Cisco IOS Cookbook 中文精简版 9-23 BGP

 9.1.  Configuring BGP
提问 在网络中启用BGP
回答
Route1在AS 65500中
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0
Router1(config-if)#ip address 192.168.55.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#router bgp 65500
Router1(config-router)#network 192.168.1.0
Router1(config-router)#neighbor 192.168.55.5 remote-as 65501
Router1(config-router)#no synchronization
Router1(config-router)#exit
Router1(config)#end
Router1#
Router2在AS 65501中
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface Serial0
Router2(config-if)#ip address 192.168.55.5 255.255.255.252
Router2(config-if)#exit
Router2(config)#router bgp 65501
Router2(config-router)#network 172.25.17.0 mask 255.255.255.0
Router2(config-router)#neighbor 192.168.55.6 remote-as 65500
Router2(config-router)#no synchronization
Router2(config-router)#exit
Router2(config)#end
Router2#
注释 在对BGP验证的时候比较有用的命令是
Router1#show ip bgp summary
BGP router identifier 192.168.99.5, local AS number 65500
BGP table version is 7, main routing table version 7
4 network entries and 4 paths using 484 bytes of memory
2 BGP path attribute entries using 196 bytes of memory
BGP activity 11/7 prefixes, 11/7 paths

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.55.5    4 65501      17      18        7    0    0 00:12:38        2
172.25.2.2      4 65531     527     526        0    0    0 21:05:23 Active
Router1#
需要注意的是理想状态是State里面是数字，尽管是Active也不代表是配置正常，反而是配置出现错误。通过neighbor 172.20.1.2 update-source Loopback0 命令来限制BGP数据包源地址为回环地址，但要确保此地址的连通性
9.2.  使用eBGP Multihop
提问 配置外部BGP，但是不是直连的路由器
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip route 172.20.1.2 255.255.255.255 192.168.1.5 2
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 172.20.1.2 remote-as 65530
Router1(config-router)#neighbor 172.20.1.2 update-source Loopback0
Router1(config-router)#neighbor 172.20.1.2 ebgp-multihop 3
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 缺省情况下eBGP的路由器必须是直连的，如果不是直连的就需要使用此命令。一种说法是此跳数越小越好，但是RFC 3682说为了安全还是越大越好，思科在12.3(7)T后也采用了这个建议，使用了neighbor 192.168.55.5 ttl-security hops 1 命令，此命令会丢弃所有TTL小于255-1＝254的BGP数据包，这时候如果对端eBGP邻居不支持此特性就必须使用下面的命令来配置neighbor 192.168.55.6 ebgp-multihop 255

9.3.  调整Next-Hop属性值
提问 在iBGP之间宣告路由时候修改下一跳属性值，使其指向内部AS的地址
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.6 remote-as 65500
Router1(config-router)#neighbor 192.168.1.6 next-hop-self
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 正常情况下iBGP之间下一跳属性值是不会修改的，只会在eBGP时会进行修改，而此地址会指向eBGP邻居的地址，而往往内部AS的路由器没有到达此地址的路由。
9.4.  连接两个ISPs  
提问 一台路由器连接两个ISP，保证网络冗余
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0
Router1(config-if)#description connection to ISP #1, ASN 65510
Router1(config-if)#ip address 192.168.1.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Serial1
Router1(config-if)#description connection to ISP #2, ASN 65520
Router1(config-if)#ip address 192.168.2.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Ethernet0
Router1(config-if)#description connection to internal network, ASN 65500
Router1(config-if)#ip address 172.18.5.2 255.255.255.0
Router1(config-if)#exit
Router1(config)#router bgp 65500
Router1(config-router)#network 172.18.5.0 mask 255.255.255.0
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.2.5 remote-as 65520
Router1(config-router)#no synchronization
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 注意此配置不是最佳配置，可能导致内部AS称为两个ISP的transit AS，同时导致自己路由器接收过多路由
9.5.  两台路由器分别连接两个ISP
提问 内部AS有两台路由器，分别连两个ISP保证网络冗余
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0
Router1(config-if)#description connection to ISP #1, ASN 65510
