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

OUT

As path

AS之间

无

短优先

IN、OUT

MED

0：通告给EBGP，告知如何进入本AS

小优先

修改MED值的方法:

2、实验拓扑

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

 ip address 2.2.2.2 255.255.255.0

 ip address 23.1.1.2 255.255.255.0

interface FastEthernet1/0

 ip address 24.1.1.2 255.255.255.0

 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

 bgp router-id 2.2.2.2

 neighbor 23.1.1.3 remote-as 3

R3配置

 ip address 3.3.3.3 255.255.255.0

interface Loopback1

 ip address 33.33.33.33 255.255.255.0        

 ip address 13.1.1.3 255.255.255.0

 ip address 23.1.1.3 255.255.255.0router bgp 3

 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

R4配置

 ip address 4.4.4.4 255.255.255.0

 ip address 14.1.1.4 255.255.255.0

 ip address 24.1.1.4 255.255.255.0

 router-id 4.4.4.4

 network 4.4.4.0 0.0.0.255 area 0

 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

4、默认选路

<a href="http://s4.51cto.com/wyfs02/M01/77/A5/wKiom1ZqffPyRzJiAAAotm06_Y8952.png" target="_blank"></a>

5、修改MED值让R3访问4.4.4.0/24的时候从R2经过

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

 neighbor 13.1.1.3 route-map MED out

<a href="http://s2.51cto.com/wyfs02/M01/77/A5/wKioL1Zqh8bhHCx5AAApXDcn_Mc103.png" target="_blank"></a>

      本文转自开源殿堂 51CTO博客，原文链接：http://blog.51cto.com/kaiyuandiantang/1722088，如需转载请自行联系原作者