Network Working Group J. Yu
Request for Comments: 2791 CoSine Communications
Category: Informational July 2000
Network Working Group J. Yu
Request for Comments: 2791 CoSine Communications
Category: Informational July 2000
Scalable Routing Design Principles
可扩展路由设计原则
Status of this Memo
本备忘录的状况
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
本备忘录为互联网社区提供信息。它没有规定任何类型的互联网标准。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The Internet Society (2000). All Rights Reserved.
版权所有（C）互联网协会（2000年）。版权所有。
Abstract
摘要
Routing is essential to a network. Routing scalability is essential to a large network. When routing does not scale, there is a direct impact on the stability and performance of a network. Therefore, routing scalability is an important issue, especially for a large network. This document identifies major factors affecting routing scalability as well as basic principles of designing scalable routing for large networks.
路由对网络至关重要。路由可伸缩性对于大型网络至关重要。当路由无法扩展时，会直接影响网络的稳定性和性能。因此，路由可伸缩性是一个重要的问题，特别是对于大型网络。本文档确定了影响路由可伸缩性的主要因素，以及为大型网络设计可伸缩路由的基本原则。
Table of Contents
目录
1 Introduction .................................. 2
2 Common Routing Design Goals ................... 3
3 Characteristics of Today's Large Networks ..... 3
4 Routing Scaling Issues .......................... 3
4.1 Router Resource Consumption ..................... 4
4.2 Routing Complexity .............................. 5
5 Routing Protocol Scalability ..................... 6
5.1 IS-IS and OSPF .................................. 6
5.2 BGP ............................................. 8
6 Scalable Routing Design Principles .............. 9
6.1 Building Hierarchy .............................. 10
6.2 Compartmentalization ............................ 13
6.3 Making Proper Trade-offs ........................ 13
6.4 Reduce Burdens of Routing Information Process ... 14
6.4.1 Routing Intelligence Placement .................. 14
6.4.2 Reduce Routes and Routing Information ........... 15
6.4.2.1 CIDR and Route Aggregation ...................... 15
6.4.2.2 Utilize Default Routing where it's Possible ..... 15
6.4.2.3 Reduce Alternative Paths ........................ 16
6.4.3 Use Static Route at Edge ......................... 16
6.4.4 Minimize the Impact of Route Flapping ............ 16
6.5 Scalable Routing Policy and Scalable Implementation 17
6.6 Out-of-band Process .............................. 19
7 Conclusion and Discussion ........................ 19
8 Security Considerations .......................... 20
9 Acknowledgement .................................. 21
10 References ....................................... 21
Author's Address .............................................. 22
Appendix A Out-of-Band Routing Processes .................... 23
Full Copyright Statement ..................................... 26
1 Introduction .................................. 2
2 Common Routing Design Goals ................... 3
3 Characteristics of Today's Large Networks ..... 3
4 Routing Scaling Issues .......................... 3
4.1 Router Resource Consumption ..................... 4
4.2 Routing Complexity .............................. 5
5 Routing Protocol Scalability ..................... 6
5.1 IS-IS and OSPF .................................. 6
5.2 BGP ............................................. 8
6 Scalable Routing Design Principles .............. 9
6.1 Building Hierarchy .............................. 10
6.2 Compartmentalization ............................ 13
6.3 Making Proper Trade-offs ........................ 13
6.4 Reduce Burdens of Routing Information Process ... 14
6.4.1 Routing Intelligence Placement .................. 14
6.4.2 Reduce Routes and Routing Information ........... 15
6.4.2.1 CIDR and Route Aggregation ...................... 15
6.4.2.2 Utilize Default Routing where it's Possible ..... 15
6.4.2.3 Reduce Alternative Paths ........................ 16
6.4.3 Use Static Route at Edge ......................... 16
6.4.4 Minimize the Impact of Route Flapping ............ 16
6.5 Scalable Routing Policy and Scalable Implementation 17
6.6 Out-of-band Process .............................. 19
7 Conclusion and Discussion ........................ 19
8 Security Considerations .......................... 20
9 Acknowledgement .................................. 21
10 References ....................................... 21
Author's Address .............................................. 22
Appendix A Out-of-Band Routing Processes .................... 23
Full Copyright Statement ..................................... 26
Routing is essential to a network. Without routing, packets cannot be delivered to desired destinations and the network would be non-functional. The challenge of designing the routing for a large network, such as a large ISP backbone network, is not only to make it work, but also to make it scale. Without a scalable routing system, a network may suffer from severe performance penalties, as unfortunately proven by disastrous events in large networks. This document attempts to analyze routing scalability issues and define a set of principles for designing scalable routing system for large networks.
路由对网络至关重要。如果没有路由，数据包无法传送到所需的目的地，网络将无法正常工作。为大型网络（如大型ISP骨干网）设计路由的挑战不仅在于使其工作，而且在于使其具有可扩展性。如果没有可扩展的路由系统，网络可能会遭受严重的性能损失，不幸的是，大型网络中的灾难性事件证明了这一点。本文试图分析路由可伸缩性问题，并为大型网络设计可伸缩路由系统定义一套原则。
The organization of this document is as follows: Section 2 describes routing functions and design goals. Sections 3 and 4 discuss the
本文件的组织结构如下：第2节描述了路由功能和设计目标。第3节和第4节讨论了
characteristics of today's large networks and the associated routing scaling issues. Section 5 explores routing protocol scalability, and Section 6 presents scalable routing design principles. Section 7 provides a conclusion to the document.
当今大型网络的特点和相关的路由扩展问题。第5节探讨了路由协议的可伸缩性，第6节介绍了可伸缩路由设计原则。第7节为本文件提供了一个结论。
The basic goals a routing system should achieve are as follows:
路由系统应实现的基本目标如下：
o Stability o Redundancy and robustness o Reasonable convergency time o Routing information integrity o Sensible and manageable routing policy
o 稳定性o冗余和鲁棒性o合理的收敛时间o路由信息完整性o合理和可管理的路由策略
The challenge of designing routing in a large network is not only to achieve these basic goals but also to make the routing system scale.
在大型网络中设计路由的挑战不仅在于实现这些基本目标，而且在于使路由系统具有可扩展性。
Today's large networks typically possess the following features:
当今的大型网络通常具有以下特点：
o They are composed of a large number of nodes (routers and/or switches), typically in the hundreds. Some provider networks include customer CPE routers within their administrative domain, which increases the number of nodes to thousands.
o 它们由大量节点（路由器和/或交换机）组成，通常有数百个。一些提供商网络在其管理域内包括客户CPE路由器，这将节点数量增加到数千个。
o They have rich connectivity to meet redundancy and robustness requirements, and they consequently have complex topologies.
o 它们具有丰富的连接性以满足冗余和健壮性要求，因此具有复杂的拓扑结构。
o They are default-free; that is, they carry all the routes known to the entire Internet. Currently, the total number is approximately 70,000.
o 它们是无违约的；也就是说，它们承载着整个互联网已知的所有路由。目前，总数约为70000人。
o The customer aggregation routers inside the large networks connect sometimes hundreds of customer routers.
o 大型网络中的客户聚合路由器有时连接数百个客户路由器。
These characteristics impose a direct challenge to the routing scalability of the network.
这些特性对网络的路由可伸缩性提出了直接挑战。
Today, the main issues surrounding routing scaling are: i) excessive router resource consumption, which can potentially increase routing convergency difficulties thus destabilize a network; and ii) routing complexity, resulting in poor management of network, producing low service quality.
今天，围绕路由扩展的主要问题是：i）过度的路由器资源消耗，这可能会增加路由收敛困难，从而破坏网络的稳定性；ii）路由复杂，导致网络管理不善，服务质量低下。
The routing process puts bursty loads on routers, especially under unstable network conditions. In the extreme case, the routing process takes all available resources from the routers, which results in slow routing convergence or no convergence. A network is paralyzed when it cannot converge internal routing information.
路由过程会给路由器带来突发性负载，尤其是在不稳定的网络条件下。在极端情况下，路由过程从路由器获取所有可用资源，这导致路由收敛缓慢或不收敛。当网络无法聚合内部路由信息时，网络将瘫痪。
It's worthy noting that routers with internal architectures that tightly couple forwarding and routing processes tend to handle the excessive routing load poorly. The emerging new generation of routers with the architecture of separating resource used for forwarding and routing could provide better routing scalability.
值得注意的是，内部架构将转发和路由过程紧密耦合的路由器往往无法很好地处理过多的路由负载。新兴的新一代路由器具有分离用于转发和路由的资源的架构，可以提供更好的路由可扩展性。
Today, a large network typically employs IS-IS [1,2] or OSPF [3] as an Interior Routing Protocol(IGP) and BGP [4] as an Exterior Routing Protocol(EGP), respectively. The IGP calculates paths across the interior of the network. BGP facilitates routing exchange between routing domains, or Autonomous Systems (AS). BGP also processes and propagates external routing information within the network. The presence of a large number of routers and adjacencies in a network, coupled with frequent topology changes due to link instability, will contribute to excessive resource consumption by the interior routing. In the case of exterior routing, a large quantity of routers in a BGP system plus frequent routing updates (route flapping) would put a heavy burden on the routers. Section 5 describes scaling issues with IS-IS, OSPF and BGP in detail.
