R1(config)#router bgp 64512 R1(config-router)#net 172.16.1.0 mask 255.255.255.0 R1(config-router)#net 192.168.2.0          R1(config-router)#net 130.130.1.0 mask 255.255.255.0 R1(config-router)#nei 172.16.1.2 remote 64512 R1(config-router)#nei 192.168.2.1 remote 64513

 

三、分析选择的下一跳：
1、查看一下R3和R4的路由表：

R3#sh ip route ………………      172.16.0.0/24 is subnetted, 1 subnets B       172.16.1.0 [20/0] via 192.168.1.1, 00:02:55      10.0.0.0/24 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1/1      130.130.0.0/24 is subnetted, 1 subnets B       130.130.1.0 [20/0] via 192.168.1.1, 00:00:13  （R3到130.130.1.1通过192.168.1.1走） C    192.168.1.0/24 is directly connected, Serial1/0 B    192.168.2.0/24 [200/0] via 10.1.1.2, 00:02:59   R4#sh ip route ………………      172.16.0.0/24 is subnetted, 1 subnets B       172.16.1.0 [20/0] via 192.168.2.2, 00:03:57      10.0.0.0/24 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1/0      130.130.0.0/24 is subnetted, 1 subnets B       130.130.1.0 [20/0] via 192.168.2.2, 00:00:56 （R4到130.130.1.0通过192.168.2.2走） B    192.168.1.0/24 [200/0] via 10.1.1.1, 00:03:57 C    192.168.2.0/24 is directly connected, FastEthernet0/0

 

 

R3#sh ip bgp BGP table version is 9, local router ID is 192.168.1.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,               r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete      Network          Next Hop            Metric LocPrf Weight Path * i10.1.1.0/24      10.1.1.2                 0    100      0 i *>                  0.0.0.0                  0         32768 i *> 130.130.1.0/24   192.168.1.1                            0 64512 i * i                 192.168.2.2              0    100      0 64512 i * i172.16.1.0/24    192.168.2.2              0    100      0 64512 i *>                  192.168.1.1              0             0 64512 i *  192.168.1.0      192.168.1.1              0             0 64512 i *>                  0.0.0.0                  0         32768 i *>i192.168.2.0      10.1.1.2                 0    100      0 i *                   192.168.1.1                            0 64512 i   R4#sh ip bgp      Network          Next Hop            Metric LocPrf Weight Path *> 10.1.1.0/24      0.0.0.0                  0         32768 i * i                 10.1.1.1                 0    100      0 i * i130.130.1.0/24   192.168.1.1              0    100      0 64512 i *>                  192.168.2.2              0             0 64512 i * i172.16.1.0/24    192.168.1.1              0    100      0 64512 i *>                  192.168.2.2              0             0 64512 i *  192.168.1.0      192.168.2.2                            0 64512 i *>i                 10.1.1.1                 0    100      0 i *> 192.168.2.0      0.0.0.0                  0         32768 i *                   192.168.2.2              0             0 64512 i R4#

 

四、理论上来讲BGP选择了自己认为的最佳路径，但我们从图中拓扑可以看到：R4àR1是快速以太网都是为100M的链路。而R3通过的链路却要通过两条串口链路。如果串口链路的带宽比较小，那么这样的选择是不合理的。R3选择了一个次路由。比较R3和R4的BGP数据库。我们应该通过什么样的方式来改变R3的选择路径呢？首先我们要了解一个概念：BGP在选择最佳路由时，并不会考虑具体链路的带宽。从上图可以看到，显示的本地优先级都为100。根据这种判断，我们可以在本地配置本地优先级来确定数据流如何流出本自治系统。
1、把R4的优先级提高，因为BGP会优先选择本地优先级值较高的路由。

 

R4(config)#router bgp 64513 R4(config-router)#bgp default local-preference 150  （配置一个高出50的优先级）

 

2、clear ip bgp * soft 软涮新一下，再来看一下R3的路由表和BGP数据库：

R3#sh ip route        172.16.0.0/24 is subnetted, 1 subnets B       172.16.1.0 [200/0] via 192.168.2.2, 00:01:09      10.0.0.0/24 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1/1      130.130.0.0/24 is subnetted, 1 subnets B       130.130.1.0 [200/0] via 192.168.2.2, 00:01:09 （现在下一跳变为:192.168.2.2） C    192.168.1.0/24 is directly connected, Serial1/0 B    192.168.2.0/24 [200/0] via 10.1.1.2, 00:58:57   R3#sh ip bgp       Network          Next Hop            Metric LocPrf Weight Path * i10.1.1.0/24      10.1.1.2                 0    150      0 i *>                  0.0.0.0                  0         32768 i *  130.130.1.0/24   192.168.1.1                            0 64512 i *>i                 192.168.2.2              0    150      0 64512 i (最佳路径为192.168.2.2) *>i172.16.1.0/24    192.168.2.2              0    150      0 64512 i *                   192.168.1.1              0             0 64512 i * i192.168.1.0      192.168.2.2              0    150      0 64512 i *                   192.168.1.1              0             0 64512 i *>                  0.0.0.0                  0         32768 i *>i192.168.2.0      10.1.1.2                 0    150      0 i *                   192.168.1.1                            0 64512 i R3#

2、再来看一下R3的路由表和拓扑数据：

R3#sh ip route ………………      172.16.0.0/24 is subnetted, 1 subnets B       172.16.1.0 [200/0] via 10.1.1.2, 00:01:32      10.0.0.0/24 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial1/1      130.130.0.0/24 is subnetted, 1 subnets B       130.130.1.0 [200/0] via 10.1.1.2, 00:01:32 C    192.168.1.0/24 is directly connected, Serial1/0 B    192.168.2.0/24 [200/0] via 10.1.1.2, 01:47:43 R3#sh ip bgp       Network          Next Hop            Metric LocPrf Weight Path * i10.1.1.0/24      10.1.1.2                 0    150      0 i *>                  0.0.0.0                  0         32768 i *  130.130.1.0/24   192.168.1.1                            0 64512 i *>i                 10.1.1.2                 0    150      0 64512 i *>i172.16.1.0/24    10.1.1.2                 0    150      0 64512 i *                   192.168.1.1              0             0 64512 i * i192.168.1.0      10.1.1.2                 0    150      0 64512 i *                   192.168.1.1              0             0 64512 i *>                  0.0.0.0                  0         32768 i *>i192.168.2.0      10.1.1.2                 0    150      0 i *                   192.168.1.1                            0 64512 i

 

六：终于完成了。前天配置的时候，本地优先级不能显示出来。很郁闷的是网上也找不到合适的显示本地优级资料。于是放下了，今天弄出来了。现在想想：可能是模拟器的问题。今天是同样的配置，然后到R3和R4上show 都能显示出来各自的本地优先级。要说明一下：只有两条或多条路径可选择的情况下才会显示出本地优先级。只有一条路径可选的话，不会显示出本地优先级。