this flag is set, then the PCP parameter is taken into account.
e. PCP（P）标志（1位）：如果设置了该标志，则会考虑PCP参数。
f. Reserved (Res) (3 bits): The reserved bits MUST be set to 0 on transmission, and the value MUST be ignored on reception.
f. 保留（Res）（3位）：传输时必须将保留位设置为0，接收时必须忽略该值。
g. Available Bandwidth (32 bits): The Available Bandwidth is specific to the traffic class identified by the PCP parameter if the PCP flag is set; otherwise, it is total bandwidth. In line with the bandwidth parameters specified in [RFC5305], the Available Bandwidth is encoded as a 32-bit IEEE floating-point number [IEEE754], and the units are bytes (not bits!) per second. When the Unreserved Bandwidth sub-TLV (sub-TLV type 11 specified by [RFC5305]) is used in a Layer 2 bridge network, the priority value encoded in the Unreserved Bandwidth sub-TLV provides the PCP, i.e., identifies a traffic class (not a setup priority level). Thus, the Available Bandwidth of a traffic class is
g. 可用带宽（32位）：如果设置了PCP标志，则可用带宽特定于PCP参数标识的流量类别；否则，它就是总带宽。根据[RFC5305]中规定的带宽参数，可用带宽被编码为32位IEEE浮点数[IEEE754]，单位为每秒字节（不是位！）。当在第2层网桥网络中使用非保留带宽子TLV（由[RFC5305]指定的子TLV类型11）时，编码在非保留带宽子TLV中的优先级值提供PCP，即，标识业务类别（而不是设置优先级）。因此，业务类别的可用带宽是
easily comparable with the Unreserved Bandwidth stored in the TED for the given traffic class. The bandwidth constraint applies for both directions in case of symmetric explicit trees. Nevertheless, a VID associated with an explicit tree can be made unidirectional by means of the T/R flags belonging to the VID in the Hop sub-TLV (item g. in Section 6.2) of the Edge Bridges. If all the VIDs of the Topology sub-TLV (Section 6.1) are unidirectional and all belong to the traffic class identified by the PCP parameter of the Bandwidth Constraint sub-TLV, then it is enough to meet the bandwidth constraint in the direction applied for those VIDs.
很容易与TED中存储的给定流量类别的无保留带宽进行比较。在对称显式树的情况下，带宽约束适用于两个方向。然而，与显式树相关联的VID可以通过边缘网桥的跃点子TLV（第6.2节中的项目g）中属于VID的T/R标志实现单向。如果拓扑子TLV（第6.1节）的所有VID都是单向的，并且都属于由带宽约束子TLV的PCP参数标识的流量类别，那么就足以满足这些VID应用方向上的带宽约束。
IS-IS PCR MAY be used for recording bandwidth assignment for explicitly placed data traffic in a domain if MSRP is not used within the domain. If MSRP is used in a domain, then only MSRP performs reservations and IS-IS does not. Both MSRP and IS-IS MUST NOT be used to make bandwidth assignments in the same domain.
如果域中未使用MSRP，IS-IS PCR可用于记录域中明确放置的数据流量的带宽分配。如果在域中使用MSRP，则只有MSRP执行保留，is-is不执行保留。MSRP和IS-IS不得用于在同一域中进行带宽分配。
The Bandwidth Assignment sub-TLV can be used to define the amount of bandwidth whose assignment is to be recorded by IS-IS PCR at each hop of the explicit tree described by the corresponding Topology sub-TLV (Section 6.1). The Bandwidth Assignment sub-TLV is used by IS-IS PCR for the recording of bandwidth assignment for a traffic class identified by the PCP parameter of a VLAN tag. If precedence order has to be determined among bandwidth assignments in a domain with multiple PCEs, then IS-IS PCR does it as described below. If the bandwidth assignment specified by the Topology sub-TLV is not possible, e.g., due to overbooking, then bandwidth recording MUST NOT be performed and a management report is generated. If the Topology sub-TLV specifies a new valid explicit tree, then the tree is installed without bandwidth assignment. The Bandwidth Assignment sub-TLV is conveyed by a Topology sub-TLV (Section 6.1). Figure 5 shows the format of the Bandwidth Assignment sub-TLV.