如今，大型网络通常分别使用IS-IS[1,2]或OSPF[3]作为内部路由协议（IGP）和BGP[4]作为外部路由协议（EGP）。IGP计算网络内部的路径。BGP促进了路由域或自治系统（AS）之间的路由交换。BGP还处理和传播网络内的外部路由信息。网络中存在大量路由器和邻接，再加上由于链路不稳定而导致的频繁拓扑变化，将导致内部路由过度消耗资源。在外部路由的情况下，BGP系统中的大量路由器加上频繁的路由更新（路由抖动）将给路由器带来沉重的负担。第5节详细描述了IS-IS、OSPF和BGP的扩展问题。
In addition, having many destinations in a routing system, combined with multiple paths associated with these routes, impose the following scaling issues on BGP:
此外，在路由系统中有多个目的地，再加上与这些路由相关联的多条路径，会给BGP带来以下扩展问题：
o A large number of routes combined with multiple paths for each increases the cost of routing processing for route selection, routing policy application and filtering.
o 大量路由与每条路由的多条路径相结合会增加路由选择、路由策略应用和筛选的路由处理成本。
o Too many routes combined with multiple paths requires large amounts of memory on routers for storage. The demand is even higher at InterExchange Points such as NAPs.
o 太多的路由与多条路径相结合，需要路由器上的大量内存进行存储。在交换点（如NAPs）的需求更高。
o The larger the number of routes, the greater the chance route flapping will occur and the more BGP routing updates will happen as a result. Based on statistics collected by [5], thousands of BGP updates in a measured 15 minute interval can occur on a typical default-free router at a NAP.
o 路由数越大，发生路由抖动的可能性越大，因此BGP路由更新的次数也越多。根据[5]收集的统计数据，在一个典型的无默认路由器上，在一个NAP中，可以在15分钟的时间间隔内进行数千次BGP更新。
Route flapping refers to frequent routing updates occurring due to network instability, for example, when the state of a physical link in the network is fluctuating, or when a BGP session is torn down and re-established numerous time within a short period of time.
路由抖动是指由于网络不稳定而频繁发生的路由更新，例如，当网络中的物理链路的状态波动时，或者当BGP会话在短时间内多次中断并重新建立时。
To facilitate fast convergence, topology change information must be propagated in a timely fashion. When a route becomes unavailable and is withdrawn, the information is typically sent immediately. If the affected routes have been announced to the global Internet, the update information is likely to be propagated to the entire Internet.
为了促进快速收敛，必须及时传播拓扑更改信息。当路由变得不可用并被撤销时，通常会立即发送信息。如果已向全球互联网公布受影响的路由，则更新信息可能会传播到整个互联网。
Route flapping has a profound impact on routers running BGP. The routers have to process routing information frequently and this consumes a tremendous amounts of the available resources. When a local route or link is oscillating, interior routing is affected as well by excessive topology information flooding and subsequent shortest path calculations. However, OSPF (or IS-IS) imposes rate limits on such activity to reduce the burden on the routers. For example, OSPF specifies that an individual SLA can be updated at most once every 5 seconds. This essentially dampens the flapping.
路由抖动对运行BGP的路由器有着深远的影响。路由器必须频繁地处理路由信息，这会消耗大量的可用资源。当本地路由或链路发生振荡时，内部路由也会受到过度拓扑信息泛滥和后续最短路径计算的影响。然而，OSPF（或IS-IS）对此类活动施加速率限制，以减轻路由器的负担。例如，OSPF指定单个SLA最多每5秒更新一次。这基本上抑制了拍打。
Moreover, large numbers of E-BGP sessions processed by a single router create another potential scaling issue. Large networks usually have huge customer subscriptions and connections. To scale the hardware and the number of nodes in the network, providers tend to dedicate a group of customer aggregation routers, each connecting as many customer CPE routers as possible. As a result, it's not uncommon for a customer aggregation router to handle hundreds of E-BGP sessions, which imposes potential problems, such as BGP session processing and maintenance, route processing, filtering and route storage.
此外，由单个路由器处理的大量E-BGP会话会产生另一个潜在的扩展问题。大型网络通常有大量的客户订阅和连接。为了扩展网络中的硬件和节点数量，提供商倾向于专用一组客户聚合路由器，每个路由器连接尽可能多的客户CPE路由器。因此，客户聚合路由器处理数百个E-BGP会话并不少见，这会带来潜在问题，例如BGP会话处理和维护、路由处理、过滤和路由存储。
Routing complexity can lead to network management difficulties, which will have an impact on trouble shooting and quick problem resolution. It can result in a less than desirable service quality across the network. Complicated routing policies and special cases or exceptions in a routing design can contribute to routing complexity in a large system.
路由复杂性会导致网络管理困难，这将影响故障排除和快速问题解决。这可能会导致整个网络的服务质量不理想。复杂的路由策略和路由设计中的特殊情况或异常会导致大型系统中的路由复杂性。
Routing Policy refers to the administrative criteria for handling routing information, commonly in the form of routing path selection and route filtering. The way routing information is handled has a direct impact on traffic flow within a network and across domains. As
路由策略是指处理路由信息的管理标准，通常采用路由路径选择和路由筛选的形式。路由信息的处理方式直接影响网络内和跨域的流量。像
a result, it affects business agreements among different networks. Therefore, the determination of routing policy is largely dominated by non-technical concerns, such as business considerations. Routing policy can be very complex, which would make management and configuration an unscalable task.
因此，它会影响不同网络之间的业务协议。因此，路由策略的确定在很大程度上取决于非技术问题，如业务考虑。路由策略可能非常复杂，这将使管理和配置成为一项无法扩展的任务。
The keys to reducing routing complexity are systematic as well as consistent routing scheme and a routing policy that is simple but meets the requirement of administrative polices.
降低路由复杂度的关键是系统的、一致的路由方案和简单但满足管理策略要求的路由策略。
Another factor contributing to the complexity of routing management is prefix-based route filtering. As is well known, prefix-based filtering is necessary in order to protect the integrity of the routing system. This becomes a challenge when the number of routes known to the Internet is as large as it is today.
导致路由管理复杂性的另一个因素是基于前缀的路由过滤。众所周知，为了保护路由系统的完整性，基于前缀的过滤是必要的。当互联网已知的路由数量与今天一样大时，这就成为了一个挑战。
Today's commonly deployed routing protocols are IS-IS or OSPF for Interior routing (aka IGP) and BGP for exterior routing (aka EGP). In terms of scaling and other aspects, these protocols are already an improvement over the previous generation of protocols, such as RIP and EGP. However, scalability is still a major issue when a network is large, when a routing design is insensitive to scaling issues, or the protocol implementation is inefficient.
目前常用的路由协议有用于内部路由的IS-IS或OSPF（又名IGP）和用于外部路由的BGP（又名EGP）。在可扩展性和其他方面，这些协议已经比上一代协议（如RIP和EGP）有所改进。然而，当网络很大、路由设计对扩展问题不敏感或协议实现效率低下时，可伸缩性仍然是一个主要问题。
As described earlier in the document, IS-IS and OSPF are Link State routing protocols. The basic components of a link state routing protocol are i) generation and maintenance of a Link-State-DataBase (LSDB) that describes the routing topology of a given routing area; and ii) route calculation based on the topology information in the database. Each node in a routing area is responsible for describing its local routing topology in a Link State Advertisement or LSA (LSP in the case of IS-IS.) Each individually generated LSA will be distributed or flooded to all the routers in the area. Each router receives LSAs from all the other routers, forming a link-state-database that reflects the routing topology of the entire routing area.
如本文档前面所述，IS-IS和OSPF是链路状态路由协议。链路状态路由协议的基本组成部分是i）生成和维护描述给定路由区域的路由拓扑的链路状态数据库（LSDB）；ii）根据数据库中的拓扑信息计算路线。路由区域中的每个节点负责在链路状态公告或LSA（在is-is的情况下为LSP）中描述其本地路由拓扑。每个单独生成的LSA将被分发或淹没到该区域中的所有路由器。每个路由器从所有其他路由器接收LSA，形成反映整个路由区域的路由拓扑的链路状态数据库。
The main associated scaling issues are the complexity of the link state flooding and routing calculation, plus the size of the LSDB which contributes to the cost of routing calculation and router memory consumption.
主要相关的扩展问题是链路状态泛洪和路由计算的复杂性，加上LSDB的大小，这会导致路由计算和路由器内存消耗的成本。
Flooding is the process by which a router distributes its self-originated LSA to the rest of the routers in the area in case of any link state change. A router will send the LSA via all its interfaces. When receiving an LSA update, a router validates the information and updates its local LSDB before sending it out via all its own interfaces, except the one from which it received the original LSA update. Given the nature of IS-IS or OSPF flooding, a full-mesh network with N routers would have O(N^2) of LSAs flooded in the network when a single link failure occurs. A single router outage would cause LSA in the order of O(N^3) to be flooded in the system.
泛洪（Flooding）是路由器在链路状态发生任何变化时将其自创LSA分发给该区域内其他路由器的过程。路由器将通过其所有接口发送LSA。当接收到LSA更新时，路由器会验证信息并更新其本地LSDB，然后再通过其自己的所有接口（从中接收原始LSA更新的接口除外）将其发送出去。鉴于IS-IS或OSPF泛洪的性质，当发生单链路故障时，具有N个路由器的全网状网络将有O（N^2）个LSA泛洪在网络中。单路由器中断将导致系统中O（N^3）级LSA被淹没。
In the case of OSPF, the protocol will refresh or flood every 30 minutes even under stable network conditions, which could increase the problem for an already highly loaded router.
在OSPF的情况下，即使在稳定的网络条件下，协议也会每30分钟刷新或泛滥一次，这可能会增加已经高负载路由器的问题。
From the above discussion, one can easily observe that the more routers and adjacencies in a Link State IGP routing area, the more CPU burden there are for each router to bear. When a network is unstable, the load will be amplified.
从上面的讨论中，我们可以很容易地观察到，链路状态IGP路由区域中的路由器和邻接越多，每个路由器要承受的CPU负担就越大。当网络不稳定时，负载将被放大。
A link-state protocol typically uses Dijkstra's Shortest Path First (SPF) algorithm for route calculation. The Dijkstra algorithm scales to the order of O(N^2), where N is the number of nodes. The algorithm could be improved to the order of O(l*logN) where l is the number of links in the network and N is the number of destinations or routers [6].