Router1(config-if)#ip address 192.168.1.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Ethernet0
Router1(config-if)#description connection to internal network, ASN 65500
Router1(config-if)#ip address 172.18.5.2 255.255.255.0
Router1(config-if)#exit
Router1(config)#ip as-path access-list 15 permit ^$
Router1(config)#router bgp 65500
Router1(config-router)#network 172.18.5.0 mask 255.255.255.0
Router1(config-router)#neighbor 172.18.5.3 remote-as 65500
Router1(config-router)#neighbor 172.18.5.3 next-hop-self
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 filter-list 15 out
Router1(config-router)#no synchronization
Router1(config-router)#exit
Router1(config)#end
Router1#
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface Serial1
Router2(config-if)#description connection to ISP #2, ASN 65520
Router2(config-if)#ip address 192.168.2.6 255.255.255.252
Router2(config-if)#exit
Router2(config)#interface Ethernet0
Router2(config-if)#description connection to internal network, ASN 65500
Router2(config-if)#ip address 172.18.5.3 255.255.255.0
Router2(config-if)#exit
Router2(config)#ip as-path access-list 15 permit ^$
Router2(config)#router bgp 65500
Router2(config-router)#network 172.18.5.0 mask 255.255.255.0
Router2(config-router)#neighbor 192.168.2.5 remote-as 65520
Router2(config-router)#neighbor 192.168.2.5 filter-list 15 out
Router2(config-router)#neighbor 172.18.5.2 remote-as 65500
Router2(config-router)#neighbor 172.18.5.2 next-hop-self
Router2(config-router)#no synchronization
Router2(config-router)#exit
Router2(config)#end
Router2#
注释
9.6.  限制向BGP 对端的网络宣告
提问 限制特定的路由公告给对端的AS
回答
有三种方法，第一种是扩展ACL
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#access-list 105 deny ip host 172.25.0.0 host 255.255.0.0
Router1(config)#access-list 105 permit ip any any
Router1(config)#route-map ACL-RT-FILTER permit 10
Router1(config-route-map)#match ip address 105
Router1(config-route-map)#exit
Router1(config)#route-map ACL-RT-FILTER deny 20
Router1(config-route-map)#exit
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 route-map ACL-RT-FILTER in
Router1(config-router)#exit
Router1(config)#end
Router1#
第二种是使用distribute-list:
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#access-list 106 deny ip host 172.25.0.0 host 255.255.0.0
Router1(config)#access-list 106 permit ip any any
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 distribute-list 106 in
Router1(config-router)#exit
Router1(config)#end
Router1#
第三种也是最常用的是使用prefix lists
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip prefix-list PREFIX-FILTER seq 10 deny 172.25.0.0/16
Router1(config)#ip prefix-list PREFIX-FILTER seq 20 permit 0.0.0.0/0 le 32
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 prefix-list PREFIX-FILTER in
Router1(config-router)#exit
Router1(config)#end
Router1#

注释 前两种使用的扩展ACL比较奇特，第一个host是子网，第二个host是子网掩码，而不是传统目的地址，所以host 172.25.0.0 host 255.255.0.0 就代表网络172.25.0.0/16，如果用正常的ACL就实现不了对无类网络的控制。所以推荐使用第三种方式prefixlist，此列表支持序列号，可以帮助你修改和插入新的条目 ge是大于，le是小于，控制子网掩码permit 0.0.0.0/0 le 32就是变相的permit any
9.7.  调整Local Preference属性值
提问 调整Local Preference属性值来控制路由选择
回答
第一种全局
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#bgp default local-preference 200
Router1(config-router)#exit
Router1(config)#end
Router1#
第二种使用route map控制
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip prefix-list LOW_LP_PREFIXES seq 10 permit 172.22.0.0/16
Router1(config)#route-map LOCALPREF permit 10
Router1(config-route-map)#match ip address prefix-list LOW_LP_PREFIXES
Router1(config-route-map)#set local-preference 50
Router1(config-route-map)#exit