带宽分配子TLV可用于定义其分配将由is-is PCR在相应拓扑子TLV所描述的显式树的每个跃点处记录的带宽量（第6.1节）。is-is PCR使用带宽分配子TLV来记录VLAN标签的PCP参数标识的流量类别的带宽分配。如果必须在具有多个PCE的域中的带宽分配之间确定优先顺序，则IS-IS PCR将按照以下所述进行。如果拓扑子TLV指定的带宽分配不可能，例如由于超售，则不得执行带宽记录并生成管理报告。如果拓扑子TLV指定了新的有效显式树，则安装该树时不会分配带宽。带宽分配子TLV由拓扑子TLV传输（第6.1节）。图5显示了带宽分配子TLV的格式。
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| PCP |D| Imp |R| (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| PCP |D| Imp |R| (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Bandwidth Assignment Sub-TLV
图5：带宽分配子TLV
The parameters of the bandwidth assignment are encoded as follows:
带宽分配的参数编码如下：
a. Type (8 bits): The type of the sub-TLV, its value is 24.
a. 类型（8位）：子TLV的类型，其值为24。
b. Length (8 bits): The total number of bytes contained in the Value field. The value of the Length field is 5 bytes.
b. 长度（8位）：值字段中包含的字节总数。长度字段的值为5字节。
c. PCP (3 bits): The PCP parameter identifies the traffic class for which the bandwidth is to be assigned.
c. PCP（3位）：PCP参数标识要为其分配带宽的通信量类别。
d. DEI (D) (1 bit): This is the Drop Eligible Indicator (DEI) parameter. If the DEI parameter is clear, then the bandwidth assignment is performed for providing the committed information rate. If the DEI parameter is set, then the bandwidth assignment is performed for providing the peak information rate.
d. DEI（D）（1位）：这是下降合格指示器（DEI）参数。如果DEI参数是清除的，则执行带宽分配以提供提交的信息速率。如果设置了DEI参数，则执行带宽分配以提供峰值信息速率。
e. Importance (Imp) (3 bits): This is the Importance parameter for determining precedence order among bandwidth assignments within a PCP as described below. A lower numerical value indicates more important bandwidth assignment within a PCP. The default value of the Importance parameter is 7.
e. 重要性（Imp）（3位）：这是用于确定PCP内带宽分配的优先顺序的重要性参数，如下所述。较低的数值表示PCP内更重要的带宽分配。重要性参数的默认值为7。
f. Reserved (R) (1 bit): The reserved bit MUST be set to 0 on transmission, and the value MUST be ignored on reception.
f. 保留（R）（1位）：传输时必须将保留位设置为0，接收时必须忽略该值。
g. Bandwidth (32 bits): This is the amount of bandwidth to be assigned for the traffic class identified by the PCP parameter. In line with the bandwidth values specified in [RFC5305], the Bandwidth parameter is encoded as a 32-bit IEEE floating-point number [IEEE754], and the units are bytes (not bits!) per second. The bandwidth assignment applies for both directions in case of symmetric explicit trees.
g. 带宽（32位）：这是要为PCP参数标识的流量类别分配的带宽量。根据[RFC5305]中规定的带宽值，带宽参数被编码为32位IEEE浮点数[IEEE754]，单位为每秒字节（不是位！）。在对称显式树的情况下，带宽分配适用于两个方向。
The PCEs are collectively responsible for making a consistent set of bandwidth assignments when IS-IS PCR is used for recording bandwidth allocations. If, despite that, precedence ordering is required among bandwidth assignments, then ordering based on the following parameters MUST be applied:
当IS-IS PCR用于记录带宽分配时，PCE共同负责进行一组一致的带宽分配。尽管如此，如果带宽分配之间需要优先排序，则必须应用基于以下参数的排序：
1. PCP parameter of Bandwidth Assignment sub-TLV,
1. 带宽分配子TLV的PCP参数，
2. Importance parameter of Bandwidth Assignment sub-TLV,
2. 带宽分配子TLV的重要参数，
3. Timestamp sub-TLV (if present in the Topology sub-TLV).
3. 时间戳子TLV（如果拓扑子TLV中存在）。
A bandwidth assignment takes precedence if it has a higher PCP, or a higher Importance within a PCP, or an earlier timestamp in case of equal Importance within a PCP. A bandwidth assignment associated with a timestamp takes precedence over a bandwidth assignment without a timestamp when PCP and Importance of different bandwidth assignments are both equal. If resolution is not possible based on the above parameters or they are not available, e.g., each bandwidth assignment lacks a timestamp or the precedence order has to be determined for the use of a VID, then the item is granted to the PCE whose LSP has the numerically lowest LSP ID.
如果带宽分配具有更高的PCP，或在PCP中具有更高的重要性，或在PCP中具有同等重要性的情况下具有更早的时间戳，则带宽分配优先。当PCP和不同带宽分配的重要性都相等时，与时间戳相关联的带宽分配优先于没有时间戳的带宽分配。如果基于上述参数的分辨率不可能，或者它们不可用，例如，每个带宽分配缺少时间戳，或者必须确定使用VID的优先顺序，则将项目授予LSP具有数字上最低LSP ID的PCE。
The Timestamp sub-TLV MAY be included in a Topology sub-TLV (Section 6.1) in order to provide precedence order among equally important bandwidth assignments within a PCP as described in Section 6.4. Figure 6 shows the format of the Timestamp sub-TLV.