链路状态协议通常使用Dijkstra的最短路径优先（SPF）算法进行路由计算。Dijkstra算法按O（N^2）的顺序进行缩放，其中N是节点数。该算法可以改进为O（l*logN）的顺序，其中l是网络中的链路数量，N是目的地或路由器的数量[6]。
Consequently, link state routing protocols do not scale to a network topology with many routers and excessive adjacencies in an area. When the network topology is unstable, the computation, processing and bandwidth costs are magnified, which causes excessive consumption of router resources. When the instability prevents IS-IS or OSPF from maintaining adjacencies, a network routing meltdown occurs.
因此，链路状态路由协议不能扩展到一个区域中有许多路由器和过度邻接的网络拓扑。当网络拓扑结构不稳定时，计算、处理和带宽成本会被放大，从而导致路由器资源的过度消耗。当不稳定性阻止IS-IS或OSPF保持邻接时，就会发生网络路由崩溃。
Node adjacencies are discovered and maintained through the exchange of HELLO messages sent periodically from each node. When a node fails to receive HELLO messages from its neighbor within a certain period of time (40 seconds for OSPF and less for IS-IS), it considers the neighbor down. When heavy flooding, re-calculation and other activities happen that make router CPU a scarce resource, a router may not be able to allocate CPU time to send or process HELLO packets. Routers in the network then lose adjacency, which magnifies the instability. As a result, an isolated instability can escalate to a routing failure across the entire network.
通过交换从每个节点定期发送的HELLO消息来发现和维护节点邻接。当一个节点在一段时间内（OSPF为40秒，IS-IS为更少）无法从其邻居接收HELLO消息时，它会认为邻居已关机。当大量的洪泛、重新计算和其他活动发生，使路由器CPU成为稀缺资源时，路由器可能无法分配CPU时间来发送或处理HELLO数据包。然后网络中的路由器失去了邻接性，这加剧了不稳定性。因此，孤立的不稳定性可能升级为整个网络的路由故障。
Link-state IGPs also do not scale well to carry a large number of routes such as the 70,000 routes known to the Internet today. Since external routes are included in the link-state-database and in LSA
链路状态IGP也不能很好地扩展以承载大量的路由，例如今天互联网上已知的70000条路由。因为外部路由包含在链路状态数据库和LSA中
(LSP for IS-IS) updates, the link bandwidth and router memory consumption will be tremendous. Moreover, due to the large size of LSA updates, it would aggravate router resource consumption in the process of LSA flooding, especially under unstable network condition.
（LSP用于IS-IS）更新，链路带宽和路由器内存消耗将是巨大的。此外，由于LSA更新的规模较大，在LSA泛洪过程中，特别是在网络不稳定的情况下，会加剧路由器资源的消耗。
To summarize, a scalable design should avoid inclusion of too many routers in an IGP routing area, a large external routes carried by IGP and, more important, excessive adjacencies in the area.
总之，一个可扩展的设计应该避免在一个IGP路由区域中包含太多的路由器、IGP承载的大量外部路由，更重要的是，避免该区域中的过度邻接。
BGP is an inter-domain routing protocol allowing the exchange of routing or reachability information between different Autonomous-System networks. Functionally, BGP is composed of External BGP(E-BGP) and Internal BGP(I-BGP). E-BGP is used for exchanging external routes while I-BGP is typically used for distributing externally learned routes within an AS.
BGP是一种域间路由协议，允许在不同的自治系统网络之间交换路由或可达性信息。在功能上，BGP由外部BGP（E-BGP）和内部BGP（I-BGP）组成。E-BGP用于交换外部路由，而I-BGP通常用于在AS中分发外部学习的路由。
The general costs of BGP are as follows:
BGP的一般成本如下：
o CPU consumption in BGP session establishment, route selection, routing information processing, and handling of routing updates
o BGP会话建立、路由选择、路由信息处理和路由更新处理中的CPU消耗
o Router memory to install routes and multiple paths associated with the routes.
o 路由器内存用于安装路由和与路由关联的多个路径。
The major scaling issue associated with BGP lie in the full mesh I-BGP connections. Since it does not scale for an IGP to carry externally learned prefixes, as mentioned in the previous section, I-BGP assumes this duty. In order to prevent routing loops, prefixes learned via I-BGP are prohibited from being advertised to another I-BGP speaker. As a result, a full mesh of I-BGP sessions among the routers within an AS is required. In an AS with N routers, each router will have to establish I-BGP sessions with N-1 routers, and the system complexity is in the order of O(N^2). Therefore, BGP scales poorly when the number of routers involved in I-BGP mesh is large.
与BGP相关的主要扩展问题在于全网状I-BGP连接。如前一节所述，由于IGP不能携带外部习得前缀，因此I-BGP承担此职责。为了防止路由循环，禁止向其他I-BGP扬声器播发通过I-BGP学习的前缀。因此，As中路由器之间的I-BGP会话需要完全网状。在具有N个路由器的AS中，每个路由器必须与N-1个路由器建立I-BGP会话，系统复杂性为O（N^2）。因此，当I-BGP网格中涉及的路由器数量较大时，BGP的扩展性较差。
A large network normally learns all the routes known to the Internet, which is approximately 70,000. I-BGP will need to carry all these routes.
一个大型网络通常学习互联网上已知的所有路由，大约70000条。I-BGP需要承载所有这些路线。
The large number of I-BGP sessions and routes consumes tremendous resources from each router, especially during BGP session establishment and during periods of heavy route flapping.
大量的I-BGP会话和路由消耗每个路由器的巨大资源，特别是在BGP会话建立期间和重路由摆动期间。
Frequent routing updates are another potential scaling problem in large networks. BGP uses incremental updates and sends out routing information about unreachable routes quickly for fast convergence. This is a great improvement from EGP, in which the whole routing table is updated at a fixed time interval. However, when a network is unstable the updates, especially those containing route withdrawals, are sent immediately, causing global BGP updates. As a result, network instability initiated anywhere in a network triggers updates all over the Internet. This effect is magnified when large amounts of routes are visible to the Internet, putting a heavy load on routers that participate in BGP.
频繁的路由更新是大型网络中另一个潜在的扩展问题。BGP使用增量更新并快速发送有关无法到达的路由的路由信息，以实现快速收敛。这是对EGP的一大改进，在EGP中，整个路由表以固定的时间间隔更新。但是，当网络不稳定时，会立即发送更新，尤其是包含路由撤回的更新，从而导致全局BGP更新。因此，网络中任何地方引发的网络不稳定会触发整个互联网的更新。当大量的路由在互联网上可见时，这种影响会被放大，给参与BGP的路由器带来沉重的负载。
The introduction of a routing hierarchy in BGP, through I-BGP Route Reflectors [7] and BGP Confederations [8], for example, will help alleviate the scaling problem caused by the requirement of full mesh I-BGP establishment.
例如，通过I-BGP路由反射器[7]和BGP联盟[8]，在BGP中引入路由层次结构，将有助于缓解由全网格I-BGP建立需求引起的缩放问题。
Another potential solution is to avoid the requirement of full mesh pairwise I-BGP connections. This will change the way that BGP distributes routing information among the I-BGP peers. Mechanisms worth considering are using multicast to distribute information or adopting flooding mechanisms similar to those used in IS-IS or OSPF. Further investigation of the implication of using such mechanism for BGP route distribution is needed.
另一个潜在的解决方案是避免要求全网成对I-BGP连接。这将改变BGP在I-BGP对等方之间分发路由信息的方式。值得考虑的机制是使用多播来分发信息，或者采用类似于IS-IS或OSPF中使用的泛洪机制。需要进一步研究使用这种机制进行BGP路由分配的含义。
Route dampening [9] is one way to reduce excessive updates triggered by route flapping. The trade-off between fast convergence and stability of the network should be considered, as discussed in section 6.3.
路由阻尼[9]是减少路由抖动触发的过度更新的一种方法。如第6.3节所述，应考虑快速收敛和网络稳定性之间的权衡。
The routing design for a large-scale network should achieve the basic goals of accuracy, stability, redundancy and convergence as described in Section 2 and moreover should achieve it in a scalable fashion.
大规模网络的路由设计应达到第2节所述的准确性、稳定性、冗余性和收敛性的基本目标，并且应以可扩展的方式实现。
How routing scales is influenced by protocol design decisions, protocol implementation decisions, and network design decisions. A network engineer has direct control over network design decisions and can have substantial influence over protocol design and implementation. The focus of this document is network design decisions.
路由规模如何受协议设计决策、协议实现决策和网络设计决策的影响。网络工程师可以直接控制网络设计决策，并对协议设计和实现产生重大影响。本文件的重点是网络设计决策。
Following is a set of design principles for making a large network routing system more scalable:
以下是一组使大型网络路由系统更具可扩展性的设计原则：
o Building hierarchy o Compartmentalization o Making proper trade-offs o Reducing route processing burdens o Defining scalable routing policies and implementation o Utilizing out-of-band routing assistance
o 构建层次结构o划分o做出适当的权衡o减少路由处理负担o定义可扩展的路由策略和实施o利用带外路由辅助
As discussed in Section 5.1, OSPF and IS-IS scale poorly when a network has a large number of routers and in particular, a large quantity of adjacencies. This has unfortunately been proven by networks that deploy IP over ATM with full mesh adjacencies among the routers. The full mesh overlay design combined with the inefficient protocol implementation led to disastrous network outages. A lesson learned from this is to avoid full mesh overlay topology in a large network with a large, flat network routing structure.
如第5.1节所述，当网络具有大量路由器，尤其是大量邻接时，OSPF和IS-IS的规模很小。不幸的是，这已经被在路由器之间采用全网状邻接的ATM上部署IP的网络所证明。全网状覆盖设计与低效的协议实现相结合导致了灾难性的网络中断。从中吸取的教训是，在具有大型扁平网络路由结构的大型网络中，避免全网格覆盖拓扑。
Building hierarchical routing structures in the network is the key to achieving routing scalability in a large network. As discussed earlier in this document, large networks are usually composed of many routers with a complex topology, which results in a large number of adjacencies. As also discussed earlier, currently available routing protocols scale poorly for handling a large number of routers in a routing domain or many adjacencies among the routers. Therefore, it is sensible to build a routing hierarchy to reduce the number of routers as well as the number of adjacencies in a routing domain.
