Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 6571 Cisco Systems
Category: Informational P. Francois, Ed.
ISSN: 2070-1721 Institute IMDEA Networks
M. Shand
Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 6571 Cisco Systems
Category: Informational P. Francois, Ed.
ISSN: 2070-1721 Institute IMDEA Networks
M. Shand
B. Decraene France Telecom J. Uttaro AT&T N. Leymann M. Horneffer Deutsche Telekom June 2012
B.Decraene法国电信J.Uttaro AT&T N.Leymann M.Horneffer德国电信2012年6月
Loop-Free Alternate (LFA) Applicability in Service Provider (SP) Networks
无环路备用（LFA）在服务提供商（SP）网络中的适用性
Abstract
摘要
In this document, we analyze the applicability of the Loop-Free Alternate (LFA) method of providing IP fast reroute in both the core and access parts of Service Provider networks. We consider both the link and node failure cases, and provide guidance on the applicability of LFAs to different network topologies, with special emphasis on the access parts of the network.
在本文中，我们分析了在服务提供商网络的核心和接入部分提供IP快速重路由的无环路备用（LFA）方法的适用性。我们考虑链路和节点故障的情况下，并提供指导的LFAS的适用性不同的网络拓扑结构，特别强调的接入部分的网络。
Status of This Memo
关于下段备忘
This document is not an Internet Standards Track specification; it is published for informational purposes.
本文件不是互联网标准跟踪规范；它是为了提供信息而发布的。
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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。并非IESG批准的所有文件都适用于任何级别的互联网标准；见RFC 5741第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6571.
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/rfc6571.
Copyright Notice
版权公告
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2012 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. Terminology .....................................................4
3. Access Network ..................................................6
3.1. Triangle ...................................................8
3.1.1. E1C1 Failure ........................................8
3.1.2. C1E1 Failure ........................................9
3.1.3. uLoop ...............................................9
3.1.4. Conclusion .........................................10
3.2. Full Mesh .................................................10
3.2.1. E1A1 Failure .......................................10
3.2.2. A1E1 Failure .......................................11
3.2.3. A1C1 Failure .......................................11
3.2.4. C1A1 Failure .......................................12
3.2.5. uLoop ..............................................12
3.2.6. Conclusion .........................................12
3.3. Square ....................................................13
3.3.1. E1A1 Failure .......................................13
3.3.2. A1E1 Failure .......................................14
3.3.3. A1C1 Failure .......................................15
3.3.4. C1A1 Failure .......................................15
3.3.5. Conclusion .........................................17
3.3.6. A Square Might Become a Full Mesh ..................17
3.3.7. A Full Mesh Might Be More Economical Than a
Square .............................................17
3.4. Extended U ................................................18
3.4.1. E1A1 Failure .......................................19
3.4.2. A1E1 Failure .......................................20
3.4.3. A1C1 Failure .......................................20
3.4.4. C1A1 Failure .......................................21
3.4.5. Conclusion .........................................21
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Access Network ..................................................6
3.1. Triangle ...................................................8
3.1.1. E1C1 Failure ........................................8
3.1.2. C1E1 Failure ........................................9
3.1.3. uLoop ...............................................9
3.1.4. Conclusion .........................................10
3.2. Full Mesh .................................................10
3.2.1. E1A1 Failure .......................................10
3.2.2. A1E1 Failure .......................................11
3.2.3. A1C1 Failure .......................................11
3.2.4. C1A1 Failure .......................................12
3.2.5. uLoop ..............................................12
3.2.6. Conclusion .........................................12
3.3. Square ....................................................13
3.3.1. E1A1 Failure .......................................13
3.3.2. A1E1 Failure .......................................14
3.3.3. A1C1 Failure .......................................15
3.3.4. C1A1 Failure .......................................15
3.3.5. Conclusion .........................................17
3.3.6. A Square Might Become a Full Mesh ..................17
3.3.7. A Full Mesh Might Be More Economical Than a
Square .............................................17
3.4. Extended U ................................................18
3.4.1. E1A1 Failure .......................................19
3.4.2. A1E1 Failure .......................................20
3.4.3. A1C1 Failure .......................................20
3.4.4. C1A1 Failure .......................................21
3.4.5. Conclusion .........................................21
3.5. Dual-Plane Core and Its Impact on the Access LFA
Analysis ..................................................21
3.6. Two-Tiered IGP Metric Allocation ..........................22
3.7. uLoop Analysis ............................................22
3.8. Summary ...................................................23
4. Core Network ...................................................24
4.1. Simulation Framework ......................................25
4.2. Data Set ..................................................26
4.3. Simulation Results ........................................26
5. Core and Access Protection Schemes Are Independent .............27
6. Simplicity and Other LFA Benefits ..............................27
7. Capacity Planning with LFA in Mind .............................28
7.1. Coverage Estimation - Default Topology ....................28
7.2. Coverage Estimation in Relation to Traffic ................29
7.3. Coverage Verification for a Given Set of Demands ..........29
7.4. Modeling - What-If Scenarios - Coverage Impact ............29
7.5. Modeling - What-If Scenarios - Load Impact ................30
7.6. Discussion on Metric Recommendations ......................31
8. Security Considerations ........................................32
9. Conclusions ....................................................32
10. Acknowledgments ...............................................32
11. References ....................................................33
11.1. Normative References .....................................33
11.2. Informative References ...................................33
3.5. Dual-Plane Core and Its Impact on the Access LFA
Analysis ..................................................21
3.6. Two-Tiered IGP Metric Allocation ..........................22
3.7. uLoop Analysis ............................................22
3.8. Summary ...................................................23
4. Core Network ...................................................24
4.1. Simulation Framework ......................................25
4.2. Data Set ..................................................26
4.3. Simulation Results ........................................26
5. Core and Access Protection Schemes Are Independent .............27
6. Simplicity and Other LFA Benefits ..............................27
7. Capacity Planning with LFA in Mind .............................28
7.1. Coverage Estimation - Default Topology ....................28
7.2. Coverage Estimation in Relation to Traffic ................29
7.3. Coverage Verification for a Given Set of Demands ..........29
7.4. Modeling - What-If Scenarios - Coverage Impact ............29
7.5. Modeling - What-If Scenarios - Load Impact ................30
7.6. Discussion on Metric Recommendations ......................31
8. Security Considerations ........................................32
9. Conclusions ....................................................32
10. Acknowledgments ...............................................32
11. References ....................................................33
11.1. Normative References .....................................33
11.2. Informative References ...................................33
In this document, we analyze the applicability of the Loop-Free Alternate (LFA) [RFC5714] [RFC5286] method of providing IP fast reroute (IPFRR) in both the core and access parts of Service Provider (SP) networks. We consider both the link and node failure cases, and provide guidance on the applicability of LFAs to different network topologies, with special emphasis on the access parts of the network.
在本文中，我们分析了在服务提供商（SP）网络的核心和接入部分提供IP快速重路由（IPFRR）的无环路备用（LFA）[RFC5714][RFC5286]方法的适用性。我们考虑链路和节点故障的情况下，并提供指导的LFAS的适用性不同的网络拓扑结构，特别强调的接入部分的网络。
We first introduce the terminology used in this document in Section 2. In Section 3, we describe typical access network designs, and we analyze them for LFA applicability. In Section 4, we describe a simulation framework for the study of LFA applicability in SP core networks, and present results based on various SP networks. We then emphasize the independence between protection schemes used in the core and at the access level of the network. Finally, we discuss the key benefits of the LFA method, which stem from its simplicity, and we draw some conclusions.
我们首先在第2节介绍本文件中使用的术语。在第3节中，我们描述了典型的接入网设计，并分析了LFA的适用性。在第4节中，我们描述了一个模拟框架，用于研究LFA在SP核心网络中的适用性，并给出了基于各种SP网络的结果。然后，我们强调在核心和网络访问级别使用的保护方案之间的独立性。最后，我们讨论了LFA方法的主要优点，这些优点源于其简单性，并得出了一些结论。
We use IS-IS [RFC1195] [IS-IS] as a reference. It is assumed that normal routing (i.e., when traffic is not being fast-rerouted around a failure) occurs along the shortest path. The analysis is equally applicable to OSPF [RFC2328] [RFC5340].
我们使用IS-IS[RFC1195][IS-IS]作为参考。假设沿最短路径发生正常路由（即，当故障周围的流量没有快速重新路由时）。该分析同样适用于OSPF[RFC2328][RFC5340]。
A per-prefix LFA for a destination D at a node S is a pre-computed backup IGP next hop for that destination. This backup IGP next hop can be link-protecting or node-protecting. In this document, we assume that all links to be protected with LFAs are point-to-point.
节点S处的目的地D的每前缀LFA是该目的地的预计算备份IGP下一跳。此备份IGP下一跳可以是链路保护或节点保护。在本文档中，我们假设所有使用LFA保护的链接都是点对点的。
Link-protecting: A neighbor N is a link-protecting per-prefix LFA for S's route to D if equation eq1 is satisfied. This is in line with the definition of an LFA in [RFC5714].
链路保护：如果满足等式eq1，则邻居N是针对S到D的路由的每个前缀LFA的链路保护。这与[RFC5714]中LFA的定义一致。
eq1: ND < NS + SD
eq1: ND < NS + SD
where XY refers to the IGP distance from X to Y
其中XY指从X到Y的IGP距离
Equation eq1
方程eq1
Node-protecting: A neighbor N is a node-protecting LFA for S's route to D with initial IGP next hop F if N is a link-protecting LFA for D and equation eq2 is satisfied. This is in line with the definition of a Loop-Free Node-Protecting Alternate (also known as a node-protecting LFA) in [RFC5714].
节点保护：如果N是保护D的LFA的链路且满足等式eq2，则邻居N是保护S到D的路由的LFA的节点，具有初始IGP下一跳F。这与[RFC5714]中无环路节点保护备用（也称为节点保护LFA）的定义一致。
eq2: ND < NF + FD
eq2: ND < NF + FD
Equation eq2
方程eq2
De facto node-protecting LFA: This is a link-protecting LFA that turns out to be node-protecting. This occurs in cases illustrated by the following examples:
事实上的节点保护LFA：这是一个链接保护LFA，结果是节点保护。以下示例说明了这种情况：
o The LFA candidate that is picked by S actually satisfies Equation eq2, but S did not verify that property. The show command issued by the operator would not indicate this LFA as "node-protecting", while in practice (de facto), it is.
o 由S选择的LFA候选者实际上满足方程eq2，但S未验证该属性。操作员发出的show命令不会将此LFA指示为“节点保护”，但实际上（事实上）是这样。
o A cascading effect of multiple LFAs can also provide de facto node protection. Equation eq2 is not satisfied, but the combined activation of LFAs by some other neighbors of the failing node F provides (de facto) node protection. In other words, it puts the data plane in a state such that packets forwarded by S ultimately
o 多个LFA的级联效应也可以提供事实上的节点保护。等式eq2不满足，但故障节点F的一些其他邻居对lfa的组合激活提供（事实上的）节点保护。换句话说，它使数据平面处于这样一种状态，即数据包最终由S转发
reach a neighbor of F that has a node-protecting LFA. Note that in this case, S cannot indicate the node-protecting behavior of the repair without running additional computations.
到达具有保护LFA的节点的F的邻居。注意，在这种情况下，如果不运行额外的计算，S不能指示修复的节点保护行为。
Per-link LFA: A per-link LFA for the link SF is one pre-computed backup IGP next hop for all of the destinations reached through SF. This is a neighbor of the repairing node that is a per-prefix LFA for all of the destinations that the repairing node reaches through SF. Note that such a per-link LFA exists if S has a per-prefix LFA for destination F.
