Border Gateway Protocol Route Summarization © 2003, Cisco Systems, Inc. All rights reserved. 1

BSCI 2.0—7-2 Objectives Upon completing this lesson, you will be able to: Explain how Border Gateway Protocol version 4 is related to classless interdomain routing(BGP4 对无类域间路由的支 持.) Explain how Border Gateway Protocol implements route summarization with the network command( 用 NETWORK 命令实施路由汇总 ) Describe how Border Gateway Protocol implements route summarization with the aggregate-address command ( 用 地址聚合命令实施路由汇总.)

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-3 CIDR and Aggregate Addresses With BGP4, routes can be aggregated by any AS on any BGP router. BGP4 is classless, supports VLSM and longest match routing, and carries a network mask for each network in the update. BGP 更新消息中包括地 址前缀和前缀长度.

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-4 Network Boundary Summarization no auto-summary Router(config-router)# BGP, RIPv1 and RIPv2, IGRP, and EIGRP perform network boundary summarization by default.( 自动汇总 ) BGP, RIPv2, and EIGRP can disable( 关闭 ) network boundary summarization. CIDR has forced the IANA to begin using class A addresses, like , in a classless manner. If you are assigned a portion of a class A, B, or C address, the no-auto summary command needs to be implemented under the BGP process or you risk claiming ownership of the whole class A, B, or C address.

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-5 network network-number [mask network-mask] Router(config-router)# This command was not designed to perform summarization by itself. The aggregate-address command was designed for summarization.( 将外部路由宣告到 BGP 中去.) BGP network Command ip route prefix mask null0( 人工创建汇总静态路由到空端口.) Router(config)# To use the network statement for summarization, the network number and mask used must already exist in the routing table. If the route was already summarized by EIGRP or OSPF, that summarization can be announced into BGP with the network and mask commands. If the route was not already summarized, a null route must be created for BGP to announce this summarization. 空端口 : 如果 BGP 路由表没有这条汇总路由时,BGP NETWORK 命令无法发布. 汇总路由不能确定从哪个物理端口走, 所以用空端口来决定来告诉路由器如何走

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-6 Cautions about Network Statement If a network statement is used for summarization, do not use the more specific entries and the summarized route as shown here. If both are used, the summarized route and the more specific routes will be announced /22 does not exist in the IP routing table without the null route. BGP will not announce the network unless the summarized route is already present in the routing table. NETWORK 也有间接汇总功能. 要求网络必须先存在, 可以使用空端口.

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-7 BGP Summarization Using the network Command and Static Route to Null0

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-8 Configuring BGP for Aggregate Addressing aggregate-address ip-address mask [summary-only] as-set Router(config-router)# Creates an aggregate (summary) entry in the BGP table( 无须先有路由.) Uses the summary-only option to advertise only the summary and not the specific routes （通告汇总路由） Adds the as-set option to include a list of all the autonomous system numbers that the more specific routes have passed through( 要求在 AS 属性中列出包含具体路 由的 AS 号，强制加上该属性起到防环的作用 ) Recommended method of summarization for BGP Null static route not needed; BGP null route automatically generated( 会自动产生一条汇总路由, 并且指向空端口.)

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-9 BGP Summarization Using the aggregate-address Command

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-10 BGP Aggregation show ip bgp Network / *>s>s>s>s> *>s>s>s>s> Next Hop LocPrfWeight Path i i i i i Metric 0 routerC# show ip bgp BGP table version is 28, local router ID is Status codes: s = suppressed, * = valid, > = best, and i = internal Origin codes : i = IGP, e = EGP, and ? = incomplete RouterC#

© 2003, Cisco Systems, Inc. All rights reserved. BSCI 2.0—7-11 BGP 汇总小结： Network: 告诉 BGP 通告什么； Neighbor: 告诉 BGP 通告给谁； Aggregate-address: 告诉 BGP 如何通告网络。