BGP配置实验之MED选路

olik

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 1.1.1.1 255.255.255.0

interface Serial0/0

ip address 13.1.1.1 255.255.255.0

interface Serial0/1

ip address 14.1.1.1 255.255.255.0

router 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 0

router 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 2.2.2.2 255.255.255.0

interface Serial0/1

ip address 23.1.1.2 255.255.255.0

interface FastEthernet1/0

ip address 24.1.1.2 255.255.255.0

router 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 0

router 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 3.3.3.3 255.255.255.0

interface Loopback1

ip address 33.33.33.33 255.255.255.0        

interface Serial0/0

ip address 13.1.1.3 255.255.255.0

interface 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 4.4.4.4 255.255.255.0

interface Serial0/1

ip address 14.1.1.4 255.255.255.0

interface FastEthernet1/0

ip address 24.1.1.4 255.255.255.0

router 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 0

router 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