每链路LFA：链路SF的每链路LFA是通过SF到达的所有目的地的一个预先计算的备份IGP下一跳。这是修复节点的邻居，是修复节点通过SF到达的所有目的地的每前缀LFA。注意，如果S对目的地F具有每前缀LFA，则存在这样的每链路LFA。
D
/ \
10 / \ 10
/ \
G H----------.
| | |
1 | 1 | |
| | |
B C | 10
| |\ |
| | \ |
| | \ 6 |
| | \ |
7 | 10 | E F
| | / /
| | / 6 / 5
| | / /
| |/ /
A-------S-----/
7
D
/ \
10 / \ 10
/ \
G H----------.
| | |
1 | 1 | |
| | |
B C | 10
| |\ |
| | \ |
| | \ 6 |
| | \ |
7 | 10 | E F
| | / /
| | / 6 / 5
| | / /
| |/ /
A-------S-----/
7
Figure 1: Example 1
图1：示例1
In Figure 1, considering the protection of link SC, we can see that A, E, and F are per-prefix LFAs for destination D, as none of them use S to reach D.
在图1中，考虑到链路SC的保护，我们可以看到A、E和F是目的地D的每个前缀lfa，因为它们都不使用S到达D。
For destination D, A and F are node-protecting LFAs, as they do not reach D through node C, while E is not node-protecting for S, as it reaches D through C.
对于目的地D，A和F是节点保护LFA，因为它们不通过节点C到达D，而E不是节点保护LFA，因为它通过节点C到达D。
If S does not compute and select node-protecting LFAs, there is a chance that S picks the non-node-protecting LFA E, although A and F were node-protecting LFAs. If S enforces the selection of node-protecting LFAs, then in the case of the single failure of link SC,
如果S不计算并选择节点保护LFA，则S有可能选择非节点保护LFA，尽管a和F是节点保护LFA。如果S强制选择节点保护LFA，则在链路SC单一故障的情况下，
S will first activate its LFA, deviate traffic addressed to D along S-A-B-G-D and/or S-F-H-D, and then converge to its post-convergence optimal path S-E-C-H-D.
S将首先激活其LFA，沿S-A-B-G-D和/或S-F-H-D偏离发送给D的流量，然后收敛到其收敛后的最优路径S-E-C-H-D。
A reaches C via S; thus, A is not a per-link LFA for link SC. E reaches C through link EC; thus, E is a per-link LFA for link SC. This per-link LFA does not provide de facto node protection. Upon failure of node C, S would fast-reroute D-destined packets to its per-link LFA (= E). E would itself detect the failure of EC; hence, it would activate its own per-link LFA (= S). Traffic addressed to D would be trapped in a loop; hence, there is no de facto node protection behavior.
A通过S到达C；因此，A不是链路SC的每链路LFA。E通过链路EC到达C；因此，E是链路SC的每链路LFA。该每链路LFA不提供事实上的节点保护。当节点C发生故障时，S将快速地将以D为目的地的分组重新路由到其每链路LFA（=E）。E本身会检测到EC的故障；因此，它将激活自己的每链路LFA（=S）。发送给D的流量将被困在一个环路中；因此，不存在事实上的节点保护行为。
If there were a link between E and F that E would pick as its LFA for destination D, then E would provide de facto node protection for S, as upon the activation of its LFA, S would deviate traffic addressed to D towards E. In turn, E deviates that traffic to F, which does not reach D through C.
如果E和F之间存在一条链路，E将选择该链路作为其目的地D的LFA，那么E将为S提供事实上的节点保护，因为一旦激活其LFA，S将使发送给D的流量偏离E。反过来，E将该流量偏离到F，而F不会通过C到达D。
F is a per-link LFA for link SC, as F reaches C via H. This per-link LFA is de facto node-protecting for destination D, as F reaches D via F-H-D.
F是链路SC的每链路LFA，因为F通过H到达C。当F通过F-H-D到达D时，该每链路LFA实际上是对目的地D的节点保护。
Micro-Loop (uLoop): the occurrence of a transient forwarding loop during a routing transition (as defined in [RFC5715]).
微循环（uLoop）：在路由转换期间发生的瞬态转发循环（如[RFC5715]中所定义）。
In Figure 1, the loss of link SE cannot create any uLoop, because of the following:
在图1中，由于以下原因，链路SE的丢失无法创建任何uLoop：
1. The link is only used to reach destination E.
1. 该链路仅用于到达目的地E。
2. S is the sole node changing its path to E upon link SE failure.
2. S是在链路SE故障时将其路径更改为E的唯一节点。
3. S's shortest path to E after the failure goes via C.
3. 故障后S到E的最短路径通过C。
4. C's best path to E (before and after link SC failure) is via CE.
4. C到E的最佳路径（链路SC故障前后）是通过CE。
On the other hand, upon failure of link AB, a micro-loop may form for traffic destined to B. Indeed, if A updates its Forwarding Information Base (FIB) before S, A will reroute B-destined traffic towards S, while S is still forwarding this traffic to A.
另一方面，在链路AB发生故障时，可能会为目的地为B的业务形成一个微环路。实际上，如果a在S之前更新其转发信息库（FIB），a将把目的地为B的业务重新路由到S，而S仍在将该业务转发到a。
The access part of the network often represents the majority of the nodes and links. It is organized in several tens or more of regions interconnected by the core network. Very often, the core acts as an IS-IS level-2 domain (OSPF area 0), while each access region is
网络的接入部分通常代表大多数节点和链路。它由几十个或更多的区域组成，通过核心网络相互连接。通常，核心充当IS-IS二级域（OSPF区域0），而每个访问区域是
confined in an IS-IS level-1 domain (OSPF non-0 area). Very often, the network topology within each access region is derived from a unique template common across the whole access network. Within an access region itself, the network is made of several aggregation regions, each following the same interconnection topologies.
受限于IS-IS 1级域（OSPF非0区域）。通常，每个接入区域内的网络拓扑都是从整个接入网络中通用的唯一模板导出的。在接入区域内，网络由几个聚合区域组成，每个聚合区域遵循相同的互连拓扑。
For these reasons, in the next sections, we base the analysis of the LFA applicability in a single access region, with the following assumptions:
出于这些原因，在下一节中，我们将在以下假设的基础上分析LFA在单个接入区域中的适用性：
o Two routers (C1 and C2) provide connectivity between the access region and the rest of the network. If a link connects these two routers in the region area, then it has a symmetric IGP metric c.
o 两个路由器（C1和C2）提供接入区域和网络其余部分之间的连接。如果一条链路连接区域中的这两个路由器，则它具有对称IGP度量c。
o We analyze a single aggregation region within the access region. Two aggregation routers (A1 and A2) interconnect the aggregation region to the two routers C1 and C2 for the analyzed access region. If a link connects A1 to A2, then it has a symmetric IGP metric a. If a link connects a router A to a router C, then for the sake of generality we will call d the metric for the directed link CA and u the metric for the directed link AC.
o 我们分析访问区域中的单个聚合区域。两个聚合路由器（A1和A2）将聚合区域互连到所分析的接入区域的两个路由器C1和C2。如果链路连接A1到A2，则它具有对称IGP度量a。如果一条链路将路由器a连接到路由器C，那么为了通用性，我们将d称为定向链路CA的度量，u称为定向链路AC的度量。
o We analyze two edge routers, E1 and E2, in the access region. Each is dual-homed directly either to C1 and C2 (Section 3.1) or to A1 and A2 (Sections 3.2, 3.3, and 3.4). The directed link metric between Cx/Ax and Ey is d and u in the opposite direction.
o 我们分析了接入区域中的两个边缘路由器E1和E2。每一个都是直接与C1和C2（第3.1节）或A1和A2（第3.2节、第3.3节和第3.4节）双宿的。Cx/Ax和Ey之间的定向链路度量是相反方向的d和u。
o We assume a multi-level IGP domain. The analyzed access region forms a level-1 (L1) domain. The core is the level-2 (L2) domain. We assume that the link between C1 and C2, if it exists, is configured as L1L2. We assume that the loopbacks of the C routers are part of the L2 topology. L1 routers learn about them as propagated routes (L2=>L1 with the Down bit set). We remind the reader that if an L1L2 router learns about X/x as an L1 path P1, an L2 path P2, and an L1L2 path P12, then it will prefer path P1. If path P1 is lost, then it will prefer path P2.
o 我们假设一个多级IGP域。所分析的接入区域形成1级（L1）域。核心是二级（L2）领域。我们假设C1和C2之间的链路（如果存在）配置为L1L2。我们假设C路由器的环回是L2拓扑的一部分。L1路由器将它们作为传播路由来了解（L2=>L1和下行位集）。我们提醒读者，如果L1L2路由器知道X/X是L1路径P1、L2路径P2和L1L2路径P12，那么它会选择路径P1。如果路径P1丢失，则它将首选路径P2。
o We assume that all of the C, A, and E routers may be connected to customers; hence, we analyze LFA coverage for the loopbacks of each type of node.
o 我们假设所有的C、A和E路由器都可以连接到客户；因此，我们分析了每种类型节点的环回LFA覆盖率。
o We assume that no useful traffic is directed to router-to-router subnets; hence, we do not analyze LFA applicability for such subnets.
o 我们假设没有有用的流量被定向到路由器到路由器子网；因此，我们不分析LFA对此类子网的适用性。
o A prefix P models an important IGP destination that is not present in the local access region. The IGP metric from C1 to P is x, and the metric from C2 to P is x + e.
o 前缀P为本地接入区域中不存在的重要IGP目的地建模。从C1到P的IGP度量是x，从C2到P的度量是x+e。
o We analyze LFA coverage against all link and node failures within the access region.
o 我们针对接入区域内的所有链路和节点故障分析LFA覆盖率。
o WxYz refers to the link from Wx to Yz.
o WxYz是指从Wx到Yz的链接。
o We assume that c < d + u and a < d + u (a commonly agreed-upon design rule).
o 我们假设c<d+u和a<d+u（一个公认的设计规则）。
o In the square access design (Section 3.3), we assume that c < a (a commonly agreed-upon design rule).
o 在方形通道设计（第3.3节）中，我们假设c<a（一个普遍同意的设计规则）。
o We analyze the most frequent topologies found in an access region.
o 我们分析访问区域中最常见的拓扑。
o We first analyze per-prefix LFA applicability and then per-link.
o 我们首先分析每个前缀LFA的适用性，然后分析每个链接。
o The topologies are symmetric with respect to a vertical axis; hence, we only detail the logic for the link and node failures of the left half of the topology.
o 拓扑相对于垂直轴对称；因此，我们仅详细说明拓扑左半部分的链路和节点故障逻辑。
We describe the LFA applicability for the failures of C1E1, E1, and C1 (Figure 2).
我们描述了LFA对C1E1、E1和C1故障的适用性（图2）。
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ /|
| \ / |
d/u| \ |d/u
| / \ |
|/ \|
E1 E2
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ /|
| \ / |
d/u| \ |d/u
| / \ |
|/ \|
E1 E2
Figure 2: Triangle
图2：三角形
Three destinations are impacted by this link failure: C1, E2, and P.
三个目的地受到此链路故障的影响：C1、E2和P。
The LFA for destination C1 is C2, because eq1: c < d + u. Node protection for route C1 is not applicable. (If C1 goes down, traffic destined to C1 is lost anyway.)
目的地C1的LFA为C2，因为eq1:c<d+u。路由C1的节点保护不适用。（如果C1故障，则前往C1的通信量仍将丢失。）
The LFA to E2 is via C2, because eq1: d < d + u + d. It is node-protecting, because eq2: d < c + d.
LFA到E2通过C2，因为eq1:d<d+u+d。它是节点保护，因为eq2:d<c+d。
The LFA to P is via C2, because c < d + u. It is node-protecting if eq2: x + e < x + c, i.e., if e < c. This relationship between e and c is an important aspect of the analysis, which is discussed in detail in Sections 3.5 and 3.6.