Router1(config)#route-map LOCALPREF permit 20
Router1(config-route-map)#exit
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 route-map LOCALPREF in
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 此local preference属性值只在内部AS有用，选路级别高于AS Path。此值越大优先级越高，缺省值为100。Show ip bgp命令可以看到各个路由的local preference属性值
9.8.  负载均衡
提问 在BGP邻居之间的多链路上负载均衡流量
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#maximum-paths 4
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 正常情况下BGP选路策略会保证只有一条路径，通过此命令可以增加到4条，不过要确保所有属性值相同，包括MED属性。同时注意此负载均衡只针对出流量而不适合入流量
9.9.  在AS Path属性值中清除私有ASNs
提问 避免内网中的私有ASN传播到互联网
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0
Router1(config-if)#description connection to ISP #1, ASN 1
Router1(config-if)#ip address 192.168.1.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Serial1
Router1(config-if)#description connection to private network, ASN 65500
Router1(config-if)#ip address 192.168.5.1 255.255.255.252
Router1(config-if)#exit
Router1(config)#router bgp 2
Router1(config-router)#neighbor 192.168.5.2 remote-as 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 1
Router1(config-router)#neighbor 192.168.1.5 remove-private-AS
Router1(config-router)#no synchronization
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 注意此命令是不能删除那些在公共ASN之间的私有ASN
9.10.  基于AS Path属性值的路由过滤  
提问 基于接收或者发送路由的AS Path属性值进行路由过滤
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip as-path access-list 15 permit ^65501$
Router1(config)#ip as-path access-list 25 permit _65530_
Router1(config)#ip as-path access-list 25 deny _65531$
Router1(config)#ip as-path access-list 25 permit .*
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 filter-list 15 in
Router1(config-router)#neighbor 192.168.2.5 remote-as 65520
Router1(config-router)#neighbor 192.168.2.5 filter-list 25 out
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 正则表达式过滤
9.11.  减少接收到的路由表大小
提问 通过汇总接收到路由的方式来减少所接收的路由表大小
回答
通过缺省路由的方式来过滤到过多的外部路由
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip route 0.0.0.0 0.0.0.0 192.168.101.0 1
Router1(config)#ip route 0.0.0.0 0.0.0.0 192.168.102.0 2
Router1(config)#ip prefix-list CREATE-DEFAULT seq 10 permit 192.168.101.0/24
Router1(config)#ip prefix-list CREATE-DEFAULT seq 20 permit 192.168.102.0/24
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65520
Router1(config-router)#neighbor 192.168.1.5 prefix-list CREATE-DEFAULT in
Router1(config-router)#exit
Router1(config)#end
Router1#
注释
9.12.  出方向路由信息汇总
提问 在向下游路由器发送路由表之前进行路由汇总
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65520
Router1(config-router)#auto-summary
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 这是缺省行为，但是是有类的汇总，并且只能针对再分发过来的路由，不能适用于network命令配置的路由。思科使用了如下命令对出方向路由进行汇总
Router3(config)#router bgp 65530
Router3(config-router)#aggregate-address 172.20.0.0 255.252.0.0 summary-only
Summaryonly选项只发布汇总路由，去掉后会发送汇总路由和子网路由，而为了避免回环建议添加as-set选项
9.13.  在AS Path属性值中添加更多ASN
提问 通过增加AS Path属性中ASN的数目来影响BGP选路
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip as-path access-list 15 permit ^$
Router1(config)#route-map PREPEND permit 10
Router1(config-route-map)#match as-path 15
Router1(config-route-map)#set as-path prepend 65500 65500 65500
Router1(config-route-map)#exit
Router1(config)#route-map PREPEND permit 20
Router1(config-route-map)#exit
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 route-map PREPEND out
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 通过这种方式来影响入流量
9.14.  再发布路由到BGP
提问 IGP和BGP之间的再分发
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router ospf 100
Router1(config-router)#network 172.26.0.0 0.0.255.255 area 0
