47、BGP配置实验之MED选路

1、as-path选路解析

属性	传播范围	默认值	优先值	方向	备注
weight	本路由器	本路由器32768，其他路由器0	大值优先	IN	CISCO私有
Local preference	AS内部	100：边界路由器，通告给IBGP邻居，告知如何离开本AS	大值优先	OUT
As path	AS之间	无	短优先	IN、OUT
MED	AS内部	0：通告给EBGP，告知如何进入本AS	小优先	OUT

修改MED值的方法:

2、实验拓扑

3、基础配置R1配置
interface Loopback0 ip address 1.1.1.1 255.255.255.0interface Serial0/0 ip address 13.1.1.1 255.255.255.0interface Serial0/1 ip address 14.1.1.1 255.255.255.0router ospf 110 router-id 1.1.1.1 log-adjacency-changes network 1.1.1.0 0.0.0.255 area 0 network 14.1.1.0 0.0.0.255 area 0router bgp 4 no synchronization bgp router-id 1.1.1.1 neighbor 4.4.4.4 remote-as 4 neighbor 4.4.4.4 update-source Loopback0 neighbor 4.4.4.4 next-hop-self neighbor 13.1.1.3 remote-as 3 no auto-summary
R2配置interface Loopback0 ip address 2.2.2.2 255.255.255.0interface Serial0/1 ip address 23.1.1.2 255.255.255.0interface FastEthernet1/0 ip address 24.1.1.2 255.255.255.0router ospf 110 router-id 2.2.2.2 network 2.2.2.0 0.0.0.255 area 0 network 24.1.1.0 0.0.0.255 area 0router bgp 4 no synchronization bgp router-id 2.2.2.2 neighbor 4.4.4.4 remote-as 4 neighbor 4.4.4.4 update-source Loopback0 neighbor 4.4.4.4 next-hop-self neighbor 23.1.1.3 remote-as 3 no auto-summary
R3配置interface Loopback0 ip address 3.3.3.3 255.255.255.0interface Loopback1 ip address 33.33.33.33 255.255.255.0 interface Serial0/0 ip address 13.1.1.3 255.255.255.0interface Serial0/1 ip address 23.1.1.3 255.255.255.0router bgp 3 no synchronization bgp router-id 3.3.3.3 network 3.3.3.0 mask 255.255.255.0 network 33.33.33.0 mask 255.255.255.0 neighbor 13.1.1.1 remote-as 4 neighbor 23.1.1.2 remote-as 4 no auto-summary
R4配置interface Loopback0 ip address 4.4.4.4 255.255.255.0interface Serial0/1 ip address 14.1.1.4 255.255.255.0interface FastEthernet1/0 ip address 24.1.1.4 255.255.255.0router ospf 110 router-id 4.4.4.4 network 4.4.4.0 0.0.0.255 area 0 network 14.1.1.0 0.0.0.255 area 0 network 24.1.1.0 0.0.0.255 area 0router bgp 4 no synchronization bgp router-id 4.4.4.4 network 4.4.4.0 mask 255.255.255.0 neighbor 1.1.1.1 remote-as 4 neighbor 1.1.1.1 update-source Loopback0 neighbor 2.2.2.2 remote-as 4 neighbor 2.2.2.2 update-source Loopback0 no auto-summary

4、默认选路


根据BGP选路原则第十条可以知道，R3到达4.4.4.0/24默认经过R1。



5、修改MED值让R3访问4.4.4.0/24的时候从R2经过
R1配置

ip prefix-list 1 seq 5 permit 4.4.4.0/24
route-map MED permit 10
match ip address prefix-list 1
set metric 10
route-map MED permit 20
router bgp 4
neighbor 13.1.1.3 route-map MED out



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