LFA到P是通过C2，因为c<d+u。如果eq2:x+e<x+c，即如果e<c，则为节点保护。e和c之间的关系是分析的一个重要方面，第3.5节和第3.6节对此进行了详细讨论。
Conclusion: All important intra-PoP (Point of Presence) routes with primary interface E1C1 benefit from LFA link and node protection. All important inter-PoP routes with primary interface E1C1 benefit from LFA link protection, and also from node protection if e < c.
结论：所有具有主要接口E1C1的重要PoP（存在点）内路由都受益于LFA链路和节点保护。所有具有主接口E1C1的重要PoP间路由都受益于LFA链路保护，如果e<c，也受益于节点保护。
We have a per-prefix LFA to C1; hence, we have a per-link LFA for link E1C1. All impacted destinations are protected against link failure. In the case of C1 node failure, the traffic to C1 is lost (by definition), the traffic to E2 is de facto protected against node failure, and the traffic to P is de facto protected when e < c.
我们有一个从LFA到C1的前缀；因此，我们为链路E1C1提供了每链路LFA。所有受影响的目的地都受到了链路故障保护。在C1节点故障的情况下，到C1的通信量丢失（根据定义），到E2的通信量事实上受到节点故障的保护，当e<c时，到P的通信量事实上受到保护。
C1 only has one primary route via C1E1: the route to E1 (because c < d + u).
C1只有一条通过C1E1的主路由：到E1的路由（因为c<d+u）。
C1's LFA to E1 is via C2, because eq1: d < c + d.
C1的LFA到E1通过C2，因为eq1:d<c+d。
Node protection upon E1's failure is not applicable, as the only impacted traffic is sinked at E1 and hence is lost anyway.
E1故障时的节点保护不适用，因为唯一受影响的流量在E1处下沉，因此无论如何都会丢失。
Conclusion: All important routes with primary interface C1E1 benefit from LFA link protection. Node protection is not applicable.
结论：所有主要接口为C1E1的重要路由均受益于LFA链路保护。节点保护不适用。
We have a per-prefix LFA to E1; hence, we have a per-link LFA for link C1E1. De facto node protection is not applicable.
我们有一个从LFA到E1的前缀；因此，对于链路C1E1，我们具有每链路LFA。事实上的节点保护不适用。
The IGP convergence cannot create any uLoop. See Section 3.7.
IGP收敛无法创建任何uLoop。见第3.7节。
All important intra-PoP routes benefit from LFA link and node protection or de facto node protection. All important inter-PoP routes benefit from LFA link protection. De facto node protection is ensured if e < c. (This is particularly the case for dual-plane core or two-tiered IGP metric design; see Sections 3.5 and 3.6.)
所有重要的内部PoP路由都受益于LFA链路和节点保护或事实上的节点保护。所有重要的PoP间路由都受益于LFA链路保护。如果e<c，则可确保事实上的节点保护。（这尤其适用于双平面铁芯或双层IGP公制设计；参见第3.5节和第3.6节。）
The IGP convergence does not cause any uLoop.
IGP收敛不会导致任何uLoop。
Per-link LFAs and per-prefix LFAs provide the same protection benefits.
每链路LFA和每前缀LFA提供相同的保护优势。
We describe the LFA applicability for the failures of C1A1, A1E1, E1, A1, and C1 (Figure 3).
我们描述了LFA对C1A1、A1E1、E1、A1和C1故障的适用性（图3）。
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ /|
| \ / |
d/u | \ | d/u
| / \ |
|/ \|
A1--a--A2
|\ /|
| \ / |
d/u| \ |d/u
| / \ |
|/ \|
E1 E2
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ /|
| \ / |
d/u | \ | d/u
| / \ |
|/ \|
A1--a--A2
|\ /|
| \ / |
d/u| \ |d/u
| / \ |
|/ \|
E1 E2
Figure 3: Full Mesh
图3：全网格
Four destinations are impacted by this link failure: A1, C1, E2, and P.
四个目的地受到此链路故障的影响：A1、C1、E2和P。
The LFA for A1 is A2: eq1: a < d + u. Node protection for route A1 is not applicable. (If A1 goes down, traffic to A1 is lost anyway.)
A1的LFA为A2:eq1:a<d+u。路由A1的节点保护不适用。（如果A1下降，到A1的交通将丢失。）
The LFA for C1 is A2: eq1: u < d + u + u. Node protection for route C1 is guaranteed: eq2: u < a + u.
C1的LFA为A2:eq1:u<d+u+u。保证路由C1的节点保护：eq2:u<a+u。
The LFA to E2 is via A2: eq1: d < d + u + d. Node protection is guaranteed: eq2: d < a + d.
LFA到E2通过A2:eq1:d<d+u+d。保证节点保护：eq2:d<a+d。
The LFA to P is via A2: eq1: u + x < d + u + u + x. Node protection is guaranteed: eq2: u + x < a + u + x.
LFA到P通过A2:eq1:u+x<d+u+u+x。保证节点保护：eq2:u+x<a+u+x。
Conclusion: All important intra-PoP and inter-PoP routes with primary interface E1A1 benefit from LFA link and node protection.
结论：所有具有主要接口E1A1的重要PoP内和PoP间路由都受益于LFA链路和节点保护。
We have a per-prefix LFA to A1; hence, we have a per-link LFA for link E1A1. All impacted destinations are protected against link failure. De facto node protection is provided for all destinations (except to A1, which is not applicable).
我们有一个从LFA到A1的前缀；因此，我们为链路E1A1提供了每链路LFA。所有受影响的目的地都受到了链路故障保护。为所有目的地提供事实上的节点保护（A1除外，A1不适用）。
A1 only has one primary route via A1E1: the route to E1 (because a < d + u).
A1只有一条通过A1E1的主路由：到E1的路由（因为a<d+u）。
A1's LFA to E1 is via A2: eq1: d < a + d.
A1的LFA至E1通过A2:eq1:d<a+d。
Node protection upon E1's failure is not applicable, as the only impacted traffic is sinked at E1 and hence is lost anyway.
E1故障时的节点保护不适用，因为唯一受影响的流量在E1处下沉，因此无论如何都会丢失。
Conclusion: All important routes with primary interface A1E1 benefit from LFA link protection. Node protection is not applicable.
结论：所有主要接口为A1E1的重要路由均受益于LFA链路保护。节点保护不适用。
We have a per-prefix LFA to E1; hence, we have a per-link LFA for link C1E1. De facto node protection is not applicable.
我们有一个从LFA到E1的前缀；因此，对于链路C1E1，我们具有每链路LFA。事实上的节点保护不适用。
Two destinations are impacted by this link failure: C1 and P.
两个目的地受到此链路故障的影响：C1和P。
The LFA for C1 is C2, because eq1: c < d + u. Node protection for route C1 is not applicable. (If C1 goes down, traffic to C1 is lost anyway.)
C1的LFA为C2，因为eq1:c<d+u。路由C1的节点保护不适用。（如果C1下降，到C1的流量仍然会丢失。）
The LFA for P is via C2, because c < d + u. It is de facto protected against node failure if eq2: x + e < x + c.
P的LFA通过C2，因为c<d+u。如果eq2:x+e<x+c，它实际上可以防止节点故障。
Conclusion: All important intra-PoP routes with primary interface A1C1 benefit from LFA link protection. (Node protection is not applicable.) All important inter-PoP routes with primary interface E1C1 benefit from LFA link protection (and from de facto node protection if e < c).
结论：所有主要接口为A1C1的重要PoP内路由均受益于LFA链路保护。（节点保护不适用。）具有主接口E1C1的所有重要PoP间路由都受益于LFA链路保护（如果e<c，则受益于事实上的节点保护）。
We have a per-prefix LFA to C1; hence, we have a per-link LFA for link A1C1. All impacted destinations are protected against link failure. In the case of C1 node failure, the traffic to C1 is lost (by definition), and the traffic to P is de facto node protected if e < c.
我们有一个从LFA到C1的前缀；因此，对于链路A1C1，我们有一个每链路LFA。所有受影响的目的地都受到了链路故障保护。在C1节点故障的情况下，到C1的通信量丢失（根据定义），如果e<c，到P的通信量实际上受到节点保护。
C1 has three routes via C1A1: A1, E1, and E2. E2 behaves like E1 and hence is not analyzed further.
C1通过C1A1有三条路由：A1、E1和E2。E2的行为类似于E1，因此不作进一步分析。
C1's LFA to A1 is via C2, because eq1: d < c + d. Node protection upon A1's failure is not applicable, as the traffic to A1 is lost anyway.
C1的LFA到A1通过C2，因为eq1:d<c+d。A1故障时的节点保护不适用，因为到A1的流量无论如何都会丢失。
C1's LFA to E1 is via A2: eq1: d < u + d + d. Node protection upon A1's failure is guaranteed, because eq2: d < a + d.
C1的LFA至E1通过A2:eq1:d<u+d+d。保证A1故障时的节点保护，因为eq2:d<a+d。
Conclusion: All important routes with primary interface C1A1 benefit from LFA link protection. Node protection is guaranteed where applicable.
结论：所有主要接口为C1A1的重要路由均受益于LFA链路保护。在适用的情况下保证节点保护。
We have a per-prefix LFA to A1; hence, we have a per-link LFA for link C1E1. De facto node protection is available.
我们有一个从LFA到A1的前缀；因此，对于链路C1E1，我们具有每链路LFA。事实上的节点保护是可用的。
The IGP convergence cannot create any uLoop. See Section 3.7.
IGP收敛无法创建任何uLoop。见第3.7节。
All important intra-PoP routes benefit from LFA link and node protection.
所有重要的内部PoP路由都受益于LFA链路和节点保护。
All important inter-PoP routes benefit from LFA link protection. They benefit from node protection upon failure of A nodes. They benefit from node protections upon failure of C nodes if e < c. (This is particularly the case for dual-plane core or two-tiered IGP metric design; see Sections 3.5 and 3.6.)
所有重要的PoP间路由都受益于LFA链路保护。在节点发生故障时，它们可以从节点保护中获益。如果e<C，则在C节点发生故障时，它们将受益于节点保护。（这尤其适用于双平面铁芯或双层IGP公制设计；参见第3.5节和第3.6节。）
The IGP convergence does not cause any uLoop.
IGP收敛不会导致任何uLoop。
Per-link LFAs and per-prefix LFAs provide the same protection benefits.
每链路LFA和每前缀LFA提供相同的保护优势。
We describe the LFA applicability for the failures of C1A1, A1E1, E1, A1, and C1 (Figure 4).
我们描述了LFA对C1A1、A1E1、E1、A1和C1故障的适用性（图4）。
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ | \
| \ | +-------+
d/u | \ | \
| +-|-----+ \
| | \ \
A1--a--A2 A3--a--A4
|\ /| | /
| \ / | | /
d/u| \ |d/u | /
| / \ | | /
|/ \| |/
E1 E2 E3
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ | \
| \ | +-------+
d/u | \ | \
| +-|-----+ \
| | \ \
A1--a--A2 A3--a--A4
|\ /| | /
| \ / | | /
d/u| \ |d/u | /
| / \ | | /
|/ \| |/
E1 E2 E3
Figure 4: Square
图4：正方形
E1 has six routes via E1A1: A1, C1, P, E2, A3, and E3.
E1通过E1A1有六条路由：A1、C1、P、E2、A3和E3。
E1's LFA route to A1 is via A2, because eq1: a < d + u. Node protection for traffic to A1 upon A1 node failure is not applicable.
E1到A1的LFA路线通过A2，因为eq1:a<d+u。A1节点故障时A1的流量的节点保护不适用。
E1's LFA route to A3 is via A2, because eq1: u + c + d < d + u + u + d. This LFA is guaranteed to be node-protecting, because eq2: u + c + d < a + u + d.
E1到A3的LFA路线通过A2，因为eq1:u+c+d<d+u+u+d。由于eq2:u+c+d<a+u+d，该LFA保证是节点保护的。
E1's LFA route to C1 is via A2, because eq1: u + c < d + u + u. This LFA is guaranteed to be node-protecting, because eq2: u + c < a + u.