Router1(config-router)#redistribute bgp 65500 metric 500 subnets
Router1(config-router)#exit
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65520
Router1(config-router)#network 172.26.0.0
Router1(config-router)#exit
Router1(config)#end
Router1#

Router2(config)#route-map REDIST permit 5
Router2(config-route-map)#match tag 123
Router2(config-route-map)#exit
Router2(config)#route-map REDIST deny 10
Router2(config-route-map)#match route-type external
Router2(config-route-map)#exit
Router2(config)#route-map REDIST permit 20
Router2(config-route-map)#exit
Router2(config)#router bgp 65520
Router2(config-router)#redistribute eigrp 99 route-map REDIST metric 500
注释
9.15. 使用Peer Groups
提问 使用组的形式来简化对多个相同属性邻居的配置
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#neighbor EBGP-PEERS peer-group
Router1(config-router)#neighbor EBGP-PEERS prefix-list PRE-RTFILTER in
Router1(config-router)#neighbor EBGP-PEERS filter-list 15 out
Router1(config-router)#neighbor 192.168.1.5 remote-as 65520
Router1(config-router)#neighbor 192.168.1.5 peer-group EBGP-PEERS
Router1(config-router)#neighbor 192.168.1.9 remote-as 65521
Router1(config-router)#neighbor 192.168.1.9 peer-group EBGP-PEERS
Router1(config-router)#neighbor 192.168.1.13 remote-as 65522
Router1(config-router)#neighbor 192.168.1.13 peer-group EBGP-PEERS
Router1(config-router)#neighbor 192.168.1.17 remote-as 65523
Router1(config-router)#neighbor 192.168.1.17 peer-group EBGP-PEERS
Router1(config-router)#exit
Router1(config)#end
Router1#
注释 当然也可以针对iBGP邻居
Router1(config)#router bgp 6550
Router1(config-router)#neighbor IBGP-PEERS peer-group
Router1(config-router)#neighbor IBGP-PEERS update-source Loopback0
Router1(config-router)#neighbor IBGP-PEERS route-reflector-client
Router1(config-router)#neighbor 192.168.101.5 remote-as 65500
Router1(config-router)#neighbor 192.168.101.5 peer-group IBGP-PEERS
Router1(config-router)#neighbor 192.168.101.9 remote-as 65500
Router1(config-router)#neighbor 192.168.101.9 peer-group IBGP-PEERS

9.16.  BGP邻居认证
提问 使用认证增加安全性
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.55.5 remote-as 65501
Router1(config-router)#neighbor 192.168.55.5 password password-1234
Router1(config-router)#exit
Router1(config)#end
Router1#
注释
9.17.  使用BGP Communities
提问 使用BGP Communities来对路由进行控制
回答
首先要通过route map的方式针对邻居设定希望的Communities值
Router3#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router3(config)#ip prefix-list 10.101/16 seq 5 permit 10.101.0.0/16
Router3(config)#ip prefix-list 10.102/16 seq 5 permit 10.102.0.0/16
Router3(config)#ip prefix-list 10.103/16 seq 5 permit 10.103.0.0/16
Router3(config)#ip prefix-list 10.104/16 seq 5 permit 10.104.0.0/16
Router3(config)#ip prefix-list 10.105/16 seq 5 permit 10.105.0.0/16
Router3(config)#route-map APPLY_COMMUNITY_A permit 10
Router3(config-route-map)#match ip address prefix-list 10.101/16
Router3(config-route-map)#set community no-advertise
Router3(config-route-map)#exit
Router3(config)#route-map APPLY_COMMUNITY_A permit 20
Router3(config-route-map)#match ip address prefix-list 10.102/16
Router3(config-route-map)#set community no-export
Router3(config-route-map)#exit
Router3(config)#route-map APPLY_COMMUNITY_A permit 30
Router3(config-route-map)#match ip address prefix-list 10.103/16
Router3(config-route-map)#set community local-AS
Router3(config-route-map)#exit
Router3(config)#route-map APPLY_COMMUNITY_A permit 40
Router3(config-route-map)#match ip address prefix-list 10.104/16
Router3(config-route-map)#set community internet
Router3(config-route-map)#exit
Router3(config)#route-map APPLY_COMMUNITY_A permit 50