时间戳子TLV可包括在拓扑子TLV（第6.1节）中，以提供PCP内同等重要带宽分配之间的优先顺序，如第6.4节所述。图6显示了时间戳子TLV的格式。
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Timestamp Sub-TLV
图6：时间戳子TLV
The timestamp represents a positive time with respect to the Precision Time Protocol (PTP) epoch, and it is encoded as follows:
时间戳表示与精确时间协议（PTP）历元相关的正时间，其编码如下：
a. Type (8 bits): The type of the sub-TLV; its value is 25.
a. 类型（8位）：子TLV的类型；它的值是25。
b. Length (8 bits): The total number of bytes contained in the Value field. The value of the Length field is 4 bytes.
b. 长度（8位）：值字段中包含的字节总数。长度字段的值为4字节。
c. Time (32 bits): This is the time in units of seconds with respect to the PTP epoch.
c. 时间（32位）：这是以秒为单位的PTP历元时间。
The Timestamp sub-TLV carries the seconds portion of PTP as specified by [IEEE1588]. The epoch is 1970-01-01 00:00:00 TAI (i.e., the PTP time does not include leap seconds).
时间戳子TLV携带[IEEE1588]指定的PTP的秒部分。历元为1970-01-01 00:00:00 TAI（即PTP时间不包括闰秒）。
The application of MRT by [IEEE8021Qca] is discussed in detail in [MRT-IEEE8021qca]. This section describes some special considerations for the use of the MRT Lowpoint algorithm [RFC7811], which are applicable both to the MRT ECT Algorithm and the MRTG ECT Algorithm. This section also explains details related to the MRTG ECT Algorithm and the application of the Topology sub-TLV in particular.
[IEEE8021Qca]对MRT的应用在[MRT-IEEE8021Qca]中进行了详细讨论。本节描述了使用MRT低点算法[RFC7811]的一些特殊注意事项，这些注意事项适用于MRT ECT算法和MRTG ECT算法。本节还详细介绍了MRTG ECT算法的相关细节，以及拓扑子TLV的应用。
IS-IS PCR does not use the MRT Profile specified in [RFC7812]. IS-IS PCR only relies on the algorithm specification in [RFC7811]. Both the MRT ECT Algorithm and the MRTG ECT Algorithm use the MRT Lowpoint algorithm specified in [RFC7811].
IS-IS PCR不使用[RFC7812]中规定的MRT配置文件。IS-IS PCR仅依赖于[RFC7811]中的算法规范。MRT ECT算法和MRTG ECT算法均使用[RFC7811]中规定的MRT低点算法。
The SPB Link Metric sub-TLV [RFC6329] specifies the metric of each link for IS-IS PCR including the MRT algorithms. If the SPB Link Metric values advertised by different ends of an adjacency are different, then the maximum value MUST be used. If equal cost (sub-)paths are found during the MRT computation, then the default tie-breaking specified by Section 11 of [RFC6329] MUST be used, which is based on the lower BridgeID. (The BridgeID is an 8-byte quantity whose upper 2 bytes are the node's BridgePriority and lower 6 bytes are the node's System ID.) Note that if MRTs are used for source-specific multicast (see [IEEE8021Qca] for details), then the bridges have to compute the MRTs of the other bridges in addition to their own in order to be able to install the appropriated FDB entries. (This is similar to the need for all pairs shortest path computation instead of Dijkstra for source-specific shortest path multicast trees.)
SPB链路度量子TLV[RFC6329]指定IS-IS PCR的每个链路的度量，包括MRT算法。如果相邻的不同端所公布的SPB链路度量值不同，则必须使用最大值。如果在MRT计算期间发现等成本（子）路径，则必须使用[RFC6329]第11节规定的默认连接中断，该中断基于较低的BridgeID。（BridgeID是一个8字节的数量，上面的2个字节是节点的BridgePriority，下面的6个字节是节点的系统ID。）注意，如果MRT用于源特定的多播（有关详细信息，请参见[IEEE8021Qca]），然后，桥梁必须计算除自身桥梁外的其他桥梁的MRT，以便能够安装适当的FDB入口。（这类似于源特定最短路径多播树需要所有对最短路径计算，而不是Dijkstra。）
The GADAG is identical for all the MRTs within a network domain, as a consequence of the use of the MRT Lowpoint algorithm [RFC7811]. Therefore, it is beneficial to compute the GADAG by a single entity, which is referred to as the GADAG Computer and is either a PCE or the GADAG Root. If the MRTG ECT Algorithm is applied, then the GADAG MUST be computed only by the GADAG Computer, which then MUST flood
由于使用MRT低点算法[RFC7811]，网络域内所有MRT的GADAG是相同的。因此，通过单个实体（称为GADAG计算机，是PCE或GADAG根）计算GADAG是有益的。如果应用MRTG ECT算法，则GADAG必须仅由GADAG计算机计算，然后必须由GADAG计算机计算
the descriptor Topology sub-TLV of the GADAG. The bridges then compute the MRTs based on the received GADAG.