E1到C1的LFA路线通过A2，因为eq1:u+c<d+u+u。由于eq2:u+c<a+u，该LFA保证是节点保护的。
E1's primary route to E2 is via ECMP(E1A1, E1A2) (Equal-Cost Multi-Path). The LFA for the first ECMP path (via A1) is the second ECMP path (via A2). This LFA is guaranteed to be node-protecting, because eq2: d < a + d.
E1到E2的主要路线是通过ECMP（E1A1、E1A2）（等成本多路径）。第一条ECMP路径（通过A1）的LFA是第二条ECMP路径（通过A2）。由于eq2:d<a+d，该LFA保证是节点保护的。
E1's primary route to E3 is via ECMP(E1A1, E1A2). The LFA for the first ECMP path (via A1) is the second ECMP path (via A2). This LFA is guaranteed to be node-protecting, because eq2: u + d + d < a + u + d + d.
E1到E3的主要路线是通过ECMP（E1A1、E1A2）。第一条ECMP路径（通过A1）的LFA是第二条ECMP路径（通过A2）。由于eq2:u+d+d<a+u+d+d，该LFA保证是节点保护的。
If e = 0: E1's primary route to P is via ECMP(E1A1, E1A2). The LFA for the first ECMP path (via A1) is the second ECMP path (via A2). This LFA is guaranteed to be node-protecting, because eq2: u + x + 0 < a + u + x.
如果e=0:E1到P的主要路线是通过ECMP（E1A1、E1A2）。第一条ECMP路径（通过A1）的LFA是第二条ECMP路径（通过A2）。由于eq2:u+x+0<a+u+x，该LFA保证是节点保护。
If e <> 0: E1's primary route to P is via E1A1. Its LFA is via A2, because eq1: u + c + x < d + u + u + x. This LFA is guaranteed to be node-protecting, because eq2: u + c + x < a + u + x.
如果e<>0:E1到P的主要路线是通过E1A1。其LFA通过A2，因为eq1:u+c+x<d+u+u+x。由于eq2:u+c+x<a+u+x，该LFA保证是节点保护。
Conclusion: All important intra-PoP and inter-PoP routes with primary interface E1A1 benefit from LFA link protection and node protection.
结论：所有具有主接口E1A1的重要PoP内和PoP间路由都受益于LFA链路保护和节点保护。
We have a per-prefix LFA for A1; hence, we have a per-link LFA for link E1A1. All important intra-PoP and inter-PoP routes with primary interface E1A1 benefit from LFA per-link protection and de facto node protection.
对于A1，我们有一个每个前缀LFA；因此，我们为链路E1A1提供了每链路LFA。具有主接口E1A1的所有重要的内部PoP和内部PoP路由都受益于LFA每链路保护和事实上的节点保护。
A1 only has one primary route via A1E1: the route to E1.
A1只有一条通过A1E1的主路由：到E1的路由。
A1's LFA for route E1 is the path via A2, because eq1: d < a + d. Node protection is not applicable.
A1对于路线E1的LFA是通过A2的路径，因为eq1:d<a+d。节点保护不适用。
Conclusion: All important routes with primary interface A1E1 benefit from LFA link protection. Node protection is not applicable.
结论：所有主要接口为A1E1的重要路由均受益于LFA链路保护。节点保护不适用。
All important routes with primary interface A1E1 benefit from LFA link protection. De facto node protection is not applicable.
具有主接口A1E1的所有重要路由均受益于LFA链路保护。事实上的节点保护不适用。
Four destinations are impacted when A1C1 fails: C1, A3, E3, and P.
A1C1故障时，四个目的地受到影响：C1、A3、E3和P。
A1's LFA to C1 is via A2, because eq1: u + c < a + u. Node protection is not applicable for traffic to C1 when C1 fails.
A1的LFA到C1通过A2，因为eq1:u+c<a+u。当C1故障时，节点保护不适用于C1的流量。
A1's LFA to A3 is via A2, because eq1: u + c + d < a + u + d. It is de facto node-protecting, as a < u + c + d (as we assumed a < u + d). Indeed, for destination A3, A2 forwards traffic to C2, and C2 has a node-protecting LFA -- A4 -- for the failure of link C2C1, as a < u + c + d. Hence, the cascading application of LFAs by A1 and C2 during the failure of C1 provides de facto node protection.
A1到A3的LFA通过A2，因为eq1:u+c+d<a+u+d。它实际上是节点保护，如<u+c+d（如我们假设的<u+d）。事实上，对于目的地A3，A2将流量转发给C2，C2有一个节点保护LFA--A4--用于链路C2C1的故障，如a<u+c+d。因此，在C1故障期间A1和C2对LFA的级联应用提供了事实上的节点保护。
A1's LFA to E3 is via A2, because eq1: u + d + d < a + u + d + d. It is node-protecting, because eq2: u + d + d < u + c + d + d.
A1到E3的LFA通过A2，因为eq1:u+d+d<a+u+d+d。它是节点保护，因为eq2:u+d+d<u+c+d+d。
A1's primary route to P is via C1 (even if e = 0, u + x < u + c + x). The LFA is via A2, because eq1: u + c + x < a + u + x (case where c <= e) and eq1: u + x + e < a + u + x (case where c >= e). This LFA is node-protecting (from the viewpoint of A1 computing eq2) if eq2: u + x + e < u + c + x. This inequality is true if e < c.
A1到P的主要路线是通过C1（即使e=0，u+x<u+c+x）。LFA通过A2，因为eq1:u+c+x<a+u+x（c<=e的情况）和eq1:u+x+e<a+u+x（c>=e的情况）。如果eq2:u+x+e<u+c+x，则该LFA是节点保护（从A1计算eq2的角度来看）。如果e<c，则该不等式成立。
Conclusion: All important intra-PoP routes with primary interface A1C1 benefit from LFA link protection and node protection. Note that A3 benefits from de facto node protection. All important inter-PoP routes with primary interface A1C1 benefit from LFA link protection. They also benefit from node protection if e < c.
结论：所有主要接口为A1C1的重要PoP内路由均受益于LFA链路保护和节点保护。请注意，A3受益于事实上的节点保护。所有具有主接口A1C1的重要PoP间路由都受益于LFA链路保护。如果e<c，它们还可以从节点保护中获益。
All important intra-PoP routes with primary interface A1C1 benefit from LFA link protection and de facto node protection. All important inter-PoP routes with primary interface A1C1 benefit from LFA link protection. They also benefit from de facto node protection if e < c.
所有具有主接口A1C1的重要内部PoP路由都受益于LFA链路保护和事实上的节点保护。所有具有主接口A1C1的重要PoP间路由都受益于LFA链路保护。如果e<c，它们还可以从事实上的节点保护中获益。
Three destinations are impacted by C1A1 link failure: A1, E1, and E2. E2's analysis is the same as E1 and hence is omitted.
三个目的地受到C1A1链路故障的影响：A1、E1和E2。E2的分析与E1相同，因此省略。
C1 has no LFA for A1. Indeed, its neighbors (C2 and A3) have a shortest path to A1 via C1. This is due to the assumption (c < a).
C1没有A1的LFA。事实上，它的邻居（C2和A3）有一条通过C1到A1的最短路径。这是由于假设（c<a）。
C1's LFA for E1 is via C2, because eq1: d + d < c + d + d. It provides node protection, because eq2: d + d < d + a + d.
E1的C1 LFA通过C2，因为eq1:d+d<c+d+d。它提供节点保护，因为eq2:d+d<d+a+d。
Conclusion: All important intra-PoP routes with primary interface A1C1, except A1, benefit from LFA link protection and node protection.
结论：除A1外，所有主要接口为A1C1的重要PoP内路由均受益于LFA链路保护和节点保护。
C1 does not have a per-prefix LFA for destination A1; hence, there is no per-link LFA for link C1A1.
C1没有针对目的地A1的每个前缀LFA；因此，链路C1A1没有每链路LFA。
The commonly agreed-upon design rule (c < a) is especially beneficial for a deployment using per-link LFA: it provides a per-link LFA for the most important direction (A1C1). Indeed, there are many more destinations reachable over A1C1 than over C1A1. As the IGP convergence duration is proportional to the number of routes to update, there is a better benefit in leveraging LFA FRR for link A1C1 than for link C1A1.
通常商定的设计规则（c<a）对于使用每链路LFA的部署特别有益：它为最重要的方向（A1C1）提供每链路LFA。事实上，通过A1C1可到达的目的地比通过C1A1可到达的目的地多得多。由于IGP收敛持续时间与要更新的路由数成正比，因此利用链路A1C1的LFA FRR比链路C1A1有更好的好处。
Note as well that the consequence of this assumption is much more important for per-link LFA than for per-prefix LFA.
还要注意，这种假设的结果对于每链路LFA比对于每前缀LFA更重要。
For per-prefix LFAs, in the case of link C1A1 failure, we do have a per-prefix LFA for E1, E2, and any node subtended below A1 and A2. Typically, most of the traffic traversing link C1A1 is directed to these E nodes; hence, the lack of per-prefix LFAs for the destination A1 might be insignificant. This is a good example of the coverage benefit of per-prefix LFAs over per-link LFAs.
对于每前缀LFA，在链路C1A1故障的情况下，对于E1、E2以及A1和A2下的任何子节点，我们都有一个每前缀LFA。通常，穿过链路C1A1的大部分业务被定向到这些E节点；因此，目的地A1缺少每前缀lfa可能是无关紧要的。这是一个很好的例子，说明了每前缀LFA相对于每链路LFA的覆盖优势。
In the remainder of this section, we analyze the consequence of not having c < a.
在本节的剩余部分中，我们将分析没有c<a的后果。
It definitely has a negative impact upon per-link LFAs.
它肯定会对每链路LFA产生负面影响。
With c > a, C1A1 has a per-link LFA, while A1C1 has no per-link LFA. The number of destinations impacted by A1C1 failure is much larger than the direction C1A1; hence, the protection is provided for the wrong direction.
With c > a, C1A1 has a per-link LFA, while A1C1 has no per-link LFA. The number of destinations impacted by A1C1 failure is much larger than the direction C1A1; hence, the protection is provided for the wrong direction.translate error, please retry
For per-prefix LFAs, the availability of an LFA depends on the topology and needs to be assessed individually for each per-prefix LFA. Some backbone topologies will lead to very good protection coverage, while some others might provide very poor coverage.
对于每前缀LFA，LFA的可用性取决于拓扑结构，需要针对每个每前缀LFA分别进行评估。一些主干拓扑将导致非常好的保护覆盖，而另一些可能提供非常差的覆盖。
More specifically, upon A1C1 failure, the coverage of a remote destination P depends on whether e < a. In such a case, A2 is de facto node-protecting per-prefix LFA for P.
更具体地说，在A1C1故障时，远程目的地P的覆盖取决于e是否<a。在这种情况下，A2实际上是保护P的每个前缀LFA的节点。
Such a study likely requires a planning tool, as each remote destination P would have a different e value (exception: all of the edge devices of other aggregation pairs within the same region, as for these e = 0 by definition, e.g., E3.)
这样的研究可能需要一个规划工具，因为每个远程目的地P将具有不同的e值（例外：同一区域内其他聚合对的所有边缘设备，根据定义，e=0，例如E3）
Finally, note that c = a is the worst choice. In this case, C1 has no per-prefix LFA for A1 (and vice versa); hence, there is no per-link LFA for C1A1 and A1C1.
最后，请注意，c=a是最糟糕的选择。在这种情况下，C1对于A1没有前缀LFA（反之亦然）；因此，C1A1和A1C1不存在每链路LFA。
All important intra-PoP routes benefit from LFA link and node protection with one exception: C1 has no per-prefix LFA to A1.