Router3(config-route-map)#match ip address prefix-list 10.105/16
Router3(config-route-map)#set community 4293328976
Router3(config-route-map)#exit
Router3(config)#route-map APPLY_COMMUNITY_A permit 100
Router3(config-route-map)#exit
Router3(config)#router bgp 65500
Router3(config-router)#no synchronization
Router3(config-router)#neighbor 172.18.5.3 remote-as 65500
Router3(config-router)#neighbor 172.18.5.3 next-hop-self
Router3(config-router)#neighbor 172.18.5.3 send-community both
Router3(config-router)#neighbor 172.18.5.10 remote-as 65500
Router3(config-router)#neighbor 172.18.5.10 next-hop-self
Router3(config-router)#neighbor 172.18.5.10 send-community both
Router3(config-router)#neighbor 192.168.1.9 remote-as 65520
Router3(config-router)#neighbor 192.168.1.9 send-community both
Router3(config-router)#neighbor 192.168.1.9 route-map APPLY_COMMUNITY_A in
Router3(config-router)#exit
Router3(config)#end
Router3#
在下游路由器上配置命令使其可以分发此Community值
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#router bgp 65500
Router2(config-router)#no synchronization
Router2(config-router)#neighbor 172.18.5.4 remote-as 65500
Router2(config-router)#neighbor 172.18.5.4 send-community both
Router2(config-router)#neighbor 172.18.5.10 remote-as 65500
Router2(config-router)#neighbor 172.18.5.10 send-community both
Router2(config-router)#no auto-summary
Router2(config-router)#exit
Router2(config)#end
Router2#
注释 通过定义local-as,no-advertise,no-export,internet四种不同community属性值的方式来限制路由公告的范围
9.18.  使用BGP Route Reflectors
提问 通过路由反射器的方式来简化iBGP邻居关系
回答
只要针对三种不同角色路由器的配置
Router1是Client Peer:
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Ethernet0/0
Router1(config-if)#ip address 172.18.5.2 255.255.255.0
Router1(config-if)#exit
Router1(config)#interface Serial0/0
Router1(config-if)#ip address 192.168.1.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Loopback0
Router1(config-if)#ip address 172.18.6.1 255.255.255.255
Router1(config-if)#exit
Router1(config)#router bgp 65500
Router1(config-router)#no synchronization
Router1(config-router)#neighbor 172.18.6.2 remote-as 65500
Router1(config-router)#neighbor 172.18.6.2 next-hop-self
Router1(config-router)#neighbor 172.18.6.2 update-source Loopback0
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#exit
Router1(config)#ip route 172.18.6.2 255.255.255.255 172.18.5.3
Router1(config)#ip route 172.18.6.3 255.255.255.255 172.18.5.4
Router1(config)#ip route 172.18.6.4 255.255.255.255 172.18.5.10
Router1(config)#end
Router1#
Router4 是Nonclient Peer:
Router4#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router4(config)#interface Ethernet0
Router4(config-if)#ip address 172.18.5.10 255.255.255.0
Router4(config-if)#exit
Router4(config)#interface Loopback0
Router4(config-if)#ip address 172.18.6.4 255.255.255.255
Router4(config-if)#exit
Router4(config)#router bgp 65500
Router4(config-router)#no synchronization
Router4(config-router)#neighbor 172.18.6.2 remote-as 65500
Router4(config-router)#neighbor 172.18.6.2 update-source Loopback0
Router4(config-router)#exit
Router4(config)#ip route 172.18.6.1 255.255.255.255 172.18.5.2
Router4(config)#ip route 172.18.6.2 255.255.255.255 172.18.5.3
Router4(config)#ip route 172.18.6.3 255.255.255.255 172.18.5.4
Router4(config)#end
Router4#
R2是 Route Reflector
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface FastEthernet0/0
Router2(config-if)#ip address 172.18.5.3 255.255.255.0
Router2(config-if)#exit
Router2(config)#interface Loopback0
Router2(config-if)#ip address 172.18.6.2 255.255.255.255
Router2(config-if)#exit