GADAG的描述符拓扑子TLV。然后，桥梁根据接收到的GADAG计算MRT。
The GADAG computation requires the selection of the GADAG Root. The bridge with the best BridgeID MUST be selected as the GADAG Root, where the numerically lower value indicates the better identifier. The Bridge Priority component of the BridgeID allows the configuration of the GADAG Root by management action. The Bridge Priority is conveyed by the SPB Instance sub-TLV [RFC6329].
GADAG计算需要选择GADAG根。必须选择具有最佳BridgeID的桥接器作为GADAG根，其中数值越小表示标识符越好。BridgeID的网桥优先级组件允许通过管理操作配置GADAG根。网桥优先级由SPB实例子TLV[RFC6329]传送。
The GADAG Computer MUST perform the GADAG computation as specified by the MRT Lowpoint algorithm [RFC7811]. The GADAG Computer then MUST encode the GADAG in a Topology sub-TLV (Section 6.1), which is then flooded throughout the domain. A GADAG is encoded in a Topology sub-TLV by means of directed ear decomposition as follows. A directed ear is a directed point-to-point path whose end points can coincide but no other element of the path is repeated in the ear. Each ear is specified by an ordered list of hops such that the order of hops is according to the direction of the arcs in the GADAG. There are no leaves in a GADAG; hence, the Leaf flag (item c.5. in Section 6.2) is used to mark the end of a topology block. (A GADAG with multiple blocks is illustrated in Figure 8.) The sequence of ears in the Topology sub-TLV is such that the end points of an ear belong to preceding ears. The GADAG Root is not marked by any flag, but the GADAG Root is the first hop in the Topology sub-TLV; correspondingly, the first ear starts and ends with the GADAG Root. MRT Roots MUST be marked by the Root flag (item c.4. in Section 6.2) and all other Edge Bridges are leaves of the given MRTs. If no MRT Root is specified, then each SPT Root is also an MRT Root.
GADAG计算机必须按照MRT低点算法[RFC7811]的规定执行GADAG计算。然后，GADAG计算机必须在拓扑子TLV（第6.1节）中对GADAG进行编码，然后该拓扑子TLV将淹没整个域。GADAG通过定向ear分解在拓扑子TLV中编码，如下所示。有向ear是一条有向点到点路径，其端点可以重合，但该路径的其他元素在ear中不重复。每个ear由有序的跃点列表指定，因此跃点的顺序取决于GADAG中弧的方向。一棵树上没有叶子；因此，叶标志（第6.2节中的项目c.5）用于标记拓扑块的结束。（图8中说明了具有多个块的GADAG。）拓扑子TLV中的耳序列使得耳的端点属于前面的耳。GADAG根没有任何标记，但GADAG根是拓扑子TLV中的第一个跃点；相应地，第一只耳朵以GADAG根开始和结束。MRT根部必须使用根部标志（第6.2节中的项目c.4）进行标记，所有其他边缘桥梁均为给定MRT的叶子。如果未指定MRT根，则每个SPT根也是MRT根。
Figure 7 shows an example GADAG. The figure also illustrates the description of the GADAG; it shows the System ID parameter of the Hop sub-TLV (Section 6.2) and the order of hops in the Topology sub-TLV (Section 6.1).