所有重要的内部PoP路由都受益于LFA链路和节点保护，但有一个例外：C1没有每个前缀LFA到A1。
All important inter-PoP routes benefit from LFA link protection. They benefit from node protection if e < c.
所有重要的PoP间路由都受益于LFA链路保护。如果e<c，它们将受益于节点保护。
Per-link LFA provides the same protection coverage as per-prefix LFA, with two exceptions: first, C1A1 has no per-link LFA at all. Second, when per-prefix LFA provides node protection (eq2 is satisfied), per-link LFA provides effective de facto node protection.
每链路LFA提供与前缀LFA相同的保护范围，但有两个例外：第一，C1A1根本没有每链路LFA。其次，当每前缀LFA提供节点保护（满足eq2）时，每链路LFA提供有效的事实节点保护。
If the vertical links of the square are made of parallel links (at the IP topology or below), then one should consider splitting these "vertical links" into "vertical and crossed links". The topology becomes "full mesh". One should also ensure that the two resulting sets of links (vertical and crossed) do not share any Shared Risk Link Group (SRLG).
如果正方形的垂直链路是由平行链路（在IP拓扑或以下）构成的，那么人们应该考虑将这些“垂直链路”分成“垂直链路和交叉链路”。拓扑变为“全网格”。还应确保产生的两组链接（垂直和交叉）不共享任何共享风险链接组（SRLG）。
A typical scenario in which this is prevented would be when the A1C1 bandwidth may be within a building while the A1C2 is between buildings. Hence, while from a router-port viewpoint the operation is cost-neutral, from a cost-of-bandwidth viewpoint it is not.
防止这种情况的典型情况是，A1C1带宽可能位于建筑物内，而A1C2位于建筑物之间。因此，虽然从路由器端口的角度来看，该操作是成本中性的，但从带宽成本的角度来看，它不是。
In a full mesh, the vertical and crossed links play the dominant role, as they support most of the primary and backup paths. The capacity of the horizontal links can be dimensioned on the basis of traffic destined to a single C node or a single A node, and to a single E node.
在全网中，垂直链路和交叉链路起主导作用，因为它们支持大多数主路径和备用路径。水平链路的容量可以基于目的地为单个C节点或单个a节点以及单个E节点的业务量来确定尺寸。
For the Extended U topology, we define the following terminology:
对于扩展U拓扑，我们定义以下术语：
C1L1: the node "C1" as seen in topology L1.
C1L1：节点“C1”，如拓扑L1中所示。
C1L2: the node "C1" as seen in topology L2.
C1L2：拓扑L2中的节点“C1”。
C1LO: the loopback of C1. This loopback is in L2.
C1LO：C1的环回。此环回位于L2中。
C2LO: the loopback of C2. This loopback is in L2.
C2LO：C2的环回。此环回位于L2中。
We remind the reader that C1 and C2 are L1L2 routers and that their loopbacks are in L2 only.
我们提醒读者C1和C2是L1L2路由器，它们的环回仅在L2中。
P
/ \
x/ \x+e
/ \
C1<...>C2
|\ | \
| \ | +-------+
d/u | \ | \
| +-|-----+ \
| | \ \
A1--a--A2 A3--a--A4
|\ /| | /
| \ / | | /
d/u| \ |d/u | /
| / \ | | /
|/ \| |/
E1 E2 E3
P
/ \
x/ \x+e
/ \
C1<...>C2
|\ | \
| \ | +-------+
d/u | \ | \
| +-|-----+ \
| | \ \
A1--a--A2 A3--a--A4
|\ /| | /
| \ / | | /
d/u| \ |d/u | /
| / \ | | /
|/ \| |/
E1 E2 E3
Figure 5: Extended U
图5：扩展U
There is no L1 link between C1 and C2. There might be an L2 link between C1 and C2. This is not relevant, as this is not seen from the viewpoint of the L1 topology, which is the focus of our analysis.
C1和C2之间没有L1链路。C1和C2之间可能存在L2链路。这是不相关的，因为这不是从L1拓扑的角度来看的，L1拓扑是我们分析的重点。
It is guaranteed that there is a path from C1LO to C2LO within the L2 topology (except if the L2 topology partitions, which is very unlikely and hence not analyzed here). We call "c" its path cost. Once again, we assume that c < a.
可以保证在L2拓扑中有一条从C1LO到C2LO的路径（除非L2拓扑分区，这是非常不可能的，因此这里不进行分析）。我们称“c”为其路径成本。我们再次假设c<a。
We exploit this property to create a tunnel T between C1LO and C2LO. Once again, as the source and destination addresses are the loopbacks of C1 and C2 and these loopbacks are in L2 only, it is guaranteed that the tunnel does not transit via the L1 domain.
我们利用这个特性在C1LO和C2LO之间创建一个隧道T。同样，由于源地址和目标地址是C1和C2的环回，并且这些环回仅在L2中，因此可以保证隧道不会通过L1域传输。
IS-IS does not run over the tunnel; hence, the tunnel is not used for any primary paths within the L1 or L2 topology.
IS-IS不在隧道上方运行；因此，隧道不用于L1或L2拓扑中的任何主路径。
Within level-1, we configure C1 (C2) with a level-1 LFA extended neighbor "C2 via tunnel T" ("C1 via tunnel T").
在1级中，我们将C1（C2）配置为1级LFA扩展邻居“C2通过隧道T”（“C1通过隧道T”）。
A router supporting such an extension learns that it has one additional potential neighbor in topology level-1 when checking for LFAs.
支持这种扩展的路由器在检查LFA时会发现它在拓扑级别1中有一个额外的潜在邻居。
The L1 topology learns about C1LO as an L2=>L1 route with the Down bit set, propagated by C1L1 and C2L1. The metric advertised by C2L1 is bigger than the metric advertised by C1L1 by "c".
L1拓扑将C1LO作为一个L2=>L1路由学习，该路由具有下行位集，由C1L1和C2L1传播。C2L1公布的度量值大于C1L1公布的度量值“c”。
The L1 topology learns about P as an L2=>L1 route with the Down bit set, propagated by C1L1 and C2L1. The metric advertised by C2L1 is bigger than the metric advertised by C1L1 by "e". This implies that e <= c.
L1拓扑将P作为L2=>L1路由和下行位集学习，由C1L1和C2L1传播。C2L1公布的度量值大于C1L1公布的度量值“e”。这意味着e<=c。
Five destinations are impacted by E1A1 link failure: A1, C1LO, E2, E3, and P.
E1A1链路故障影响五个目的地：A1、C1LO、E2、E3和P。
The LFA for A1 is via A2, because eq1: a < d + u. Node protection for traffic to A1 upon A1 node failure is not applicable.
A1的LFA通过A2，因为eq1:a<d+u。A1节点故障时A1的流量的节点保护不适用。
The LFA for E2 is via A2, because eq1: d < d + u + d. Node protection is guaranteed, because eq2: d < a + d.
E2的LFA通过A2，因为eq1:d<d+u+d。节点保护是有保证的，因为eq2:d<a+d。
The LFA for E3 is via A2, because eq1: u + d + d < d + u + d + d. Node protection is guaranteed, because eq2: u + d + d < a + u + d + d.
E3的LFA通过A2，因为eq1:u+d+d<d+u+d+d。节点保护是有保证的，因为eq2:u+d+d<a+u+d+d。
The LFA for C1LO is via A2, because eq1: u + c < d + u + u. Node protection is guaranteed, because eq2: u + c < a + u.
C1LO的LFA通过A2，因为eq1:u+c<d+u+u。节点保护是有保证的，因为eq2:u+c<a+u。
If e = 0: E1's primary route to P is via ECMP(E1A1, E1A2). The LFA for the first ECMP path (via A1) is the second ECMP path (via A2). Node protection is possible, because eq2: u + x < a + u + x.
如果e=0:E1到P的主要路线是通过ECMP（E1A1、E1A2）。第一条ECMP路径（通过A1）的LFA是第二条ECMP路径（通过A2）。节点保护是可能的，因为eq2:u+x<a+u+x。
If e <> 0: E1's primary route to P is via E1A1. Its LFA is via A2, because eq1: a + c + x < d + u + u + x. Node protection is guaranteed, because eq2: u + x + e < a + u + x <=> e < a. This is true, because e <= c and c < a.
如果e<>0:E1到P的主要路线是通过E1A1。其LFA通过A2，因为eq1:a+c+x<d+u+u+x。节点保护是有保证的，因为eq2:u+x+e<a+u+x<=>e<a。这是真的，因为e<=c，c<a。
Conclusion: Same as that for the square topology.
结论：与方形拓扑相同。
Same as the square topology.
与方形拓扑相同。
Same as the square topology.
与方形拓扑相同。
Same as the square topology.
与方形拓扑相同。
Three destinations are impacted when A1C1 fails: C1, E3, and P.
当A1C1故障时，三个目的地受到影响：C1、E3和P。
A1's LFA to C1LO is via A2, because eq1: u + c < a + u. Node protection is not applicable for traffic to C1 when C1 fails.
A1的LFA到C1LO通过A2，因为eq1:u+c<a+u。当C1故障时，节点保护不适用于C1的流量。
A1's LFA to E3 is via A2, because eq1: u + d + d < d + u + u + d + d. Node protection is guaranteed, because eq2: u + d + d < a + u + d + d.
A1到E3的LFA通过A2，因为eq1:u+d+d<d+u+u+d+d。节点保护是有保证的，因为eq2:u+d+d<a+u+d+d。
A1's primary route to P is via C1 (even if e = 0, u + x < a + u + x). The LFA is via A2, because eq1: u + x + e < a + u + x <=> e < a (which is true; see above). Node protection is guaranteed, because eq2: u + x + e < a + u + x.
A1到P的主要路线是通过C1（即使e=0，u+x<a+u+x）。LFA通过A2，因为eq1:u+x+e<a+u+x<=>e<a（这是正确的；见上文）。节点保护是有保证的，因为eq2:u+x+e<a+u+x。
Conclusion: Same as that for the square topology.
结论：与方形拓扑相同。
Same as the square topology.
与方形拓扑相同。
Three destinations are impacted by C1A1 link failure: A1, E1, and E2. E2's analysis is the same as E1 and hence is omitted.
三个目的地受到C1A1链路故障的影响：A1、E1和E2。E2的分析与E1相同，因此省略。
C1L1 has an LFA for A1 via the extended neighbor C2L1 reachable via tunnel T. Indeed, eq1 is true: d + a < d + a + u + d. From the viewpoint of C1L1, C2L1's path to C1L1 is C2L1-A2-A1-C1L1. Remember that the tunnel is not seen by IS-IS for computing primary paths! Node protection is not applicable for traffic to A1 when A1 fails.
C1L1通过可通过隧道T到达的扩展邻居C2L1对A1具有LFA。事实上，eq1为真：d+a<d+a+u+d。从C1L1的角度来看，C2L1到C1L1的路径是C2L1-A2-A1-C1L1。请记住，is-is在计算主路径时不会看到隧道！当A1发生故障时，节点保护不适用于A1的流量。
C1L1's LFA for E1 is via extended neighbor C2L1 (over tunnel T), because eq1: d + d < d + a + u + d + d. Node protection is guaranteed, because eq2: d + d < d + a + d.
E1的C1L1的LFA通过扩展邻居C2L1（通过隧道T），因为eq1:d+d<d+a+u+d+d。节点保护是有保证的，因为eq2:d+d<d+a+d。
C1 has a per-prefix LFA for destination A1; hence, there is a per-link LFA for the link C1A1. Node resistance is applicable for traffic to E1 (and E2).
C1对于目的地A1具有每个前缀LFA；因此，对于链路C1A1存在每链路LFA。节点电阻适用于E1（和E2）的流量。
The Extended U topology is as good as the square topology.
扩展U拓扑与方形拓扑一样好。
It does not require any crossed links between the A and C nodes within an aggregation region. It does not need an L1 link between the C routers in an access region. Note that a link between the C routers might exist in the L2 topology.