在网络中建立分层路由结构是在大型网络中实现路由可伸缩性的关键。正如本文档前面所讨论的，大型网络通常由许多具有复杂拓扑结构的路由器组成，这会导致大量的邻接。正如前面所讨论的，当前可用的路由协议在处理路由域中的大量路由器或路由器之间的许多邻接时伸缩性较差。因此，建立一个路由层次结构以减少路由域中路由器的数量以及邻接的数量是明智的。
The current common practice is to build a two-tiered hierarchy in a network with a center component (or transit core network) to which a number of outskirt components (or access networks) attach. The transit core network covers the entire geographical area the network serves; each access network (aka regional network) covers one region. There are usually no direct link connections among the regional components. Traffic from one regional network to another traverses the transit core. Customer networks connect only to access or regional networks. There are a number of ways to build a routing hierarchy in the above described hierarchical network topology.
当前的常见做法是在网络中构建两层层次结构，其中包含一个中心组件（或公交核心网络），多个郊区组件（或接入网络）连接到该中心组件。公交核心网络覆盖网络服务的整个地理区域；每个接入网络（又名区域网络）覆盖一个区域。区域组成部分之间通常没有直接联系。从一个区域网络到另一个区域网络的流量穿过运输中心。客户网络仅连接到接入或区域网络。在上述分层网络拓扑中，有许多方法可以构建路由层次结构。
1) Completely Separate Routing Domains
1) 完全分离的路由域
This design treats the transit core network and each regional network as completely independent ASs with respect to routing, and each AS runs an independent IGP. Each regional network E-BGP with the transit core for exchanging routing knowledge. Full I-BGP
该设计将公交核心网和每个区域网视为路由方面完全独立的ASs，每个as运行一个独立的IGP。每个区域网络E-BGP都有一个中转核心，用于交换路由知识。全I-BGP
connections need to be established only within each component network. With this design, the maximum number of routers in an IGP domain is the total number of routers in each component. As a result, the IGP processing load is reduced, and the number of routers in an I-BGP mesh in the network routing system is decreased dramatically.
只需在每个组件网络内建立连接。在这种设计中，IGP域中路由器的最大数量是每个组件中路由器的总数。结果，IGP处理负载降低，并且网络路由系统中的I-BGP网格中的路由器数量显著减少。
Another advantage of this design is that it compartmentalizes the routing system so that instability in one such component has less impact on the entire system. See the discussion in section 6.2.
这种设计的另一个优点是，它将路由系统划分为多个部分，因此其中一个部分的不稳定性对整个系统的影响较小。见第6.2节中的讨论。
The main disadvantage of this scheme is that it inserts one extra AS in the routing path when routes are advertised to the Internet via BGP. This extra AS in the path may cause route selection difficulties for other providers.
该方案的主要缺点是，当路由通过BGP播发到Internet时，在路由路径中插入一个额外的AS。路径中的这种额外AS可能会导致其他提供商的路由选择困难。
2) One Domain with IGP and BGP Hierarchy
2) 一个具有IGP和BGP层次结构的域
This method includes the transit core and each regional network into one AS domain. The routing hierarchy is realized by utilizing multi-level IS-IS or OSPF areas and either BGP Confederation or I-BGP Reflector or a combination of the two.
该方法将公交核心网和各区域网络合并为一个AS域。路由层次结构通过利用多级is-is或OSPF区域以及BGP联盟或I-BGP反射器或两者的组合来实现。
This mechanism avoids the introduction of an extra AS in the routing path, which is an advantage over the method described in Point 1). However, multi-area hierarchical IGP is rarely used now-a-days in large networks since most of them are using IS-IS for internal routing, which does not have sufficient multi-level support. Although IS-IS supports multi-area routing, it imposes a strict hierarchy between backbone and sub-areas and allows only the advertisement of a default route from the backbone area to the sub-areas instead of specific prefixes. This restriction may be suitable for a network with a simple sub-area topology. A sub-area in a large network, typically a regional or access network, itself has a complicated topology. Receiving highly abstract routing information, such as a default route, would affect the sub-area's ability to make route selections required for traffic engineering. It would also limit the information passed to external ASs, for example, IGP-derived BGP Multi-Exit-Discriminator (MED) information.
这种机制避免了在路由路径中引入额外的AS，这是第1点所述方法的优势。然而，目前在大型网络中很少使用多区域分层IGP，因为大多数网络使用is-is进行内部路由，而内部路由没有足够的多级支持。虽然IS-IS支持多区域路由，但它在主干和子区域之间施加了严格的层次结构，并且只允许从主干区域到子区域的默认路由的公布，而不允许特定前缀。此限制可能适用于具有简单子区域拓扑的网络。大型网络（通常是区域或接入网络）中的子区域本身具有复杂的拓扑结构。接收高度抽象的路由信息（如默认路由）将影响子区域选择交通工程所需路由的能力。它还将限制传递给外部ASs的信息，例如IGP派生的BGP多出口鉴别器（MED）信息。
Efforts are being made to modify the IS-IS protocol to allow the distribution of specific route from backbone area to sub-areas. A mechanism facilitates such distribution is specified in [15]. When implementation of such mechanism become available, implementing multi-level IGP will be an attractive option for building routing hierarchy within a large network.
正在努力修改IS-IS协议，以允许将特定路由从主干区域分配到子区域。[15]中规定了促进此类分配的机制。当实现这种机制成为可能时，实现多级IGP将是在大型网络中构建路由层次结构的一个有吸引力的选择。
3) One IGP Area with BGP Hierarchy
3) 一个具有BGP层次结构的IGP区域
In lieu of multi-area IS-IS, the routing hierarchy could be achieved by defining one IGP domain for the entire network while employing a BGP hierarchy. Fortunately, the hierarchical topology of the network in this case helps reduce adjacencies in the routing domain (recall there are no connections among the second-level network components). In addition, improvements could be made to further reduce the adjacency by carefully arranging the adjacencies to keep them at a minimum but still achieve good redundancy. However, this is less than ideal since the number of routers remains unchanged, which increases the load on the SPF calculation. Moreover, instability within any regional network would still affect the entire network (that is, there would be no fault isolation).
代替多区域IS-IS，可以通过在使用BGP层次结构的同时为整个网络定义一个IGP域来实现路由层次结构。幸运的是，在这种情况下，网络的分层拓扑有助于减少路由域中的邻接（回想一下，二级网络组件之间没有连接）。此外，还可以进行改进，通过仔细安排邻接来进一步减少邻接，使其保持在最小值，但仍能实现良好的冗余。然而，这并不理想，因为路由器的数量保持不变，这增加了SPF计算的负载。此外，任何区域网络内的不稳定性仍然会影响整个网络（即，不会有故障隔离）。
Even with one IGP domain, it is possible to build BGP hierarchy to make I-BGP more scalable in the network. BGP Reflectors and BGP Confederations are existing mechanisms to address the scaling problem of full-mesh I-BGP.
即使只有一个IGP域，也可以构建BGP层次结构，使I-BGP在网络中更具可扩展性。BGP反射器和BGP联盟是解决全网格I-BGP缩放问题的现有机制。
Further, a BGP reflector provides the ability to build more than two levels of hierarchy, as long as the interactions among the different levels of the hierarchy are carefully arranged to avoid the possibility of creating routing loops.
此外，BGP反射器提供构建两个以上层次的能力，只要层次的不同层次之间的交互被仔细地安排以避免创建路由循环的可能性。
Questions worth asking are: "Are two levels of routing hierarchy sufficient for handling scaling issues?" "Is there really a need for more than two levels of hierarchy?"
值得一提的问题是：“两级路由层次结构是否足以处理扩展问题？”“是否真的需要两级以上的层次结构？”
When a second-tier sub-domain of a large network, such as a regional network, grows too big for routing protocols to handle, either another layer of hierarchy needs to be introduced or the sub-domain needs to be split into multiple second-tiered sub-domains.
当大型网络的第二层子域（如区域网络）变得太大而无法处理路由协议时，需要引入另一层层次结构，或者需要将子域拆分为多个第二层子域。
Keeping two levels of hierarchy and adding more sub-domains appears to be more manageable than adding another level to the hierarchy. However, one concern is to avoid adding more nodes to the top-level or transit core network to make it less scalable. Connecting the split sub-areas to the same core router would eliminate the need to add more nodes in the core area than is recommended.
保持两个层次结构并添加更多子域似乎比向层次结构添加另一个层次结构更易于管理。然而，一个问题是避免向顶层或传输核心网络添加更多节点，从而降低其可扩展性。将分割的子区域连接到同一个核心路由器将无需在核心区域中添加比推荐数量更多的节点。
Having more than two levels of hierarchy would exceed the capability of IGPs as they are defined today. In OSPF, for example, all the areas must be connected via the backbone area, which eliminates the possibility of having more than two levels of hierarchy. IS-IS has the same limitation. Therefore, the protocols need to be redefined should more than two hierarchical layers in IGP be desirable.
拥有两个以上的层次结构将超过IGP今天定义的能力。例如，在OSPF中，所有区域都必须通过主干区域连接，这消除了具有两个以上层次结构的可能性。IS-IS具有相同的限制。因此，如果IGP中需要两个以上的层次结构，则需要重新定义协议。
The complexity of protocols and management will increase with the number of levels added to the hierarchy. According to [6], most of the OSPF protocol bugs found over the years are related to routing area support. Because the interaction among the multiple levels increases management and debugging complexity, it is desirable to keep the levels within a hierarchy to a minimum.