图7显示了一个示例GADAG。该图还说明了GADAG的说明；它显示了跃点子TLV的系统ID参数（第6.2节）和拓扑子TLV中的跃点顺序（第6.1节）。
Leaf
Hop flag
+-----------+---+
| A | |
+-----------+---+
| B | |
+-----------+---+
| C | |
+-----------+---+
| F | |
[B]<---[A]<---[I] +-----------+---+
| ^ ^ | A | |
| | | +-----------+---+
V | | | C | |
[C]--->[F]--->[H] +-----------+---+
| ^ | D | |
| | +-----------+---+
V | | E | |
[D]--->[E]--->[G] +-----------+---+
| G | |
+-----------+---+
| H | |
+-----------+---+
| I | |
+-----------+---+
| A | |
+-----------+---+
| F | |
+-----------+---+
| H | X |
+-----------+---+
Leaf
Hop flag
+-----------+---+
| A | |
+-----------+---+
| B | |
+-----------+---+
| C | |
+-----------+---+
| F | |
[B]<---[A]<---[I] +-----------+---+
| ^ ^ | A | |
| | | +-----------+---+
V | | | C | |
[C]--->[F]--->[H] +-----------+---+
| ^ | D | |
| | +-----------+---+
V | | E | |
[D]--->[E]--->[G] +-----------+---+
| G | |
+-----------+---+
| H | |
+-----------+---+
| I | |
+-----------+---+
| A | |
+-----------+---+
| F | |
+-----------+---+
| H | X |
+-----------+---+
Figure 7: A GADAG and Its Description; GADAG Root = Node A
Figure 7: A GADAG and Its Description; GADAG Root = Node A
A topology can comprise multiple blocks, like the one illustrated in Figure 8(a). This example topology comprises four blocks as each cut-link is a block. A-B-C-D-E-F is a block, D-G is another block, G-H, and H-J-K are further blocks. A GADAG for this topology is shown in Figure 8(b). Note that two arcs with opposite directions represent a cut-link in a GADAG; see, for example, the cut-link between D and G. The encoding starts with the block (ADAG) involving the GADAG Root as illustrated in Figure 8. The first hop in the Topology sub-TLV is the GADAG Root (node A in this example). The ADAG of the first block is then described using the ear decomposition, as described above. In this example, the first block has been completely traversed at the second occurrence of node A in the GADAG descriptor. The end of a block is indicated by setting the Leaf flag for the last hop of the block, e.g., for the second
拓扑可以包括多个块，如图8（A）所示。此示例拓扑包含四个块，因为每个切割链接都是一个块。A-B-C-D-E-F是一个块，D-G是另一个块，G-H和H-J-K是进一步的块。该拓扑的GADAG如图8（b）所示。请注意，两个方向相反的圆弧表示GADAG中的切割连接；例如，参见D和G之间的切割链接。编码从涉及GADAG根的块（ADAG）开始，如图8所示。拓扑子TLV中的第一个跃点是GADAG根（本例中为节点A）。然后，如上所述，使用ear分解来描述第一块的ADAG。在本例中，第一个块在GADAG描述符中节点A的第二次出现时被完全遍历。通过为块的最后一跳（例如第二跳）设置叶标志来指示块的结束
occurrence of node A in the example GADAG descriptor. The next node that appears in the GADAG descriptor (D in this case) is the localroot for the nodes in the next block. Continuing this process, the Leaf flag is set for the third occurrence of D, the third occurrence of G, and the third occurrence of H, each indicating the end of a block. The first hop of the first block is the GADAG Root, the fist hop in the rest of the blocks is the localroot. The position of the set Leaf flags helps to determine the localroot, which is the next hop. In the example GADAG descriptor, one can determine that A is the localroot for B, C, D, E, F (and A is the GADAG Root). D is the localroot for G. G is the localroot for H. And H is the localroot for J and K. The GADAG Root is assigned a localroot of None.
在示例GADAG描述符中出现节点A。GADAG描述符中出现的下一个节点（本例中为D）是下一个块中节点的localroot。继续该过程，为第三次出现的D、第三次出现的G和第三次出现的H设置叶标志，每个都指示块的结束。第一个块的第一个跃点是GADAG根，其余块的第一个跃点是localroot。设置叶标志的位置有助于确定localroot，这是下一个跃点。在示例GADAG描述符中，可以确定A是B、C、D、E、F的本地根（A是GADAG根）。D是G的localroot。G是H的localroot。H是J和K的localroot。GADAG根的localroot为None。
Block IDs are reconstructed while parsing a Topology sub-TLV specifying a GADAG. The current Block ID starts at 0 and is assigned to the GADAG Root. A node appearing in the GADAG descriptor without a previously assigned Block ID value is assigned the current Block ID. And the current Block ID is incremented by 1 after processing the localroot of a block. Note that the localroot of a block will keep the Block ID of the first block in which it is assigned a Block ID. In the example in Figure 8, A has Block ID=0. B, C, D, E, and F have Block ID=1. G has Block ID=2. H has Block ID=3. J and K have Block ID=4.