它不需要聚合区域内A和C节点之间的任何交叉链路。它不需要接入区域中的C路由器之间的L1链路。请注意，L2拓扑中可能存在C路由器之间的链路。
A dual-plane core is defined as follows:
双平面铁芯的定义如下：
o Each access region k is connected to the core by two C routers (C(1,k) and C(2,k)).
o 每个接入区域k通过两个C路由器（C（1，k）和C（2，k））连接到核心。
o C(1,k) is part of plane-1 of the dual-plane core.
o C（1，k）是双平面核的平面1的一部分。
o C(2,k) is part of plane-2 of the dual-plane core.
o C（2，k）是双平面核的平面2的一部分。
o C(1,k) has a link to C(2, l) iff k = l.
o 当k=l时，C（1，k）与C（2，l）有一个链接。
o {C(1,k) has a link to C(1, l)} iff {C(2,k) has a link to C(2, l)}.
o {C（1，k）与C（1，l）有链接}iff{C（2，k）与C（2，l）有链接}。
In a dual-plane core design, e = 0; hence, the LFA node-protection coverage is improved in all of the analyzed topologies.
在双平面堆芯设计中，e=0；因此，在所有分析的拓扑中，LFA节点保护覆盖率都得到了提高。
A two-tiered IGP metric allocation scheme is defined as follows:
两层IGP度量分配方案定义如下：
o All of the link metrics used in the L2 domain are part of range R1.
o L2域中使用的所有链路度量都是范围R1的一部分。
o All of the link metrics used in an L1 domain are part of range R2.
o L1域中使用的所有链路度量都是范围R2的一部分。
o Range R1 << range R2 such that the difference e = C2P - C1P is smaller than any link metric within an access region.
o 范围R1<<范围R2，使得差值e=C2P-C1P小于接入区域内的任何链路度量。
Assuming such an IGP metric allocation, the following properties are guaranteed: c < a, e < c, and e < a.
假设这样一个IGP度量分配，则保证以下属性：c<a、e<c和e<a。
In this section, we analyze a case where the routing transition following the failure of a link may have some uLoop potential for one destination. Then, we show that all of the other cases do not have uLoop potential.
在本节中，我们将分析一种情况，即链路故障后的路由转换可能对一个目的地具有一些潜在的uLoop。然后，我们证明所有其他情况都不具有uLoop电位。
In the square design, upon the failure of link C1A1, traffic addressed to A1 can undergo a transient forwarding loop as C1 reroutes traffic to C2, which initially reaches A1 through C1, as c < a. This loop will actually occur when C1 updates its FIB for destination A1 before C2.
在方形设计中，当链路C1A1发生故障时，寻址到A1的业务可以经历一个瞬态转发环路，因为C1将业务重新路由到C2，C2最初通过C1到达A1，因为c<a。当C1在C2之前更新其目的地A1的FIB时，实际上会发生此循环。
It can be shown that all of the other routing transitions following a link failure in the analyzed topologies do not have uLoop potential. Indeed, in each case, for all destinations affected by the failure, the rerouting nodes deviate their traffic directly to adjacent nodes whose paths towards these destinations do not change. As a consequence, all of these routing transitions cannot undergo transient forwarding loops.
可以证明，在分析的拓扑中，链路故障之后的所有其他路由转换都不具有uLoop潜力。事实上，在每种情况下，对于受故障影响的所有目的地，重路由节点将其流量直接偏离到其通向这些目的地的路径不变的相邻节点。因此，所有这些路由转换都不能经历瞬时转发循环。
For example, in the square topology, the failure of directed link A1C1 does not lead to any uLoop. The destinations reached over that directed link are C1 and P. A1's and E1's shortest paths to these destinations after the convergence go via A2. A2's path to C1 and P is not using A1C1 before the failure; hence, no uLoop may occur.
例如，在方形拓扑中，定向链路A1C1的故障不会导致任何uLoop。通过该定向链路到达的目的地是C1和P.A1和E1在会聚后通过A2到达这些目的地的最短路径。A2到C1和P的路径在故障前未使用A1C1；因此，不可能发生uLoop。
In this section, we summarize the applicability of LFAs detailed in the previous sections. For link protection, we use "Full" to refer to the applicability of LFAs for each destination, reached via any link of the topology. For node protection, we use "Yes" to refer to the fact that node protection is achieved for a given node.
在本节中，我们总结了前面几节中详细介绍的LFA的适用性。对于链路保护，我们使用“完整”表示LFA对每个目的地的适用性，通过拓扑的任何链路达到。对于节点保护，我们使用“是”来表示为给定节点实现节点保护的事实。
1. Intra-Area Destinations
1. 区域内目的地
Link Protection
链路保护
+ Triangle: Full + Full Mesh: Full + Square: Full, except C1 has no LFA for dest A1 + Extended U: Full
+ 三角形：完整+完整网格：完整+正方形：完整，除了C1没有目标A1+扩展U:完整的LFA
Node Protection
节点保护
+ Triangle: Yes
+ 三角：是的
+ Full Mesh: Yes + Square: Yes + Extended U: Yes
+ 全网格：是+正方形：是+扩展U:是
2. Inter-Area Destinations
2. 区域间目的地
Link Protection
链路保护
+ Triangle: Full + Full Mesh: Full + Square: Full + Extended U: Full
+ 三角形：完全+完全网格：完全+正方形：完全+扩展U:完全
Node Protection
节点保护
+ Triangle: Yes, if e < c + Full Mesh: Yes for A failure, if e < c for C failure + Square: Yes for A failure, if e < c for C failure + Extended U: Yes, if e <= c and c < a
+ 三角形：是，如果e<c+全网格：是，如果e<c表示c故障+正方形：是，如果e<c表示c故障+扩展U：是，如果e<=c和c<A
3. uLoops
3. 乌洛普斯
* Triangle: None * Full Mesh: None * Square: None, except traffic to A1 when C1A1 fails * Extended U: None, if a > e
* 三角形：无*全网格：无*正方形：无，C1A1故障时A1的流量除外*扩展U:无，如果a>e
4. Per-Link LFA vs. Per-Prefix LFA
4. 每链路LFA与每前缀LFA
* Triangle: Same * Full Mesh: Same * Square: Same, except C1A1 has no per-link LFA. In practice, this means that per-prefix LFAs will be used. (Hence, C1 has no LFA for dest = E1 and dest = A1.) * Extended U: Same
* 三角形：相同*完整网格：相同*正方形：相同，但C1A1没有每个链接LFA。实际上，这意味着将使用每个前缀LFA。（因此，C1对于dest=E1和dest=A1没有LFA。）*扩展U：相同
In the backbone, the optimization of the network design to achieve the maximum LFA protection is less straightforward than in the case of the access/aggregation network.
在主干网中，与接入/聚合网络相比，优化网络设计以实现最大LFA保护并不那么简单。
The main optimization objectives for backbone topology design are cost, latency, and bandwidth, constrained by the availability of fiber. Optimizing the design for local IP restoration is more likely to be considered as a non-primary objective. For example, the way the fiber is laid out and the resulting cost to change it lead to ring topologies in some backbone networks.
主干网拓扑设计的主要优化目标是成本、延迟和带宽，受光纤可用性的限制。优化本地IP恢复的设计更可能被视为非主要目标。例如，光纤的布置方式以及由此产生的更改成本会导致某些骨干网络中出现环形拓扑。
Also, the capacity-planning process is already complex in the backbone. The process needs to make sure that the traffic matrix (demand) is supported by the underlying network (capacity) under all possible variations of the underlying network (what-if scenario related to one-SRLG failure). Classically, "supported" means that no congestion is experienced and that the demands are routed along the appropriate latency paths. Selecting the LFA method as a deterministic FRR solution for the backbone would require enhancement of the capacity-planning process to add a third constraint: Each variation of the underlying network should lead to sufficient LFA coverage. (We detail this aspect in Section 7.)
此外，主干网的容量规划过程已经很复杂。该过程需要确保在底层网络的所有可能变化（与一个SRLG故障相关的假设情况）下，底层网络（容量）支持流量矩阵（需求）。传统上，“受支持”意味着没有遇到拥塞，需求沿着适当的延迟路径路由。选择LFA方法作为主干网的确定性FRR解决方案需要增强容量规划过程，以添加第三个约束：基础网络的每个变化都应导致足够的LFA覆盖。（我们将在第7节中详细介绍这一方面。）
On the other hand, the access network is based on many replications of a small number of well-known (well-engineered) topologies. The LFA coverage is deterministic and is independent of additions/ insertions of a new edge device, a new aggregation sub-region, or a new access region.
另一方面，接入网络基于少量已知（精心设计的）拓扑的许多复制。LFA覆盖是确定的，并且与新边缘设备、新聚集子区域或新接入区域的添加/插入无关。
In practice, we believe that there are three profiles for the backbone applicability of the LFA method:
在实践中，我们认为LFA方法的主干适用性有三个方面：
In the first profile, the designer plans all of the network resilience on IGP convergence. In such a case, the LFA method is a free bonus. If an LFA is available, then the loss of connectivity is likely reduced by a factor of 10 (50 msec vs. 500 msec); otherwise, the loss of connectivity depends on IGP
在第一个概要文件中，设计师规划了IGP融合上的所有网络恢复能力。在这种情况下，LFA方法是免费的奖金。如果LFA可用，则连接损失可能减少10倍（50毫秒vs.500毫秒）；否则，连通性的丧失取决于IGP
convergence, which is the initial target anyway. The LFA method should be very successful here, as it provides a significant improvement without any additional cost.
收敛，这是最初的目标。LFA方法在这里应该非常成功，因为它在没有任何额外成本的情况下提供了显著的改进。
In the second profile, the designer seeks a very high and deterministic FRR coverage, and he either does not want or cannot engineer the topology. The LFA method should not be considered in this case. MPLS Traffic Engineering (TE) FRR would perform much better in this environment. Explicit routing ensures that a backup path exists, whatever the underlying topology.
在第二个配置文件中，设计师寻求非常高且确定的FRR覆盖率，他不想要或不能设计拓扑。在这种情况下，不应考虑LFA方法。MPLS流量工程（TE）FRR将在这种环境中表现得更好。显式路由确保备份路径存在，无论底层拓扑如何。
In the third profile, the designer seeks a very high and deterministic FRR coverage, and he does engineer the topology. The LFA method is appealing in this scenario, as it can provide a very simple way to obtain protection. Furthermore, in practice, the requirement for FRR coverage might be limited to a certain part of the network (e.g., a given sub-topology) and/or is likely limited to a subset of the demands within the traffic matrix. In such a case, if the relevant part of the network natively provides a high degree of LFA protection for demands of interest, it might actually be straightforward to improve the topology and achieve the level of protection required for the sub-topology and the demands that matter. Once again, the practical problem needs to be considered (which sub-topology, and which real demands need 50 msec), as it is often simpler than the theoretical generic one.
在第三个配置文件中，设计师寻求非常高且确定的FRR覆盖率，并且他设计了拓扑。LFA方法在这种情况下很有吸引力，因为它可以提供一种非常简单的获得保护的方法。此外，在实践中，对FRR覆盖的要求可能限于网络的某个部分（例如，给定的子拓扑）和/或可能限于业务矩阵内的需求子集。在这种情况下，如果网络的相关部分本机为感兴趣的需求提供高度LFA保护，那么实际上可以直接改进拓扑并实现子拓扑和重要需求所需的保护级别。再一次，需要考虑实际问题（哪个子拓扑，哪个实际需求需要50毫秒），因为它通常比理论上的一般问题简单。
For the reasons explained previously, the backbone applicability should be analyzed on a case-by-case basis, and it is difficult to derive generic rules.
出于前面解释的原因，主干网的适用性应该根据具体情况进行分析，很难推导出通用规则。
In order to help the reader to assess the LFA applicability in his own case, we provide some simulation results based on 11 real backbone topologies in the next section.