协议和管理的复杂性将随着层次结构中级别的增加而增加。根据[6]，多年来发现的大多数OSPF协议错误都与路由区域支持有关。由于多个级别之间的交互增加了管理和调试的复杂性，因此希望将层次结构中的级别保持在最低限度。
A scalable routing design of a large network should be able to localize problems or failures, thus preventing them from spreading to the entire network, consuming resources of network routers, and causing network wide instability. This is compartmentalization. Network compartmentalization makes fault isolation possible which contributes the stability of a large network.
大型网络的可扩展路由设计应该能够定位问题或故障，从而防止它们扩散到整个网络，消耗网络路由器的资源，并造成网络范围的不稳定。这就是划分。网络划分使故障隔离成为可能，这有助于大型网络的稳定性。
To achieve compartmentalization in routing design for a large network, one needs to avoid a design where the whole large network is one flat routing system or routing domain. This is the reason for the architecture of dividing interior and exterior routing in the global routing system. Within a network, it is best to divide the network into multiple routing domains or multiple routing areas. For example, in OSPF, only summary route SLAs, rather than individual area routes, are flooded beyond the area. When an area border router aggregates the routes in its sub-area, instability of any route included in the summary route would not cause flooding of SLAs to other areas. As a result, router resources in other areas would not be consumed for handling flooding and the SPF recalculation. In other words, instability within each individual area would be prevented from spreading to the entire routing domain.
为了在大型网络的路由设计中实现分区，需要避免整个大型网络是一个平面路由系统或路由域的设计。这就是在全局路由系统中划分内部路由和外部路由架构的原因。在网络中，最好将网络划分为多个路由域或多个路由区域。例如，在OSPF中，只有摘要路由SLA而不是单个区域路由被淹没在该区域之外。当区域边界路由器聚合其子区域中的路由时，摘要路由中包含的任何路由的不稳定性都不会导致SLA泛滥到其他区域。因此，其他区域的路由器资源不会被用于处理泛洪和SPF重新计算。换言之，将防止每个单独区域内的不稳定性扩散到整个路由域。
Since building a routing hierarchy essentially divides a big routing area into smaller areas or domains, it help achieve the goal of compartmentalization.
由于构建路由层次结构本质上是将一个较大的路由区域划分为较小的区域或域，因此它有助于实现划分的目标。
When designing routing for a large network, the overall goal should be set with considerations of routing scalability and stability. The trade-offs between conflicting goals should be taken into account. Examples of such trade-offs are redundancy vs. scalability and convergence vs. stability.
在为大型网络设计路由时，应考虑路由的可伸缩性和稳定性。应该考虑相互冲突的目标之间的权衡。这种权衡的例子是冗余与可伸缩性、收敛与稳定性。
Redundancy introduces complexity and increased adjacencies to the network topology. Redundancy also imposes the need for as many alternative paths as possible for each route, which increases route
冗余增加了网络拓扑的复杂性和相邻性。冗余还要求每条路由需要尽可能多的备选路径，这增加了路由的可用性
processing and storage burdens. Because of these problems, it may be necessary to sacrifice absolute redundancy in favor of a reasonable level that scales better for the routing system.
加工和储存负担。由于这些问题，可能有必要牺牲绝对冗余，以获得更适合路由系统的合理级别。
Fast convergence requires that changes in network topology be propagated to the network as quickly as possible. Such action increases routing updates and, consequently, the route processing burden. The burden is aggravated when a network carries full Internet routing information, as large networks usually do, and topology changes happen frequently. Route dampening may be necessary to achieve stability at the expense of absolute fast convergence.
快速收敛要求将网络拓扑中的更改尽快传播到网络。这样的操作会增加路由更新，从而增加路由处理负担。当网络承载完整的Internet路由信息时（如大型网络通常所做的那样），并且拓扑结构经常发生变化，这种负担就会加重。路线阻尼可能是必要的，以牺牲绝对快速收敛来实现稳定性。
The tasks of reducing routing processing burdens includes: i) strategically place the routing intelligence within the network, ii) avoid carrying unnecessary routing information and iii) reduce the impact of route flapping.
减少路由处理负担的任务包括：i）战略性地将路由智能放置在网络中，ii）避免携带不必要的路由信息，以及iii）减少路由抖动的影响。
A router that executes routing policies, performs route filtering and dampening is said to posses routing intelligence. Routing intelligence is needed for a network i) to enforce the business agreement between network entities in the form of routing policies; ii) to protect the integrity of the routing information within the network and sometimes iii) to shield a network from instability happening elsewhere in the Internet.
执行路由策略、执行路由过滤和抑制的路由器被称为具有路由智能。网络需要路由智能i）以路由策略的形式强制执行网络实体之间的业务协议；ii）保护网络内路由信息的完整性，有时iii）保护网络免受互联网其他地方发生的不稳定。
The more routing intelligence a router has, the more resources of the router are needed to perform those tasks. It is logical, then, to place as little routing intelligence as possible on routers that already are heavily burdened with other tasks.
路由器拥有的路由智能越多，执行这些任务所需的路由器资源就越多。因此，合理的做法是，尽可能少地将路由智能放在已经承担了大量其他任务的路由器上。
Usually, traffic is heavily concentrated in the core of the network. Because traffic aggregates from the edge of the network toward the core, traffic is less concentrated near the edge of the network. Consequently, to build a scalable routing system, it is wise to place routing intelligence at the edge of the network, especially in the networks deployed with routers that do not sufficiently decouple forwarding and routing. In addition, pushing routing intelligency as close to the edge of the network as possible also serves the purpose of distributing computational and configuration burdens across all routers.
通常，流量主要集中在网络的核心。由于流量从网络边缘向核心聚集，因此流量较少集中在网络边缘附近。因此，为了构建一个可伸缩的路由系统，明智的做法是将路由智能放在网络的边缘，特别是在部署了路由器的网络中，这些路由器没有充分地解耦转发和路由。此外，将路由智能推到尽可能靠近网络边缘的位置也有助于在所有路由器之间分配计算和配置负担。
It is also desirable to move the heavy burden of processing routes to out-of-band processors, freeing more resources in network routers for packet forwarding and handling.
还希望将处理路由的沉重负担转移到带外处理器，从而释放网络路由器中更多的资源用于数据包转发和处理。
As discussed in Section 4.1, a large number of routes in the system is one of the major culprits in route scaling problems. Therefore, it is best to reduce the number of routes in the system without losing necessary routing information.
如第4.1节所述，系统中的大量路由是路由扩展问题的主要原因之一。因此，最好在不丢失必要的路由信息的情况下减少系统中的路由数量。
CIDR as specified in [10] provides a mechanism to aggregate routes for efficiently utilizing IP address space as well as reducing the number of routes in the global routing table. CIDR offers a way to summarize routing information, which is one of the keys for routing scalability in today's Internet.
[10]中规定的CIDR提供了一种聚合路由的机制，以有效利用IP地址空间并减少全局路由表中的路由数。CIDR提供了一种总结路由信息的方法，这是当今互联网中路由可伸缩性的关键之一。
Route aggregation would not only help global Internet scalability but would also contribute to scalability in local networks. The overall goal is to keep the routes in the backbone to a minimum.
路由聚合不仅有助于全球互联网的可扩展性，而且有助于本地网络的可扩展性。总体目标是将主干中的路由保持在最低限度。
To achieve better aggregation within the network; that is, to reduce the number of routes in the network, a block of consecutive IP addresses should be allocated to each access or regional network so that when a regional network announces its routes to the transit core network, they can be aggregated. This way, the core and other regional networks would not need to know the specific prefixes of any particular access network. Although assignment of customer addresses from a provider block would have to be planned to support aggregation, the effort would be worthwhile.
在网络内实现更好的聚合；也就是说，为了减少网络中的路由数量，应该为每个接入或区域网络分配一个连续的IP地址块，以便当区域网络向传输核心网络宣布其路由时，可以聚合它们。这样，核心网络和其他区域网络就不需要知道任何特定接入网络的特定前缀。虽然必须计划从提供者块分配客户地址以支持聚合，但这项工作是值得的。
The use of a default route achieves ultimate route summarization, which reduces routing information to minimum. Route summarization also masks the instability associated with an individual route, for example, in the case of route flapping. It's beneficial for a network to utilize default routing when appropriate. For example, if a second-tiered regional network is a stub and there is no connected customer requesting full Internet routing information, the regional network can simply point default to its connected core network. However, over-summarization of routing information has the danger of losing routing granularity and as a result, management of network such as traffic engineering would be adversely affected. Therefore, caution needs to be exercised when using default routing.
使用默认路由可以实现最终的路由摘要，从而将路由信息减少到最小。路由摘要还掩盖了与单个路由相关的不稳定性，例如，在路由抖动的情况下。在适当的时候利用默认路由对网络是有益的。例如，如果第二层区域网络是存根，并且没有连接的客户请求完整的Internet路由信息，则区域网络可以简单地将默认值指向其连接的核心网络。然而，路由信息的过度汇总有丢失路由粒度的危险，因此，网络管理（如流量工程）将受到不利影响。因此，在使用默认路由时需要谨慎。
Due to the requirement of reliability, the connectivity in the Internet is rich, resulting in many paths toward a particular destination. In other words, there are many alternate paths in the BGP routing table towards the same destination, which consumes router memory and adds to the routing processing burden.
由于可靠性的要求，互联网中的连接是丰富的，导致通往特定目的地的许多路径。换句话说，BGP路由表中有许多通向同一目的地的备用路径，这会消耗路由器内存并增加路由处理负担。
To make routing scale, it is desirable to reduce alternate paths while preserving reasonable redundancy. For example, on a given border router (such as a NAP router), one primary path plus an alternate path should provide reasonable redundancy. In this case, a third or a fourth alternate route could be discarded for the sake of scaling. This is a trade-off decision every network administrator needs to make based on the particular needs of her network.