在解析指定GADAG的拓扑子TLV时重构块ID。当前块ID从0开始，并分配给GADAG根。在GADAG描述符中出现的节点，如果之前没有分配块ID值，则会分配当前块ID。在处理块的localroot后，当前块ID将增加1。请注意，块的localroot将保留第一个块的块ID，在第一个块中它被分配了块ID。在图8中的示例中，一个块ID=0。B、 C、D、E和F的块ID为1。G具有块ID＝2。H具有块ID＝3。J和K具有块ID=4。
Leaf
Hop flag
[F]--[E] +--[K] +-----------+---+
| | | | | A | |
| | | | +-----------+---+
[A] [D]--[G]--[H] | | B | |
| | | | +-----------+---+
| | | | | C | |
[B]--[C] +--[J] +-----------+---+
| D | |
(a) Topology +-----------+---+
| E | |
+-----------+---+
| F | |
+-----------+---+
| A | X |
+-----------+---+
+-+ +-+ +-+ | D | |
|F|<-|E| +--|K| +-----------+---+
+-+ +-+ | +-+ | G | |
| ^ | ^ +-----------+---+
| | V | | D | X |
V +-+ +-+ +-+ | +-----------+---+
+-+ | |->| |->| | | | G | |
|A| |D| |G| |H| | +-----------+---+
+-+ | |<-| |<-| | | | H | |
| +-+ +-+ +-+ | +-----------+---+
| ^ | | | G | X |
V | | | +-----------+---+
+-+ +-+ | +-+ | H | |
|B|->|C| +->|J| +-----------+---+
+-+ +-+ +-+ | J | |
+-----------+---+
(b) GADAG | K | |
+-----------+---+
| H | X |
+-----------+---+
Leaf
Hop flag
[F]--[E] +--[K] +-----------+---+
| | | | | A | |
| | | | +-----------+---+
[A] [D]--[G]--[H] | | B | |
| | | | +-----------+---+
| | | | | C | |
[B]--[C] +--[J] +-----------+---+
| D | |
(a) Topology +-----------+---+
| E | |
+-----------+---+
| F | |
+-----------+---+
| A | X |
+-----------+---+
+-+ +-+ +-+ | D | |
|F|<-|E| +--|K| +-----------+---+
+-+ +-+ | +-+ | G | |
| ^ | ^ +-----------+---+
| | V | | D | X |
V +-+ +-+ +-+ | +-----------+---+
+-+ | |->| |->| | | | G | |
|A| |D| |G| |H| | +-----------+---+
+-+ | |<-| |<-| | | | H | |
| +-+ +-+ +-+ | +-----------+---+
| ^ | | | G | X |
V | | | +-----------+---+
+-+ +-+ | +-+ | H | |
|B|->|C| +->|J| +-----------+---+
+-+ +-+ +-+ | J | |
+-----------+---+
(b) GADAG | K | |
+-----------+---+
| H | X |
+-----------+---+
(c) GADAG Descriptor
(c) 伽达格描述符
Figure 8: A GADAG with Cut-Links and Its Description; GADAG Root = Node A
图8：带有切割链接的GADAG及其说明；GADAG根=节点A
This document specifies IS-IS sub-TLVs for the control of explicit trees in Layer 2 networks. These sub-TLVs can be also used for the distribution of a centrally computed GADAG or MRTs if MFT-FRR is used.
本文件规定了用于控制第2层网络中显式树的IS-IS子TLV。如果使用MFT-FRR，这些子TLV也可用于集中计算的GADAG或MRT的分配。
This document defines the following IS-IS sub-TLVs within the MT-Capability TLV (type 144). They are listed in the "IS-IS TLV Codepoints" registry.
本文件定义了MT能力TLV（144型）内的以下IS-IS子TLV。它们列在“IS-IS TLV代码点”注册表中。
Type Description Length
---- ---------------------------- --------
21 Topology variable
22 Hop variable
23 Bandwidth Constraint 5
24 Bandwidth Assignment 5
25 Timestamp 4
Type Description Length
---- ---------------------------- --------
21 Topology variable
22 Hop variable
23 Bandwidth Constraint 5
24 Bandwidth Assignment 5
25 Timestamp 4
This document adds no additional security risks to IS-IS, nor does it provide any additional security for IS-IS when used in a configured environment or a single-operator domain such as a data center. IS-IS PCR is not for zero-configuration environments.
本文档不会给IS-IS增加额外的安全风险，也不会为IS-IS在配置的环境或单个运营商域（如数据中心）中使用时提供任何额外的安全性。IS-IS PCR不适用于零配置环境。
Any mechanism that chooses forwarding paths, and allocates resources to those paths, is potentially vulnerable to attack. The security considerations section of [RFC4655] describes the risks associated with the use of PCE for this purpose and should be referred to. Use of any other means to determine paths should only be used after considering similar concerns.
