BGP总是优先选择具有最短AS路径的路由。本案例中，我们将配置R1，将1.0.0.0通告给R2、R3之前预先添加两个额外的AS路径到1.0.0.0上（AS300、AS400）
具体配置如下：
//// r1 ////
int f2/0
ip ad 192.1.1.1 255.255.255.0
int f3/0
ip ad 193.1.1.1 255.255.255.0
int lo0
ip ad 1.1.1.1 255.255.255.0
int lo1
ip ad 2.2.2.2 255.255.255.0
router bgp 100
no syn
neighbor 192.1.1.2 remote-as 200
neighbor 193.1.1.3 remote-as 200
network 1.0.0.0
network 1.0.0.0
neighbor 193.1.1.3 route-map aspath out
access-list 1 permit 1.0.0.0 0.255.255.255
route-map aspath 10
match ip ad 1
set as-path prepend 300 400
route-map aspath 20
//// r2 ////
int f2/0
ip ad 192.1.1.2 255.255.255.0
int f4/0
ip ad 194.1.1.2 255.255.255.0
router os 1
netw 0.0.0.0 255.255.255.255 a 0
passive-interface f2/0
router bgp 200
no syn
neighbor 192.1.1.1 remote-as 100
neighbor 194.1.1.4 remote-as 200
neighbor 195.1.1.3 remote-as 200
neighbor 194.1.1.4 next-hop-self
//// r3 ////
int f3/0
ip ad 193.1.1.3 255.255.255.0
int f5/0
ip ad 195.1.1.3 255.255.255.0
router os 1
netw 0.0.0.0 255.255.255.255 a 0
passive-interface f3/0
router bgp 200
no syn
neighbor 193.1.1.1 remote-as 100
neighbor 195.1.1.4 remote-as 200
neighbor 195.1.1.4 next-hop-self
neighbor 194.1.1.2 remote-as 200
//// r4 ////
int f4/0
ip ad 194.1.1.4 255.255.255.0
int f5/0
ip ad 195.1.1.4 255.255.255.0
int lo0
ip ad 4.4.4.4 255.255.255.0
router os 1
netw 0.0.0.0 255.255.255.255 a 0
router bgp 200
no syn
neighbor 194.1.1.2 remote-as 200
neighbor 195.1.1.3 remote-as 200
netw 4.0.0.0
验证：
r3#sh ip b
Network Next Hop Metric LocPrf Weight Path
*>i1.0.0.0 192.1.1.1 0 100 0 100 i
* 193.1.1.1 0 0 100 300 400 i
* i2.0.0.0 192.1.1.1 0 100 0 100 i
*> 193.1.1.1 0 0 100 i
*>i4.0.0.0 195.1.1.4 0 100 0 i
Network Next Hop Metric LocPrf Weight Path
*>i1.0.0.0 192.1.1.1 0 100 0 100 i
* 193.1.1.1 0 0 100 300 400 i
* i2.0.0.0 192.1.1.1 0 100 0 100 i
*> 193.1.1.1 0 0 100 i
*>i4.0.0.0 195.1.1.4 0 100 0 i
r3#sh ip ro
B 1.0.0.0/8 [200/0] via 192.1.1.1, 00:05:23
B 2.0.0.0/8 [20/0] via 193.1.1.1, 00:05:23
4.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O 4.4.4.4/32 [110/65] via 195.1.1.4, 00:10:38, Serial5/0
B 4.0.0.0/8 [200/0] via 195.1.1.4, 00:10:13
C 193.1.1.0/24 is directly connected, Serial3/0
O 192.1.1.0/24 [110/192] via 195.1.1.4, 00:10:38, Serial5/0
C 195.1.1.0/24 is directly connected, Serial5/0
O 194.1.1.0/24 [110/128] via 195.1.1.4, 00:10:38, Serial5/0
B 2.0.0.0/8 [20/0] via 193.1.1.1, 00:05:23
4.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O 4.4.4.4/32 [110/65] via 195.1.1.4, 00:10:38, Serial5/0
B 4.0.0.0/8 [200/0] via 195.1.1.4, 00:10:13
C 193.1.1.0/24 is directly connected, Serial3/0
O 192.1.1.0/24 [110/192] via 195.1.1.4, 00:10:38, Serial5/0
C 195.1.1.0/24 is directly connected, Serial5/0
O 194.1.1.0/24 [110/128] via 195.1.1.4, 00:10:38, Serial5/0
本文转自 ciscodocu51CTO博客，原文链接：http://blog.51cto.com/ciscodocu/52841，如需转载请自行联系原作者