为了实现路由规模，需要在保持合理冗余的同时减少备用路径。例如，在给定的边界路由器（如NAP路由器）上，一条主路径加上一条备用路径应提供合理的冗余。在这种情况下，为了扩展，可以放弃第三或第四个备用路由。这是每个网络管理员都需要根据其网络的特定需求做出的权衡决定。
As mentioned earlier, one of the scaling issues in large networks is that a single router may fan out to hundreds of customer routers. As a result, resource consumption will be very intensive if all the customer routers communicate via BGP with the edge router. Is it necessary for the edge router to BGP with all of its attached customer routers?
如前所述，大型网络中的扩展问题之一是单个路由器可能会分散到数百个客户路由器。因此，如果所有客户路由器都通过BGP与边缘路由器通信，则资源消耗将非常密集。边缘路由器是否需要将BGP与其所有连接的客户路由器连接起来？
At first glance, it seems necessary for a customer network in a different Autonomous System(AS) to exchange routing information with the provider network via BGP. However, this is not necessarily the case. When a customer network is single-homed (that is, if the sole network connection for a customer is via its provider network), BGP is not necessary and static routing can work. Since the customer network is single-homed, static routing will not have any negative impact on services. The advantages are that the customer aggregation router will have fewer E-BGP sessions to handle, and no route flapping can result from the statically configured customer routes.
乍一看，不同自治系统（AS）中的客户网络似乎有必要通过BGP与提供商网络交换路由信息。然而，情况未必如此。当客户网络是单宿网络时（即，如果客户的唯一网络连接是通过其提供商网络），则不需要BGP，静态路由可以工作。由于客户网络是单宿网络，静态路由不会对服务产生任何负面影响。其优点是，客户聚合路由器将有较少的E-BGP会话需要处理，并且静态配置的客户路由不会导致路由抖动。
Configuration of the customer's static routes on the provider's aggregation router may add management overhead, especially if a customer advertises a large number of routes. On the other hand, the set of routes a customer announces to the provider usually changes infrequently; thus it requires low maintenance once it is configured.
在提供商的聚合路由器上配置客户的静态路由可能会增加管理开销，特别是当客户公布大量路由时。另一方面，客户向提供商宣布的路线集通常很少改变；因此，一旦配置，它就需要低维护。
As discussed earlier, route flapping is largely caused by link instability and/or BGP session instability that results in excessive routing updates across the Internet. Route flapping can originate anywhere in the global Internet and affect every network in the
如前所述，路由抖动主要是由链路不稳定和/或BGP会话不稳定引起的，这会导致Internet上的路由更新过多。路由抖动可以起源于全球互联网的任何地方，并影响到世界上的每一个网络
Internet routing mesh (BGP mesh). Given that there are over 70,000 routes known to the Internet and there is little isolation for route flapping, handling route flapping could be overwhelming to routers in any network.
互联网路由网格（BGP网格）。考虑到互联网上已知的路由超过70000条，并且路由抖动几乎没有隔离，处理路由抖动对任何网络中的路由器来说都可能是压倒性的。
One way to reduce the effect of route flapping is to turn on route dampening as specified in [10]. Essentially, dampening suppresses an unstable route until it becomes stable. The current practice is for each ISP to enable route dampening on its border routers. This way, excessive routing updates can be stopped at the border.
减少路线摆动影响的一种方法是按照[10]中的规定打开路线阻尼。本质上，阻尼会抑制不稳定的路线，直到它变得稳定。目前的做法是每个ISP在其边界路由器上启用路由抑制。这样，可以在边界处停止过多的路由更新。
An ideal model is to suppress the announcement of a flapping route right at the source. One way to implement this is to have a router recognize instability associated with its directly connected links and suppress the announcement of the route. So far, there is no such implementation. This approach should be explored.
理想的模型是在震源处抑制拍打路线的宣布。实现这一点的一种方法是让路由器识别与其直接连接的链路相关联的不稳定性，并抑制路由的宣布。到目前为止，还没有这样的实施。应该探索这种方法。
Route aggregation often masks route flapping since components of an aggregated route (more specific routes) would not cause the aggregated route to flap. Therefore using CIDR can also help to alleviate route flapping.
路由聚合通常会掩盖路由抖动，因为聚合路由（更具体的路由）的组件不会导致聚合路由抖动。因此，使用CIDR也有助于减轻路径抖动。
Routing policy involves routing decisions about acceptance and advertisement of certain routes to or from other networks and about routing preference when more than one route becomes available. Routing policy enforces business agreements between network entities and is largely governed by non-technical criteria. In essence, routing policy involves defining criteria for route filtering and route selection.
路由策略涉及到关于接受和公布某些路由到或来自其他网络的路由决策，以及关于当多条路由可用时的路由偏好的路由决策。路由策略强制网络实体之间的业务协议，并且主要由非技术标准控制。本质上，路由策略涉及定义路由筛选和路由选择的标准。
One aspect of route filtering has to do with traffic control between routing domains or between different provider networks. Making policy based on individual prefixes should be avoided in this case because, with the large number of prefixes in the Internet, it does not scale. Making policy based on ASs that administratively represent a set of prefixes scales better.
路由过滤的一个方面与路由域之间或不同提供商网络之间的流量控制有关。在这种情况下，应避免基于单个前缀制定策略，因为互联网上有大量前缀，因此无法扩展。基于在管理上表示一组前缀的ASs来制定策略的扩展性更好。
Another purpose of route filtering is to protect the integrity of routing information by preventing the acceptance of falsely advertised routing information that would lead traffic to 'black holes'. In this case, only prefix-based filtering will sufficiently achieve the goal. Prefix-based filtering needs to occur at the borders between a network and its direct customers or peer networks. The filtering is harder to manage at the boundary of the peer networks since a peer network usually advertises a large amount of prefixes. As mentioned earlier, there are about 70,000 routes known
路由过滤的另一个目的是通过防止接受虚假广告的路由信息来保护路由信息的完整性，这将导致流量进入“黑洞”。在这种情况下，只有基于前缀的过滤才能充分实现这一目标。基于前缀的过滤需要在网络与其直接客户或对等网络之间的边界处进行。由于对等网络通常播发大量前缀，因此在对等网络边界处的过滤更难管理。如前所述，已知路线约为70000条
to the Internet. This means a large default-free network would need to filter on the order of hundred of thousands of prefixes or even more since a route could be advertised by more than one sources. The sheer amount of the prefixes to be filtered imposes challenges for router configuration memory and configuration management. To make it scale, one would need to rely on the help from an out-of-band process to sort out which prefixes should be accepted or denied from which source. IRR [11] and DNS [12] are among the current proposed mechanisms for implementing prefix-based filtering.
上网。这意味着一个大型的无默认网络需要过滤数十万个前缀，甚至更多，因为一条路由可能由多个来源发布。要过滤的前缀数量之多给路由器配置内存和配置管理带来了挑战。要使其具有可扩展性，需要依赖带外进程的帮助来确定哪些前缀应该从哪个来源接受或拒绝。IRR[11]和DNS[12]是当前提出的用于实现基于前缀的过滤的机制之一。
Route selection policy determines which path should be used to send traffic toward a certain destination. This is important, for example, when a network has two connections to another network and learns routes from both connections. The decision involves which path to select to send traffic to the customers behind the other network. The choices are typically:
路由选择策略确定应使用哪个路径将流量发送到特定目的地。例如，当一个网络有两个到另一个网络的连接并从两个连接中学习路由时，这一点很重要。决策涉及选择哪条路径将流量发送到其他网络后面的客户。这些选择通常是：
o Directing traffic to the closest interconnection point for traffic to exit the network. This policy is also known as Hot-Potato-Routing
o 将流量定向到最近的互连点，以便流量退出网络。此策略也称为烫手山芋路由
o Directing traffic to the optimal network exit point. The optimal exit point is determined based on certain criteria by the network administrator and is not necessary the closest exit point
o 将流量定向到最佳网络出口点。最佳退出点由网络管理员根据某些标准确定，而不是最近的退出点
o Always preferring routes advertised by directly connected customers
o 始终优先选择直连客户宣传的路线
o Allowing other network or customer to determine the path
o 允许其他网络或客户确定路径
When a policy is defined, its implications for scalable implementation need to be considered. For example, if the policy allows customers to determine which paths traffic follows, customers, not the provider, should be required to set routing parameters to make the routing favor their preferred path. Customers can use the BGP community or mechanisms such as MED to set routing preferences in a much more scalable way. This avoids putting such routing management burdens solely on the provider. Distributing the routing management burden makes the policy implementation more scalable.
定义策略时，需要考虑其对可伸缩实现的影响。例如，如果策略允许客户确定流量遵循的路径，则应要求客户（而不是提供商）设置路由参数，以使路由偏向其首选路径。客户可以使用BGP社区或MED等机制以更具可扩展性的方式设置路由首选项。这避免了将这样的路由管理负担完全放在提供者身上。分配路由管理负担使策略实现更具可扩展性。
Another scaling measure is to avoid making complex policy. When routing policy is complex, management, such as configuration of the router and debugging, would be a problem. The ultimate goal is to make the network manageable.
另一个扩展措施是避免制定复杂的政策。当路由策略复杂时，管理（如路由器的配置和调试）将成为一个问题。最终目标是使网络易于管理。
The following basic principles would help scale the routing policy management.
以下基本原则将有助于扩展路由策略管理。
o Making policies as simple as possible but meet the requirements
o 使政策尽可能简单，但满足要求
o Automating as much as possible to avoid error-prone manual work
o 尽可能自动化以避免容易出错的手动工作
o Avoiding policy based on individual prefixes as much as possible with the exception of prefix-based route filtering for protecting routing integrity
o 尽可能避免基于单个前缀的策略，但基于前缀的路由过滤除外，以保护路由完整性
o Avoiding making exceptions
o 避免例外
o Using out-of-band routing policy processing where possible
o 尽可能使用带外路由策略处理
A typical router assumes both routing and forwarding functions. However, conceptually, routing and forwarding are two separate processes. A router's ultimate task is to forward packets based on its forwarding table, which is derived from routing information. One of the main causes of route scaling problems is that routers run out of processing power because routing requires too much processing. While a router has to forward packets, it does not necessarily have to exchange and process routing information or execute routing policy; these tasks can be performed elsewhere. Thus the question should be: Would it be possible to remove the routing process from a router to reduce its burden? Moving the routing process from the routers to other systems is referred to as out-of-band route processing.