任何选择转发路径并将资源分配给这些路径的机制都可能容易受到攻击。[RFC4655]的“安全注意事项”一节描述了与为此目的使用PCE相关的风险，应参考。只有在考虑类似问题后，才能使用任何其他方法确定路径。
Because the mechanism assumed for distributing tree information relies on IS-IS routing, IS-IS routing security considerations (Section 6, [RFC1195]) and mechanisms (e.g., [RFC5310]) used to authenticate peer advertisements apply.
由于假定用于分发树信息的机制依赖于IS-IS路由，IS-IS路由安全注意事项（第6节，[RFC1195]）和用于认证对等广告的机制（例如，[RFC5310]）适用。
[IEEE8021Qca] IEEE, "IEEE Standard for Local and metropolitan area networks - Bridges and Bridged Networks - Amendment 24: Path Control and Reservation", IEEE 802.1Qca-2015, DOI 10.1109/IEEESTD.2016.7434544, 2016, <https://standards.ieee.org/findstds/ standard/802.1Qca-2015.html>.
[IEEE8021Qca]IEEE，“局域网和城域网IEEE标准-网桥和桥接网络-修改件24：路径控制和保留”，IEEE 802.1Qca-2015，DOI 10.1109/IEEESTD.2016.74345442016<https://standards.ieee.org/findstds/ 标准/802.1Qca-2015.html>。
[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>.
[RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303, DOI 10.17487/RFC5303, October 2008, <http://www.rfc-editor.org/info/rfc5303>.
[RFC5303]Katz，D.，Saluja，R.，和D.Eastlake 3rd，“IS-IS点对点邻接的三方握手”，RFC 5303，DOI 10.17487/RFC5303，2008年10月<http://www.rfc-editor.org/info/rfc5303>.
[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>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008, <http://www.rfc-editor.org/info/rfc5307>.
[RFC5307]Kompella，K.，Ed.和Y.Rekhter，Ed.，“支持通用多协议标签交换（GMPLS）的IS-IS扩展”，RFC 5307，DOI 10.17487/RFC5307，2008年10月<http://www.rfc-editor.org/info/rfc5307>.
[RFC6329] Fedyk, D., Ed., Ashwood-Smith, P., Ed., Allan, D., Bragg, A., and P. Unbehagen, "IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging", RFC 6329, DOI 10.17487/RFC6329, April 2012, <http://www.rfc-editor.org/info/rfc6329>.
[RFC6329]Fedyk，D.，Ed.，Ashwood Smith，P.，Ed.，Allan，D.，Bragg，A.，和P.Unbehagen，“支持IEEE 802.1aq最短路径桥接的IS-IS扩展”，RFC 6329，DOI 10.17487/RFC6329，2012年4月<http://www.rfc-editor.org/info/rfc6329>.
[RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions", RFC 7810, DOI 10.17487/RFC7810, May 2016, <http://www.rfc-editor.org/info/rfc7810>.
[RFC7810]Previdi，S.，Ed.，Giacalone，S.，Ward，D.，Drake，J.，和Q.Wu，“IS-IS交通工程（TE）度量扩展”，RFC 7810，DOI 10.17487/RFC7810，2016年5月<http://www.rfc-editor.org/info/rfc7810>.
[RFC7811] Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and A. Gopalan, "An Algorithm for Computing IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-FRR)", RFC 7811, DOI 10.17487/RFC7811, June 2016, <http://www.rfc-editor.org/info/rfc7811>.
[RFC7811]Enyedi，G.，Csaszar，A.，Atlas，A.，Bowers，C.，和A.Gopalan，“使用最大冗余树计算IP/LDP快速重路由的算法（MRT-FRR）”，RFC 7811，DOI 10.17487/RFC78112016年6月<http://www.rfc-editor.org/info/rfc7811>.
[IEEE1588] IEEE, "IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems", IEEE 1588-2008, DOI 10.1109/IEEESTD.2008.4579760, 2008, <http://standards.ieee.org/findstds/ standard/1588-2008.html>.
[IEEE1588]IEEE，“网络测量和控制系统精密时钟同步协议的IEEE标准”，IEEE 1588-2008，DOI 10.1109/IEEESTD.2008.4579760，2008<http://standards.ieee.org/findstds/ 标准/1588-2008.html>。
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE 754-2008, DOI 10.1109/IEEESTD.2008.4610935, 2008, <http://standards.ieee.org/findstds/ standard/754-2008.html>.
