BGP路由决择6(MED)
bgp bestpath med missing-as-worst
IOS Version 12.4(13b)
默认情况下，Cisco路由器BGP设置没有的MED值为0
相应配置如下：
R4 router bgp 4 no synchronization bgp log-neighbor-changes network 4.4.4.4 mask 255.255.255.255 neighbor 14.0.0.1 remote-as 1 neighbor 34.0.0.3 remote-as 3 no auto-summary |
R1 router bgp 1 no synchronization bgp router-id 1.1.1.1 bgp log-neighbor-changes neighbor 12.0.0.2 remote-as 2 neighbor 12.0.0.2 route-map xx out neighbor 14.0.0.4 remote-as 4 no auto-summary neighbor 12.0.0.2 route-map xx out route-map xx permit 10 match ip address 1 set metric 20 access-list 1 permit 4.4.4.4 |
R3 router bgp 3 no synchronization bgp router-id 3.3.3.3 bgp log-neighbor-changes neighbor 23.0.0.2 remote-as 2 neighbor 34.0.0.4 remote-as 4 no auto-summary |
R2 router bgp 2 no synchronization bgp always-compare-med bgp log-neighbor-changes bgp bestpath as-path ignore neighbor 12.0.0.1 remote-as 1 neighbor 23.0.0.3 remote-as 3 no auto-summary |
路由器R2正在接收路由器R1和R3对网络4.4.4.4/32的广播
R2#show ip bgp BGP table version is 12, local router ID is 2.2.2.2 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 *4.4.4.4/32 12.0.0.1 20 0 1 4 i *>23.0.0.3 0 3 4 i |
最好的路径选择算法已经选择了从路由器R3来的路由作为最好的路由，因为我们比较了它们的MED，路由器R1没有发送网络4.4.4.4/32的MED值。默认情况下，BGP设置从路由器R3来的前缀MED值为0。
现在，修改路由器R2上的BGP配置，把没有的MED（注意是”没有”，不是为”0”）值当作无穷大：
R2 router bgp 2 bgp bestpath med missing-as-worst |
证实
进一步证实最好路径选择算法已经选择了从路由器R1来的路由作为最好的路由，因为它有更低的MED值
R2 R2#show ip bgp BGP table version is 2, local router ID is 2.2.2.2 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 *> 4.4.4.4/32 12.0.0.1 20 0 1 4 i *23.0.0.3 4294967295 0 3 4 i |
