1. 通过修改weight属性改变选路
R1、R4分别与R2、R3建立EBGP邻居关系
在R1上建loopback0 IP：
R1 router-id：
R2 router-id：
默认情况下R4的bgp表中：
R4#sh ip bgp
BGP table version is 2, local router ID is 44.44.44.44
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
*
*> 24.1.1.2 0 23 1 i
根据选路原则，prefer the path with the lowest neighbor BGP router ID（因为前面的都相同），所以默认选择router-id小的为最优路径，即R2。
下面更改weight值来改变选路：
R4：
R4(config)#ip prefix-list 1 per
R4(config)#route-map weight per 10
R4(config-route-map)#mat ip add prefix-list 1
R4(config-route-map)#set weight 100
R4(config)#router bgp 4
R4(config-router)#nei 34.1.1.3 route-map weight in
R4 BGP表
R4#sh ip bgp
BGP table version is 3, local router ID is 44.44.44.44
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
*>
* 24.1.1.2 0 23 1 i
注：修改weight的route-map只能在bgp的inbound方向应用