典型的路由器同时具有路由和转发功能。然而，从概念上讲，路由和转发是两个独立的过程。路由器的最终任务是根据路由信息导出的转发表转发数据包。路由扩展问题的主要原因之一是路由器的处理能力不足，因为路由需要太多的处理。当路由器必须转发数据包时，它不必交换和处理路由信息或执行路由策略；这些任务可以在其他地方执行。因此，问题应该是：是否有可能从路由器中删除路由过程以减轻其负担？将路由过程从路由器移动到其他系统称为带外路由处理。
Out-of-band route processes would, in short, perform the heavy-duty routing tasks. They would build a forwarding table for the router, select routes based on pre-defined policy, filter routes, and shield the router from route flapping attacks.
简言之，带外路由过程将执行繁重的路由任务。他们将为路由器建立一个转发表，根据预定义的策略选择路由，过滤路由，并保护路由器免受路由抖动攻击。
The shortcomings of out-of-band route processing are the possible introduction of delays in routing changes; the de-coupling of routing and forwarding paths, which could introduce inaccurate routing information; and the cost of extra equipment.
带外路由处理的缺点是在路由更改中可能引入延迟；路由和转发路径的去耦合，可能引入不准确的路由信息；以及额外设备的费用。
Appendix A presents a current example of out-of-band route processing. It also suggests other possible solutions.
附录A给出了带外路由处理的当前示例。它还提出了其他可能的解决办法。
How routing scales has a direct impact on network stability and performance. With the fast growth of the Internet and consequent expansion of providers' networks, routing scaling become increasingly
路由规模如何直接影响网络的稳定性和性能。随着互联网的快速发展和供应商网络的不断扩展，路由规模越来越大
an important issue to address. This document identifies the major factors that affect route scalability and establishes basic principles for designing scalable routing in large networks.
这是一个需要解决的重要问题。本文档确定了影响路由可伸缩性的主要因素，并建立了在大型网络中设计可伸缩路由的基本原则。
The major routing scaling issues we are facing today are excessive router resource consumption due to routing processing burdens causing routing convergency difficulties thus introducing network instability; and routing complexity resulting in difficulties of management and trouble shooting causing degradation of service.
我们今天面临的主要路由扩展问题是由于路由处理负担导致的过度路由器资源消耗，导致路由收敛困难，从而导致网络不稳定；路由的复杂性导致管理和故障排除的困难，从而导致服务降级。
The outlined principles for designing a scalable routing system are building routing hierarchy; introducing fault isolation; reducing routing processing burden where possible; defining manageable routing policies and using the assistance of available out-of-band routing process.
设计可扩展路由系统的基本原则是建立路由层次结构；引入故障隔离；尽可能减少路由处理负担；定义可管理的路由策略，并使用可用带外路由过程的帮助。
The use of out-of-band resources to assist routing processing is a concept only been used in the Internet Exchange Points (IXPs). However, it could potentially be used to advantage within a network to help addressing routing scaling issues. This is a topic worthy of further exploration.
使用带外资源来辅助路由处理是一个仅在Internet交换点（IXP）中使用的概念。然而，它有可能在网络中发挥优势，帮助解决路由扩展问题。这是一个值得进一步探讨的课题。
Routing protocols and/or their implementations can still be improved or enhanced for better handling of the scaling issues. For example, the IS-IS multiple level mechanism is needed in order to scale the IGP in large network. Also, using multicast or a reliable flooding mechanism for I-BGP updates instead of pairwise full mesh peering is something worth investigating.
路由协议和/或其实现仍然可以改进或增强，以便更好地处理扩展问题。例如，为了在大型网络中扩展IGP，需要IS-IS多级机制。此外，使用多播或可靠的泛洪机制进行I-BGP更新而不是成对全网格对等是值得研究的。
It is our belief that even with the deployment of new technologies such as DWDM, MPLS and others in the future, the fundamental routing scheme will remain the current IGP/BGP paradigm. Therefore, the scalable routing design principles outlined in this document should still apply with the deployment of new technologies.
我们相信，即使将来部署了DWDM、MPLS等新技术，基本路由方案仍将是当前的IGP/BGP模式。因此，本文档中概述的可扩展路由设计原则仍应适用于新技术的部署。
This document deals with routing scaling issues and thus is unlikely to have a direct impact on security.
本文档涉及路由扩展问题，因此不太可能对安全性产生直接影响。
However, certain routing scaling improvement mechanisms suggested in the document, such as network compartmentalization, will possibly alleviate network outages caused by denial-of-service attacks since it would help prevent such outages from spreading to the entire network.
然而，文件中建议的某些路由扩展改进机制，如网络划分，可能会缓解拒绝服务攻击造成的网络中断，因为这将有助于防止此类中断蔓延到整个网络。
Although the mechanisms described in this document do not enhance or weaken the security aspect of routing protocols, it is worth indicating here that security enhancement of routing protocols or routing mechanisms may impact routing scalability. Therefore, when applying security enhancement in routing, one has to be aware of the implications on scalability.
尽管本文档中描述的机制不会增强或削弱路由协议的安全性，但值得指出的是，路由协议或路由机制的安全性增强可能会影响路由可伸缩性。因此，当在路由中应用安全增强时，必须了解对可伸缩性的影响。
For example, TCP MD5 signature option is proposed to be a mechanism to protect BGP sessions from being spoofed [13]. It is done on a per-session basis and the overhead of MD-5 extensions are minimal thus has no direct impact on scalability. There have been concerns about doing per-prefix AS path verification as any one ISP along a path could have forged or modified information (maliciously or not). One extreme solution is to have a signature for each prefix which gives very strong security but presents enormous scaling issues in terms of processing, memory and administrative overhead.
例如，TCP MD5签名选项是一种保护BGP会话不被欺骗的机制[13]。它是在每个会话的基础上完成的，MD-5扩展的开销最小，因此对可伸缩性没有直接影响。一直有人担心将每个前缀作为路径验证，因为路径上的任何一个ISP都可能伪造或修改信息（恶意或非恶意）。一个极端的解决方案是每个前缀都有一个签名，这提供了非常强的安全性，但在处理、内存和管理开销方面存在巨大的扩展问题。
Special thanks to Curtis Villamizar and Dave Katz for the extensive review of the document and many helpful comments. Many thanks to Yakov Rekhter, Noel Chiappa and Rob Coltun for their insightful comments. The author also like to thank Susan R. Harris for the much needed polishing of English language in the document.
特别感谢Curtis Villamizar和Dave Katz对该文件的广泛审查和许多有益的评论。非常感谢亚科夫·雷克特、诺埃尔·基亚帕和罗布·科尔顿的富有洞察力的评论。作者还想感谢苏珊·R·哈里斯（Susan R.Harris）在文件中对英语语言进行了必要的润色。
The author was made aware after the publication of this document that there is a relevant and independent presentation made by Enke Chen on the subject. The presentation is thus referenced in [14].
作者在本文件出版后获悉，陈恩科就这一主题作了相关的独立陈述。因此，在[14]中引用了该演示文稿。
[1] "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)", ISO DP 10589, February 1990.
[1] “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）”，ISO DP 10589，1990年2月。
[2] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments", RFC 1195, December 1990.
[2] Callon，R.，“OSI IS-IS在TCP/IP和双环境中的路由使用”，RFC1195，1990年12月。
[3] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[3] Moy，J.，“OSPF版本2”，RFC 23281998年4月。
[4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995.
[4] Rekhter，Y.和T.Li，“边境网关协议4（BGP-4）”，RFC 17711995年3月。
[5] C. Labovitz, R. Malan, F. Jahanian, "Origins of Internet Routing Instability," in the Proceedings of INFOCOM99, New York, NY, June, 1999
[5] C.Labovitz，R.Malan，F.Jahanian，“互联网路由不稳定性的起源”，摘自《信息通信99》，纽约，纽约，1999年6月
[6] J. Moy, "OSPF-Anatomy of an Internet Routing Protocol", Addison-Wesley, January 1998.
[6] J.Moy，“互联网路由协议的OSPF剖析”，Addison-Wesley，1998年1月。
[7] Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection - An alternative to full mesh IBGP", RFC 2796, April 2000.
[7] Bates，T.，Chandra，R.和E.Chen，“BGP路线反射-全网格IBGP的替代方案”，RFC 2796，2000年4月。
[8] Traina, P., "Autonomous System Confederation Approach to Solving the I-BGP Scaling Problem", RFC 1965, June 1996.
[8] Trana，P.，“解决I-BGP缩放问题的自治系统联合方法”，RFC 1965，1996年6月。
[9] Curtis, V., Chandra, R. and R. Govindan, "BGP Route Flap Damping", RFC 2439, November 1998.
[9] Curtis，V.，Chandra，R.和R.Govindan，“BGP路线襟翼阻尼”，RFC 2439，1998年11月。
[10] Fuller, V., Li, T., Yu, J. and K. Varadhan "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, September 1993.
[10] Fuller，V.，Li，T.，Yu，J.和K.Varadhan，“无类域间路由（CIDR）：地址分配和聚合策略”，RFC 1519，1993年9月。
[11] Villamizar, C., Alaettinoglu, C., Govindan, R. and D. Meyer, "Routing Policy System Replication", RFC 2769, February 2000.
[11] Villamizar，C.，Alaettinoglu，C.，Govindan，R.和D.Meyer，“路由策略系统复制”，RFC 2769，2000年2月。
[12] Bates, T., Bush, R., Li, T. and Y. Rekhter, "DNS-based NLRI origin AS verification in BGP", Work in Progress.