[IEEE754]IEEE，“IEEE浮点运算标准”，IEEE 754-2008，DOI 10.1109/IEEESTD.2008.46109352008<http://standards.ieee.org/findstds/ 标准/754-2008.html>。
[IEEE8021aq] IEEE, "IEEE Standard for Local and metropolitan area networks -- Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks -- Amendment 20: Shortest Path Bridging", IEEE 802.1aq-2012, DOI 10.1109/IEEESTD.2012.6231597, 2012, <https://standards.ieee.org/findstds/ standard/802.1aq-2012.html>.
[IEEE8021aq]IEEE，“局域网和城域网IEEE标准——媒体访问控制（MAC）网桥和虚拟桥接局域网——修改件20：最短路径桥接”，IEEE 802.1aq-2012，DOI 10.1109/IEEESTD.2012.62315972012<https://standards.ieee.org/findstds/ 标准/802.1aq-2012.html>。
[IEEE8021Q] IEEE, "IEEE Standard for Local and metropolitan area networks--Bridges and Bridged Networks", IEEE 802.1Q-2014, DOI 10.1109/IEEESTD.2014.6991462, 2014, <https://standards.ieee.org/findstds/ standard/802.1Q-2014.html>.
[IEEE8021Q]IEEE，“局域网和城域网的IEEE标准——网桥和桥接网络”，IEEE 802.1Q-2014，DOI 10.1109/IEEESTD.2014.6991462，2014<https://standards.ieee.org/findstds/ 标准/802.1Q-2014.html>。
[MRT-IEEE8021qca] Bowers, C. and J. Farkas, "Applicability of Maximally Redundant Trees to IEEE 802.1Qca Path Control and Reservation", Work in Progress, draft-bowers-rtgwg-mrt-applicability-to-8021qca-01, July 2015.
[MRT-IEEE8021qca]Bowers，C.和J.Farkas，“最大冗余树对IEEE 802.1Qca路径控制和保留的适用性”，正在进行的工作，草稿-Bowers-rtgwg-MRT-Applicability-to-8021qca-01，2015年7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, <http://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<http://www.rfc-editor.org/info/rfc1195>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, <http://www.rfc-editor.org/info/rfc4655>.
[RFC4655]Farrel，A.，Vasseur，J.，和J.Ash，“基于路径计算元素（PCE）的体系结构”，RFC 4655，DOI 10.17487/RFC4655，2006年8月<http://www.rfc-editor.org/info/rfc4655>.
[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>.
[RFC7812] Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016, <http://www.rfc-editor.org/info/rfc7812>.
[RFC7812]Atlas，A.，Bowers，C.，和G.Enyedi，“使用最大冗余树的IP/LDP快速重路由架构（MRT-FRR）”，RFC 7812，DOI 10.17487/RFC7812，2016年6月<http://www.rfc-editor.org/info/rfc7812>.
Acknowledgements
致谢
The authors would like to thank Don Fedyk and Eric Gray for their comments and suggestions.
作者要感谢Don Fedyk和Eric Gray的评论和建议。
Authors' Addresses
作者地址
Janos Farkas (editor) Ericsson Konyves Kalman krt. 11/B Budapest 1097 Hungary
Janos Farkas（编辑）爱立信Konyves Kalman krt。11/B布达佩斯1097匈牙利
Email: janos.farkas@ericsson.com
Email: janos.farkas@ericsson.com
Nigel Bragg Ciena 43-51 Worship Street London EC2A 2DX United Kingdom
英国伦敦朝拜街43-51号奈杰尔·布拉格·西纳EC2A 2DX
Email: nbragg@ciena.com
Email: nbragg@ciena.com
Paul Unbehagen Jr Avaya 1300 W. 120th Avenue Westminster, CO 80234 United States
Paul Unbehagen Jr Avaya美国科罗拉多州威斯敏斯特大街120号西1300号，邮编80234
Email: unbehagen@avaya.com
Email: unbehagen@avaya.com
Glenn Parsons Ericsson 349 Terry Fox Drive Ottawa ON, K2K 2V6 Canada
格伦·帕森斯·爱立信加拿大渥太华Terry Fox大道349号，K2K 2V6
Email: glenn.parsons@ericsson.com
Email: glenn.parsons@ericsson.com
Peter Ashwood-Smith Huawei Technologies 303 Terry Fox Drive, Suite 400 Ottawa ON, K2K 3J1 Canada
Peter Ashwood Smith华为技术有限公司加拿大渥太华市Terry Fox大道303号400室K2K 3J1
Email: Peter.AshwoodSmith@huawei.com
Email: Peter.AshwoodSmith@huawei.com
Chris Bowers Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 United States
Chris Bowers Juniper Networks 1194 N.Mathilda Ave.Sunnyvale，加利福尼亚州94089
Email: cbowers@juniper.net
Email: cbowers@juniper.net