2. 单台边界路由连接到单一ISP环境中的负载共享
本场景展示了当在远程AS和本地AS之间存在多条链路时如何完成负载共享。链路建立在本地AS中的一个路由器和远程AS中的多个路由器之间。
拓扑图如下：
默认情况下BGP只从可能存在的等价路径中选择出一条非常好的的路径，通过命令maximum-path对可选择的等价路径数目（1~6）进行修改。
配置如下：
RouterA
RouterA (config)#int s1/0
RouterA (config-if)#ip add 160.20.20.1 255.255.255.0
RouterA (config-if)#no sh
RouterA (config)#int s1/2
RouterA (config-if)#ip add 150.10.10.1 255.255.255.0
RouterA (config-if)#no sh
RouterA (config-if)#int lo0
RouterA (config-if)#ip add 1.1.1.1 255.255.255.0
RouterA (config-if)#router bgp 11
RouterA (config-router)#network 1.0.0.0
RouterA (config-router)#neighbor 150.10.10.2 remote-as 10
RouterA (config-router)#neighbor 160.20.20.2 remote-as 10
RouterA (config-router)#maximum-paths 2
RouterB
RourterB(config)#int s1/2
RourterB(config-if)#ip add 160.20.20.2 255.255.255.0
RourterB(config-if)#no sh
RourterB(config-if)#int fa0/0
RourterB(config-if)#ip add 2.2.2.1 255.255.255.0
RourterB(config-if)#no sh
RourterB(config)#router bgp 10
RourterB(config-router)#network 2.0.0.0
RourterB(config-router)#neighbor 160.20.20.1 remote-as 11
RourterB(config-router)#auto-summary
RouterC
RourterB(config)#int s1/2
RourterB(config-if)#ip add 150.10.10.2 255.255.255.0
RourterB(config-if)#no sh
RourterB(config-if)#int fa0/0
RourterB(config-if)#ip add 2.2.2.2 255.255.255.0
RourterB(config-if)#no sh
RourterB(config)#router bgp 10
RourterB(config-router)#network 2.0.0.0
RourterB(config-router)#neighbor 150.10.10.1 remote-as 11
RourterB(config-router)#auto-summary
验证：
RouterA# sh ip bgp
BGP table version is 5, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
? ?Network? ?? ?? ? Next Hop? ?? ?? ?? ?Metric LocPrf Weight Path
*> 1.0.0.0? ?? ?? ? 0.0.0.0? ?? ?? ?? ?? ?? ?0? ?? ?? ?32768 i
*> 2.0.0.0? ?? ?? ? 160.20.20.2? ?? ?? ?? ???0? ?? ?? ?? ? 0 10 i
*? ?? ?? ?? ?? ?? ? 150.10.10.2? ?? ?? ?? ???0? ?? ?? ?? ? 0 10 i
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RouterA#sh ip ro
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
B 2.0.0.0/8 [20/0] via 150.10.10.2, 00:16:32
[20/0] via 160.20.20.2, 00:15:47
160.20.0.0/24 is subnetted, 1 subnets
C 160.20.20.0 is directly connected, Serial1/0
150.10.0.0/24 is subnetted, 1 subnets
C 150.10.10.0 is directly connected, Serial1/2
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RouterA#
RouterA#traceroute 2.2.2.2
Type escape sequence to abort.
Tracing the route to 2.2.2.2
1 150.10.10.2 64 msec
160.20.20.2 92 msec *
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