为了帮助读者评估LFA在自己案例中的适用性，我们在下一节中提供了基于11种实际主干拓扑的一些模拟结果。
In order to perform an analysis of LFA applicability in the core, we usually receive the complete IS-IS/OSPF linkstate database taken on a core router. We parse it to obtain the topology. During this process, we eliminate all nodes connected to the topology with a single link and all prefixes except a single "node address" per router. We compute the availability of per-prefix LFAs to all of these node addresses, which we hereafter call "destinations". We treat each link in each direction.
为了对LFA在核心中的适用性进行分析，我们通常会收到在核心路由器上获取的完整IS-IS/OSPF链路状态数据库。我们解析它以获得拓扑。在这个过程中，我们使用单个链路和每个路由器的唯一“节点地址”之外的所有前缀消除了连接到拓扑的所有节点。我们计算每个前缀LFA对所有这些节点地址的可用性，我们在下文中称之为“目的地”。我们对待每个方向的每个环节。
For each (directed) link, we compute whether we have a per-prefix LFA to the next hop. If so, we have a per-link LFA for the link.
对于每个（定向）链路，我们计算是否有到下一跳的每个前缀LFA。如果是这样，我们有一个链接的每链接LFA。
The per-link-LFA coverage for a topology T is the fraction of the number of links with a per-link LFA divided by the total number of links.
拓扑T的每条链路LFA覆盖率是每条链路LFA的链路数除以链路总数的分数。
For each link, we compute the number of destinations whose primary path involves the analyzed link. For each such destination, we compute whether a per-prefix LFA exists.
对于每个链路，我们计算其主要路径涉及分析链路的目的地数量。对于每个这样的目的地，我们计算每个前缀LFA是否存在。
The per-prefix LFA coverage for a topology T is the following fraction:
拓扑T的每前缀LFA覆盖率为以下分数：
(the sum across all links of the number of destinations with a primary path over the link and a per-prefix LFA)
（链路上具有主路径和每个前缀LFA的目的地数量在所有链路上的总和）
divided by
除以
(the sum across all links of the number of destinations with a primary path over the link)
（链路上具有主路径的目的地数量在所有链路上的总和）
Our data set is based on 11 SP core topologies with different geographical scopes: worldwide, national, and regional. The number of nodes ranges from 600 to 16. The average link-to-node ratio is 2.3, with a minimum of 1.2 and maximum of 6.
我们的数据集基于11个SP核心拓扑，具有不同的地理范围：全球、国家和地区。节点数从600到16不等。平均链路与节点的比率为2.3，最小为1.2，最大为6。
+----------+--------------+----------------+
| Topology | Per-Link LFA | Per-Prefix LFA |
+----------+--------------+----------------+
| T1 | 45% | 76% |
| T2 | 49% | 98% |
| T3 | 88% | 99% |
| T4 | 68% | 84% |
| T5 | 75% | 94% |
| T6 | 87% | 98% |
| T7 | 16% | 67% |
| T8 | 87% | 99% |
| T9 | 67% | 79% |
| T10 | 98% | 99% |
| T11 | 59% | 77% |
| Average | 67% | 89% |
| Median | 68% | 94% |
+----------+--------------+----------------+
+----------+--------------+----------------+
| Topology | Per-Link LFA | Per-Prefix LFA |
+----------+--------------+----------------+
| T1 | 45% | 76% |
| T2 | 49% | 98% |
| T3 | 88% | 99% |
| T4 | 68% | 84% |
| T5 | 75% | 94% |
| T6 | 87% | 98% |
| T7 | 16% | 67% |
| T8 | 87% | 99% |
| T9 | 67% | 79% |
| T10 | 98% | 99% |
| T11 | 59% | 77% |
| Average | 67% | 89% |
| Median | 68% | 94% |
+----------+--------------+----------------+
Table 1: Core LFA Coverages
表1：核心LFA覆盖范围
In Table 1, we observe a wide variation in terms of LFA coverage across topologies: from 67% to 99% for the per-prefix LFA coverage, and from 16% to 98% for the per-link LFA coverage. Several topologies have been optimized for LFAs (T3, 6, 8, and 10). This illustrates the need for case-by-case analysis when considering LFAs for core networks.
在表1中，我们观察到不同拓扑的LFA覆盖率差异很大：每个前缀LFA覆盖率从67%到99%，每个链路LFA覆盖率从16%到98%。已经针对LFA优化了几种拓扑（T3、6、8和10）。这说明了在考虑核心网络的LFA时需要进行个案分析。
It should be noted that, contrary to the access/aggregation topologies, per-prefix LFA outperforms per-link LFA in the backbone.
应该注意的是，与访问/聚合拓扑相反，在主干中，每前缀LFA优于每链路LFA。
Specifically, a design might use LFA FRR in the access and MPLS TE FRR in the core.
具体地说，设计可能在接入中使用LFA FRR，在核心中使用MPLS TE FRR。
The LFA method provides great benefits for the access network, due to its excellent access coverage and its simplicity.
LFA方法由于其优良的接入覆盖率和简单性，为接入网络提供了巨大的好处。
MPLS TE FRR's topology independence might prove beneficial in the core when the LFA FRR coverage is judged too small and/or the designer feels unable to optimize the topology to improve the LFA coverage.
当LFA FRR覆盖范围被判断为太小和/或设计者感觉无法优化拓扑以提高LFA覆盖时，MPLS TE FRR的拓扑独立性可能在核心中被证明是有益的。
The LFA solution provides significant benefits that mainly stem from its simplicity.
LFA解决方案提供了主要源于其简单性的显著优势。
Behavior of LFAs is an automated process that makes fast restoration an intrinsic part of the IGP, with no additional configuration burden in the IGP or any other protocol.
LFA的行为是一个自动化过程，使快速恢复成为IGP的固有部分，而IGP或任何其他协议中没有额外的配置负担。
Thanks to this integration, the use of multiple areas in the IGP does not make fast restoration more complex to achieve than in a single area IGP design.
由于这种集成，在IGP中使用多个区域不会使快速恢复比单区域IGP设计更复杂。
There is no requirement for network-wide upgrade, as LFAs do not require any protocol change and hence can be deployed router by router.
不需要网络范围的升级，因为LFA不需要任何协议更改，因此可以逐个路由器部署。
With LFAs, the backup paths are pre-computed and installed in the data plane in advance of the failure. Assuming a fast enough FIB update time compared to the total number of (important) destinations, a "<50-msec repair" requirement becomes achievable. With a prefix-independent implementation, LFAs have a fixed repair time, as the repair time depends on the failure detection time and the time required to activate the behavior of an LFA, which does not scale with the number of destinations to be fast-rerouted.
使用LFA，备份路径是预先计算的，并在发生故障之前安装在数据平面中。假设与（重要）目的地总数相比，FIB更新时间足够快，“小于50毫秒的修复”要求是可以实现的。对于前缀无关的实现，LFA具有固定的修复时间，因为修复时间取决于故障检测时间和激活LFA行为所需的时间，而LFA的行为不随要快速重新路由的目的地数量而扩展。
Link and node protection are provided together and without any operational differences. (As a comparison, MPLS TE FRR link and node protections require different types of backup tunnels and different grades of operational complexity.)
链路和节点保护一起提供，没有任何操作差异。（作为比较，MPLS TE FRR链路和节点保护需要不同类型的备份隧道和不同级别的操作复杂性。）
Also, compared to MPLS TE FRR, an important simplicity aspect of the LFA solution is that it does not require the introduction of yet another virtual layer of topology. Maintaining a virtual topology of explicit MPLS TE tunnels clearly increases the complexity of the network. MPLS TE tunnels would have to be represented in a network management system in order to be monitored and managed. In large networks, this may significantly contribute to the number of network entities polled by the network management system and monitored by operational staff. An LFA, on the other hand, only has to be monitored for its operational status once per router, and it needs to be considered in the network-planning process. If the latter is done based on offline simulations for failure cases anyway, the incremental cost of supporting LFAs for a defined set of demands may be relatively low.
此外，与MPLS TE FRR相比，LFA解决方案的一个重要的简单性方面是，它不需要引入另一个虚拟拓扑层。维护显式MPLS TE隧道的虚拟拓扑显然会增加网络的复杂性。MPLS TE隧道必须在网络管理系统中表示，以便进行监视和管理。在大型网络中，这可能会大大增加网络管理系统轮询并由运营人员监控的网络实体的数量。另一方面，LFA只需对每个路由器的运行状态进行一次监控，并且需要在网络规划过程中加以考虑。如果后者是基于失效案例的离线模拟进行的，那么为一组定义的需求支持LFA的增量成本可能相对较低。
The per-prefix mode of LFAs allows for simpler and more efficient capacity planning. As the backup path of each destination is optimized individually, the load to be fast-rerouted can be spread on a set of shortest repair paths (as opposed to a single backup tunnel). This leads to a simpler and more efficient capacity-planning process that takes congestion during protection into account.
LFAs的每前缀模式允许更简单、更高效的容量规划。由于每个目的地的备份路径都是单独优化的，因此要快速重新路由的负载可以分布在一组最短的修复路径上（与单个备份隧道相反）。这将导致更简单、更高效的容量规划过程，该过程将保护期间的拥塞考虑在内。
We briefly describe the functionality a designer should expect from a capacity-planning tool that supports LFAs, and the related capacity-planning process.
我们简要描述了设计师应该从支持LFA的容量规划工具中获得的功能，以及相关的容量规划过程。
Per-Link LFA Coverage Estimation: The tool would color each unidirectional link in, depending on whether or not per-link LFAs are available.
每链路LFA覆盖估计：该工具将根据每链路LFA是否可用，为每个单向链路添加颜色。
Per-Prefix LFA Coverage Estimation: The tool would color each unidirectional link with a colored gradient, based on the percent of destinations that have a per-prefix LFA.
每前缀LFA覆盖估计：该工具将根据具有每前缀LFA的目的地的百分比，使用彩色渐变为每个单向链接着色。
In addition to the visual GUI reporting, the tool should provide detailed tables that list, on a per-interface basis, the percentage of LFAs, the number of prefixes with LFAs, the number of prefixes without LFAs, and a list of those prefixes without LFAs.
除了可视化GUI报告外，该工具还应提供详细的表格，以每个接口为基础列出LFA的百分比、带有LFA的前缀数量、没有LFA的前缀数量以及没有LFA的前缀列表。
Furthermore, the tool should list and provide percentages for the traffic matrix demands with less than 100% source-to-destination LFA coverage, as well as average coverage (number of links on which a demand has an LFA/number of links traversed by this demand) for every demand (using a threshold).
此外，该工具应列出并提供源到目标LFA覆盖率小于100%的流量矩阵需求百分比，以及每个需求（使用阈值）的平均覆盖率（需求具有LFA的链路数/该需求穿过的链路数）。
The user should be able to alter the color scheme to show whether these LFAs are guaranteed node-protecting or de facto node-protecting, or only link-protecting.
用户应该能够改变颜色方案，以显示这些LFA是保证节点保护还是事实上的节点保护，还是仅链路保护。
This functionality provides the same level of information as we described in Sections 4.1 to 4.3.
此功能提供与我们在第4.1节至第4.3节中描述的相同级别的信息。
Instead of reporting the coverage as a ratio of the number of destinations with a backup, one might prefer a ratio of the amount of traffic on a link that benefits from protection.
与其将覆盖率报告为具有备份的目的地数量的比率，不如报告链路上受益于保护的通信量的比率。
This is likely much more relevant, as not all destinations are equal, and it is much more important to have an LFA for a destination attracting lots of traffic rather than an unpopular destination.
这可能更为相关，因为并非所有目的地都是平等的，对于吸引大量交通的目的地而言，LFA比不受欢迎的目的地更为重要。
Depending on the requirements on the network, it might be more relevant to verify the complete LFA coverage of a given sub-topology, or a given set of demands, rather than to calculate the relative coverage of the overall traffic. This is most likely true for the third engineering profile described in Section 4.