[12] Bates，T.，Bush，R.，Li，T.和Y.Rekhter，“基于DNS的NLRI来源作为BGP中的验证”，工作正在进行中。
[13] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.
[13] Heffernan，A.，“通过TCP MD5签名选项保护BGP会话”，RFC 2385，1998年8月。
[14] E. Chen, "Routing Scalability in Backbone Networks." Nanog
Presentation: http://www.nanog.org/mtg-9901/ppt/enke/index.htm
[14] E. Chen, "Routing Scalability in Backbone Networks." Nanog
Presentation: http://www.nanog.org/mtg-9901/ppt/enke/index.htm
[15] T. Li, T. Przygienda, H. Smit, "Domain-wide Prefix Distribution with Two-Level IS-IS", Work in Progress.
[15] T.Li，T.Przygienda，H.Smit，“具有两级IS-IS的域范围前缀分布”，工作正在进行中。
Author's Address
作者地址
Jieyun (Jessica) Yu CoSine Communications 1200 Bridge Parkway Redwood City, CA 94065
杰云（杰西卡）余余余弦通信1200桥公园路加利福尼亚州红木市94065
EMail: jyy@cosinecom.com
EMail: jyy@cosinecom.com
The use of a Route Server(RS) at NAPs is an example of achieving routing scalability through an out-of-band routing process. A NAP is a public inter-connection point where ISP networks exchange traffic. ISP routers at a NAP establish BGP peer sessions with each other. The result is full mesh E-BGP peering with a complexity of O(N^2) system wide. When the RS is in place, each router peers only with the RS (and its backup) to obtain necessary routing information (or more precisely, the necessary forwarding information). In addition, the RS also filters routes and executes policy for each provider's router, which further reduces the burden on all routers involved.
在NAPs使用路由服务器（RS）是通过带外路由过程实现路由可伸缩性的一个示例。NAP是ISP网络交换流量的公共互连点。处于NAP的ISP路由器彼此建立BGP对等会话。其结果是全网格E-BGP对等，系统范围内的复杂度为O（N^2）。当RS就位时，每个路由器仅与RS（及其备份）对等以获得必要的路由信息（或者更准确地说，必要的转发信息）。此外，RS还为每个提供商的路由器过滤路由并执行策略，这进一步减轻了所有相关路由器的负担。
The concept of the Route Server can also be used to help address routing scalability in a large network.
路由服务器的概念也可用于帮助解决大型网络中的路由可伸缩性问题。
1) RS Assisted Peering between Customer Aggregation Router and Customer Routers
1) 客户聚合路由器和客户路由器之间的RS辅助对等
Currently, in a typical large provider network, it's not unusual that a customer aggregation router connects up to hundreds of customer routers. That means the router has to handle hundreds of E-BGP sessions and filter a large number of prefixes. These tasks impose a heavy burden on the aggregation router. Reducing the number of customer routers per aggregation router is not an optimal option, since this would introduce more routers in the routing system of the whole network, which is neither scalable for backbone routing, nor cost efficient. Using an RS between customers and the providers' customer aggregation router become an attractive option to reduce the burden on the router.
目前，在一个典型的大型提供商网络中，一个客户聚合路由器连接数百个客户路由器并不罕见。这意味着路由器必须处理数百个E-BGP会话，并过滤大量前缀。这些任务给聚合路由器带来了沉重的负担。减少每个聚合路由器的客户路由器数量不是一个最佳选择，因为这将在整个网络的路由系统中引入更多的路由器，这对于主干路由既不可扩展，也不具有成本效益。在客户和提供商的客户聚合路由器之间使用RS成为减少路由器负担的一个有吸引力的选择。
Figure 1 shows one way of incorporating an RS router between a provider's customer aggregation router and customer routers.
图1显示了在提供商的客户聚合路由器和客户路由器之间合并RS路由器的一种方法。
--------------------------- LAN Media in a POP
| |
----- -----
|CR | |RS |
----- -----
/ | \
/ | \
C1 C2..Cn
--------------------------- LAN Media in a POP
| |
----- -----
|CR | |RS |
----- -----
/ | \
/ | \
C1 C2..Cn
Figure 1: RS serving customer aggregation router connecting customer routers
图1:RS服务客户聚合路由器连接客户路由器
In a scenario without an RS, the customer aggregation router(CR) has to peer with customer routers C1, C2 ... Cn (where n could be in the hundreds). When an RS router is introduced, CR, C1, C2 ... Cn peer with the RS router instead, and the RS passes the processed routing information (or forwarding information) to all of them, according to policy and filters.
在没有RS的情况下，客户聚合路由器（CR）必须与客户路由器C1、C2对等。。。Cn（其中n可以是数百）。当引入RS路由器时，CR、C1、C2。。。Cn与RS路由器对等，RS根据策略和筛选器将处理后的路由信息（或转发信息）传递给所有这些路由器。
The advantages are obvious:
优势显而易见：
o The customer aggregation router peers only with the RS router instead of with hundreds of customer routers.
o 客户聚合路由器仅与RS路由器对等，而不是与数百个客户路由器对等。
o The customer aggregation router does not need to filter prefixes or process routing policies, which frees resources for packet forwarding and handling.
o 客户聚合路由器不需要过滤前缀或处理路由策略，这为数据包转发和处理释放了资源。
One general concern with the use of an RS router is the possibility of a mismatch of routing connectivity and the physical connectivity. For example, if the link between the CR and C1 is down and if the RS router is not aware of the outage, it will continue to pass routes from C1 to the CR, and the traffic following these routes will be black holed. However, this is not a problem in the specific application described here. This is because the RS router has to go through the CR to peer with C1, C2 ... Cn. When the link is down, C1 is inaccessible from the RS router, and no routing information can be exchanged between the two. Consequently, the RS will announce no routes related to C1.
使用RS路由器的一个普遍问题是路由连接和物理连接可能不匹配。例如，如果CR和C1之间的链路断开，并且如果RS路由器没有意识到中断，它将继续从C1向CR传递路由，并且遵循这些路由的流量将被黑洞覆盖。然而，这在这里描述的特定应用中不是问题。这是因为RS路由器必须通过CR与C1、C2对等。。。中国。当链路断开时，RS路由器无法访问C1，并且两者之间无法交换路由信息。因此，RS将不会宣布与C1相关的路由。
Another concern is the creation of single point of failure. If the RS router is down, no routing information can be exchanged between the customer aggregation router and C1, C2 ... Cn, and no traffic will flow between them. This problem could be addressed by adding a second RS router as a backup.
另一个问题是单点故障的产生。如果RS路由器关闭，则客户聚合路由器和C1、C2之间无法交换路由信息。。。Cn，它们之间不会有流量。这个问题可以通过添加第二个RS路由器作为备份来解决。
In this scenario, since RS peers with C1 ... Cn via CR, it requires that when the RS router passes routing information to C1...Cn, it designates the IP address of the CR as the next hop. Likewise, when the RS router passes routes from each customer router to the customer aggregation router, it needs to place the correct next hop on the route. Modifications need to be made to the RS code to include this function.
在这种情况下，由于RS与C1对等。。。Cn通过CR，它要求当RS路由器将路由信息传递给C1…Cn时，它将CR的IP地址指定为下一跳。同样，当RS路由器将路由从每个客户路由器传递到客户聚合路由器时，它需要在路由上放置正确的下一跳。需要对RS代码进行修改以包含此功能。
2) Private RS Router at InterExchange Point
2) 交换点的专用RS路由器
A large provider network often has many BGP peers at the Interexchange Point, NAP or private interconnection. This means a border router has to handle many E-BGP sessions. Since an
大型提供商网络通常在交换点、NAP或专用互连处有许多BGP对等点。这意味着边界路由器必须处理许多E-BGP会话。自从
Interconnect points is usually the administrative boundary between ISPs, policy and route filtering are very demanding. This imposes a scaling problem on the border router.
互连点通常是ISP之间的管理边界，策略和路由过滤要求很高。这给边界路由器带来了缩放问题。
Deploying many routers to distribute the load among them is an expensive solution: extra hardware and extra ports cost money. Shifting the routing burden to an RS router is a promising alternative solution. In the case of using RS for multiple peers at a private interexchange point, the scenario is similar to RS used between customer aggregation router and customer routers as described in 1) above. In the case of such peering at a NAP, the private RS could be placed either on the same NAP media or a private media between the ISP's NAP router and the RS.
部署许多路由器来在它们之间分配负载是一个昂贵的解决方案：额外的硬件和额外的端口需要花钱。将路由负担转移到RS路由器是一种很有前途的替代解决方案。在专用交换点对多个对等点使用RS的情况下，该场景类似于上文1）中所述的客户聚合路由器和客户路由器之间使用的RS。在这种对NAP的窥视情况下，专用RS可以放置在相同的NAP介质上，也可以放置在ISP的NAP路由器和RS之间的专用介质上。
3) RS Routers at Each POP in a Large Network
3) 大型网络中每个POP的RS路由器
Even in a network with a hierarchical routing structure, a sub-area may become too large, and I-BGP full meshing may impose a scaling problem. One way to address this would be to split the sub-area or add yet another tier of I-BGP reflector structure. Another possible solution would be to use an RS router as an I-BGP Server. Depending on the topology of a POP, this solution may or may not be suitable. The use of RS routers at network POPs need to be investigated further.
即使在具有分层路由结构的网络中，子区域也可能变得太大，并且I-BGP完全网格化可能带来缩放问题。解决这一问题的一种方法是分割子区域或添加另一层I-BGP反射器结构。另一种可能的解决方案是使用RS路由器作为I-BGP服务器。根据POP的拓扑结构，此解决方案可能适用，也可能不适用。需要进一步调查RS路由器在网络POP中的使用情况。
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确认
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