Router2(config)#router bgp 65500
Router2(config-router)#no synchronization
Router2(config-router)#neighbor 172.18.6.1 remote-as 65500
Router2(config-router)#neighbor 172.18.6.1 route-reflector-client
Router2(config-router)#neighbor 172.18.6.1 update-source Loopback0
Router2(config-router)#neighbor 172.18.6.3 remote-as 65500
Router2(config-router)#neighbor 172.18.6.3 route-reflector-client
Router2(config-router)#neighbor 172.18.6.3 update-source Loopback0
Router2(config-router)#neighbor 172.18.6.4 remote-as 65500
Router2(config-router)#neighbor 172.18.6.4 update-source Loopback0
Router2(config-router)#no auto-summary
Router2(config-router)#exit
Router2(config)#ip route 172.18.6.1 255.255.255.255 172.18.5.2
Router2(config)#ip route 172.18.6.3 255.255.255.255 172.18.5.4
Router2(config)#ip route 172.18.6.4 255.255.255.255 172.18.5.10
Router2(config)#end
Router2#

注释 路由反射器是解决要求iBGP全互联的问题。不过为了保证冗余性还是要配置多个路由反射器，使用bgp cluster-id 1234命令来定义cluster
9.19.       汇总实验
提问 结合前面的方法，重新配置一台路由器两个冗余链路的情况
回答
Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0
Router1(config-if)#description connection to ISP #1, ASN 65510
Router1(config-if)#ip address 192.168.1.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Serial1
Router1(config-if)#description connection to ISP #2, ASN 65520
Router1(config-if)#ip address 192.168.2.6 255.255.255.252
Router1(config-if)#exit
Router1(config)#interface Ethernet0
Router1(config-if)#description connection to internal network, ASN 65500
Router1(config-if)#ip address 172.18.5.2 255.255.255.0
Router1(config-if)#exit
Router1(config)#ip as-path access-list 15 permit ^$
Router1(config)#ip route 0.0.0.0 0.0.0.0 192.168.101.0 1
Router1(config)#ip route 0.0.0.0 0.0.0.0 192.168.102.0 2
Router1(config)#ip prefix-list CREATE-DEFAULT seq 10 permit 192.168.101.0/24
Router1(config)#ip prefix-list CREATE-DEFAULT seq 20 permit 192.168.102.0/24
Router1(config)#ip prefix-list BLOCK-DEFAULT seq 10 permit 0.0.0.0/0 ge 1
Router1(config)#route-map PREPEND permit 10
Router1(config-route-map)#set as-path prepend 65500 65500
Router1(config-route-map)#exit
Router1(config)#route-map LOCALPREF permit 10
Router1(config-route-map)#set local-preference 75
Router1(config-route-map)#exit
Router1(config)#route-map DEFAULT-ROUTE permit 10
Router1(config-route-map)#match ip address prefix-list CREATE-DEFAULT
Router1(config-route-map)#exit
Router1(config)#router bgp 65500
Router1(config-router)#network 172.18.5.0 mask 255.255.255.0
Router1(config-router)#neighbor 172.18.5.3 remote-as 65500
Router1(config-router)#neighbor 172.18.5.3 password password_number1
Router1(config-router)#neighbor 172.18.5.3 default-origniate route-map DEFAULT-ROUTE
Router1(config-router)#neighbor 192.168.1.5 remote-as 65510
Router1(config-router)#neighbor 192.168.1.5 password password_number2
Router1(config-router)#neighbor 192.168.1.5 filter-list 15 out
Router1(config-router)#neighbor 192.168.1.5 prefix-list CREATE-DEFAULT in
Router1(config-router)#neighbor 192.168.1.5 prefix-list BLOCK-DEFAULT out
Router1(config-router)#neighbor 192.168.2.5 remote-as 65520
Router1(config-router)#neighbor 192.168.2.5 password password_number3
Router1(config-router)#neighbor 192.168.2.5 filter-list 15 out
Router1(config-router)#neighbor 192.168.2.5 prefix-list CREATE-DEFAULT in
Router1(config-router)#neighbor 192.168.2.5 prefix-list BLOCK-DEFAULT out
Router1(config-router)#neighbor 192.168.2.5 route-map PREPEND out
Router1(config-router)#neighbor 192.168.2.5 route-map LOCALPREF in
Router1(config-router)#no synchronization
Router1(config-router)#exit
Router1(config)#end
Router1#