根据网络上的需求，验证给定子拓扑或给定需求集的完整LFA覆盖可能比计算总体流量的相对覆盖更为相关。对于第4节中描述的第三个工程剖面，这很可能是正确的。
In that case, the tool should be able to separately report the LFA coverage on a given set of demands and highlight each part of the network that does not support 100% coverage for any of those demands.
在这种情况下，该工具应该能够单独报告给定需求集上的LFA覆盖率，并突出显示不支持这些需求100%覆盖率的网络的每个部分。
The tool should be able to compute the coverage for all of the possible topologies that result from a set of expected failures (i.e., one-SRLG failure).
该工具应能够计算由一组预期故障（即一个SRLG故障）导致的所有可能拓扑的覆盖率。
Filtering the key information from the huge amount of generated data should be a key property of the tool.
从大量生成的数据中过滤关键信息应该是该工具的关键属性。
For example, the user could set a threshold (at least 80% per-prefix LFA coverage in all one-SRLG what-if scenarios), and the tool would report only the cases where this condition is not met, hopefully with some assistance on how to remedy the problem (IGP metric optimization).
例如，用户可以设置一个阈值（在所有一个SRLG假设情景中，每个前缀LFA覆盖率至少为80%），该工具将仅报告不满足此条件的情况，希望在如何解决问题（IGP度量优化）方面提供一些帮助。
As an application example, a designer who is not able to ensure that c < a could leverage such a tool to assess the per-prefix LFA coverage for square aggregation topologies grafted to the backbone of his network. The tool would analyze the per-prefix LFA availability for each remote destination and would help optimize the backbone topology to increase the LFA protection coverage for failures within the square aggregation topologies.
作为一个应用示例，无法确保c<a的设计师可以利用这样的工具来评估嫁接到其网络主干的方形聚合拓扑的每前缀LFA覆盖率。该工具将分析每个远程目的地的每前缀LFA可用性，并有助于优化主干拓扑，以增加方形聚合拓扑内故障的LFA保护覆盖率。
The tool should be able to compute the link load for all routing states that result from a set of expected failures (i.e., one-SRLG failure).
该工具应该能够计算由一组预期故障（即一个SRLG故障）导致的所有路由状态的链路负载。
The routing states that should be supported are 1) network-wide converged state before the failure, 2) state in which all of the LFAs protecting the failure are active, and 3) network-wide converged state after the failure.
应支持的路由状态为1）故障前的网络范围聚合状态，2）保护故障的所有LFA都处于活动状态的状态，以及3）故障后的网络范围聚合状态。
Filtering the key information from the huge amount of generated data should be a key property of the tool.
从大量生成的数据中过滤关键信息应该是该工具的关键属性。
For example, the user could set a threshold (at most 100% link load in all one-SRLG what-if scenarios), and the tool would report only the cases where this condition is violated, hopefully with some assistance on how to remedy the problem (IGP metric optimization).
例如，用户可以设置一个阈值（在所有一个SRLG假设情景中最多100%的链路负载），该工具将只报告违反此条件的情况，希望在如何解决问题（IGP度量优化）方面提供一些帮助。
The tool should be able to do this for the aggregate load, and on a per-class-of-service basis as well.
该工具应该能够对总负载以及每类服务执行此操作。
Note: In cases where the traffic matrix is unknown, an intermediate solution consists of identifying the destinations that would attract traffic (i.e., Provider Edge (PE) routers), and those that would not (i.e., Provider (P) routers). One could achieve this by creating a traffic matrix with equal demands between the sources/destinations that would attract traffic (PE to PE). This will be more relevant than considering all demands between all prefixes (e.g., when there is no customer traffic from P to P).
注：在流量矩阵未知的情况下，中间解决方案包括确定会吸引流量的目的地（即提供商边缘（PE）路由器）和不会吸引流量的目的地（即提供商（P）路由器）。可以通过创建一个流量矩阵来实现这一点，该矩阵在吸引流量的源/目的地之间具有相等的需求（PE到PE）。这将比考虑所有前缀之间的所有需求（例如，当P到P之间没有客户流量时）更相关。
While LFA FRR has many benefits (Section 6), LFA FRR's applicability depends on topology.
虽然LFA FRR有许多优点（第6节），但LFA FRR的适用性取决于拓扑结构。
The purpose of this document is to show how to introduce a level of control over this topology parameter.
本文档旨在说明如何对该拓扑参数引入控制级别。
On the one hand, we wanted to show that by adopting a small set of IGP metric constraints and a repetition of well-behaved patterns, the designer could deterministically guarantee maximum link and node protection for the vast majority of the network (the access/ aggregation). By doing so, he would obtain an extremely simple resiliency solution.
一方面，我们希望表明，通过采用一小组IGP度量约束和良好行为模式的重复，设计者可以确定地为绝大多数网络（访问/聚合）提供最大的链路和节点保护。通过这样做，他将获得一个极其简单的弹性解决方案。
On the other hand, we also wanted to show that it might not be so bad to not apply (all of) these constraints.
另一方面，我们还想表明，不应用（所有）这些约束可能不是那么糟糕。
Indeed, we explained in Section 3.3.4.3 that the per-prefix LFA coverage in a square where c >= a might still be very good, depending on the backbone topology.
事实上，我们在第3.3.4.3节中解释了c>=a的正方形中的每前缀LFA覆盖可能仍然非常好，这取决于主干拓扑。
We showed in Section 4.3 that the median per-prefix LFA coverage for 11 SP backbone topologies still provides 94% coverage. (Most of these topologies were built without any idea of LFA!)
我们在第4.3节中显示，11 SP主干拓扑的每前缀LFA覆盖率中值仍提供94%的覆盖率。（这些拓扑中的大多数都是在不考虑LFA的情况下构建的！）
Furthermore, we showed that any topology may be analyzed with an LFA-aware capacity-planning tool. This would readily assess the coverage of per-prefix LFAs and would assist the designer in fine-tuning it to obtain the level of protection he seeks.
此外，我们还表明，任何拓扑都可以使用LFA感知的容量规划工具进行分析。这将很容易评估每个前缀LFA的覆盖范围，并将帮助设计者对其进行微调，以获得他所寻求的保护级别。
While this document highlights LFA applicability and benefits for SP networks, it also notes that LFAs are not meant to replace MPLS TE FRR.
虽然本文档强调了LFA对SP网络的适用性和好处，但也指出LFA并不意味着取代MPLS TE FRR。
With a very LFA-unfriendly topology, a designer seeking guaranteed <50-msec protection might be better off leveraging the explicit-routed backup capability of MPLS TE FRR to provide 100% protection while ensuring no congestion along the backup paths during protection.
对于非常不友好的LFA拓扑，寻求保证<50毫秒保护的设计师最好利用MPLS TE FRR的显式路由备份功能提供100%保护，同时确保保护期间备份路径上没有拥塞。
But when LFAs provide 100% link and node protection without any uLoop, then clearly the LFA method seems a technology to consider to drastically simplify the operation of a large-scale network.
但是当LFAS提供100%链路和节点保护而没有任何ULoOP时，那么显然LFA方法似乎是一种考虑大幅度简化大规模网络的操作的技术。
The security considerations applicable to LFAs are described in [RFC5286]. This document does not introduce any new security considerations.
[RFC5286]中描述了适用于LFA的安全注意事项。本文档没有引入任何新的安全注意事项。
The LFA method is an important protection alternative for IP/MPLS networks.
LFA方法是IP/MPLS网络的一种重要保护方案。
Its simplicity benefit is significant, in terms of automation and integration with the default IGP behavior and the absence of any requirement for network-wide upgrade. The technology does not require any protocol change and hence can be deployed router by router.
在自动化和与默认IGP行为的集成以及不需要任何网络范围的升级方面，它的简单性优势是显著的。该技术不需要任何协议更改，因此可以逐个路由器部署。
At first sight, these significant simplicity benefits are negated by the topological dependency of its applicability.
乍一看，这些显著的简单性优势被其适用性的拓扑依赖性所否定。
The purpose of this document is to highlight that very frequent access and aggregation topologies benefit from excellent link and node LFA coverage.
本文档的目的是强调非常频繁的访问和聚合拓扑受益于出色的链路和节点LFA覆盖。
A second objective consists of describing the three different profiles of LFA applicability for the IP/MPLS core networks and illustrating them with simulation results based on real SP core topologies.
第二个目标包括描述IP/MPLS核心网络LFA适用性的三种不同配置，并用基于真实SP核心拓扑的仿真结果对其进行说明。
We would like to thank Alvaro Retana and especially Stewart Bryant for their valuable comments on this work.
我们要感谢阿尔瓦罗·雷塔纳，特别是斯图尔特·布莱恩特对这项工作的宝贵评论。
[RFC5286] Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5286]Atlas，A.，Ed.，和A.Zinin，Ed.“IP快速重路由的基本规范：无环路替代”，RFC 5286，2008年9月。
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, January 2010.
[RFC5714]Shand，M.和S.Bryant，“IP快速重路由框架”，RFC 5714，2010年1月。
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free Convergence", RFC 5715, January 2010.
[RFC5715]Shand，M.和S.Bryant，“无环收敛框架”，RFC 5715，2010年1月。
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990.
[RFC1195]Callon，R.，“OSI IS-IS在TCP/IP和双环境中的路由使用”，RFC 11951990年12月。
[IS-IS] ISO/IEC 10589:2002, Second Edition, "Intermediate System to Intermediate System Intra-Domain Routeing Exchange Protocol for use in Conjunction with the Protocol for Providing the Connectionless-mode Network Service (ISO 8473)", 2002.
[IS-IS]ISO/IEC 10589:2002，第二版，“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交换协议（ISO 8473）”，2002年。
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2328]Moy，J.，“OSPF版本2”，STD 54，RFC 2328，1998年4月。
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, July 2008.
[RFC5340]Coltun，R.，Ferguson，D.，Moy，J.，和A.Lindem，“IPv6的OSPF”，RFC 53402008年7月。
Authors' Addresses
作者地址
Clarence Filsfils (editor) Cisco Systems Brussels 1000 BE
Clarence Filsfils（编辑）思科系统布鲁塞尔1000 BE
EMail: cf@cisco.com
EMail: cf@cisco.com
Pierre Francois (editor) Institute IMDEA Networks Avda. del Mar Mediterraneo, 22 Leganese 28918 ES
Pierre Francois（编辑）IMDEA网络Avda研究所。德尔马尔地中海，22勒加内斯28918 ES
EMail: pierre.francois@imdea.org
EMail: pierre.francois@imdea.org
Mike Shand
迈克·尚德
EMail: imc.shand@googlemail.com
EMail: imc.shand@googlemail.com
Bruno Decraene France Telecom 38-40 rue du General Leclerc 92794 Issy Moulineaux cedex 9 FR
Bruno Decarene法国电信公司Leclerc将军街38-40号92794 Issy Moulineaux cedex 9 FR
EMail: bruno.decraene@orange.com
EMail: bruno.decraene@orange.com
James Uttaro AT&T 200 S. Laurel Avenue Middletown, NJ 07748 US
James Uttaro AT&T美国新泽西州米德尔敦月桂大道200号，邮编07748
EMail: uttaro@att.com
EMail: uttaro@att.com
Nicolai Leymann Deutsche Telekom Winterfeldtstrasse 21 10781, Berlin DE
Nicolai Leymann德国电信公司Winterfeldtstrasse 21 10781，德国柏林
EMail: N.Leymann@telekom.de
EMail: N.Leymann@telekom.de
Martin Horneffer Deutsche Telekom Hammer Str. 216-226 48153, Muenster DE
马丁·霍内弗德国电信哈默街216-226 48153号，明斯特德
EMail: Martin.Horneffer@telekom.de
EMail: Martin.Horneffer@telekom.de