4.6 优选限最小MED值路由

1.R1配置OSPF

[R1]ospf 1 router-id 1.1.1.1
[R1-ospf-1]area 0
[R1-ospf-1-area-0.0.0.0]network 1.1.1.1 0.0.0.0
[R1-ospf-1-area-0.0.0.0]network 12.1.1.0 0.255.255.255

2.R2配置OSPF

[R2]ospf 1 router-id 2.2.2.2
[R2-ospf-1]area 0
[R2-ospf-1-area-0.0.0.0]network 2.2.2.2 0.0.0.0
[R2-ospf-1-area-0.0.0.0]network 12.1.1.0 0.255.255.255

3.R1配置IBGP、EBGP

[R1]bgp 12
[R1-bgp]router-id 1.1.1.1
[R1-bgp]peer 2.2.2.2 as-number 12
[R1-bgp]peer 2.2.2.2 connect-interface LoopBack 0
[R1-bgp]peer 2.2.2.2 next-hop-local
[R1-bgp]peer 13.1.1.3 as-number 300

4.R2配置IBGP、EBGP

[R2]bgp 12
[R2-bgp]router-id 2.2.2.2 
[R2-bgp]peer 1.1.1.1 as-number 12
[R2-bgp]peer 1.1.1.1 connect-interface LoopBack 0
[R2-bgp]peer 1.1.1.1 next-hop-local
[R2-bgp]peer 23.1.1.3 as-number 300

5.R3配置EBGP，宣告3.3.3.3/32到BGP

[R3]bgp 300
[R3-bgp]router-id 3.3.3.3
[R3-bgp]peer 13.1.1.1 as-number 12
[R3-bgp]peer 23.1.1.2 as-number 12
[R3-bgp]network 3.3.3.3 32

6.R3针对R1使用策略方法修改MED值

[R3]ip ip-prefix dsrw.com permit 3.3.3.3 32
[R3]route-policy R1 permit node 10
[R3-route-policy]if-match ip-prefix dsrw.com
[R3-route-policy]apply cost 500
[R3-route-policy]quit
[R3]route-policy R1 permit node 20

7.R3针对R2使用策略方法修改MED值，在BGP应用策略

[R3]route-policy R2 permit node 10
[R3-route-policy]if-match ip-prefix dsrw.com
[R3-route-policy]apply cost 1000
[R3-route-policy]quit
[R3]route-policy R2 permit node 20

[R3]bgp 300
[R3-bgp]peer 13.1.1.1 route-policy R1 export 
[R3-bgp]peer 23.1.1.2 route-policy R2 export

8.R1、R2查看BGP信息，选路都是从R1进入到AS300

<R3>refresh bgp all export 

<R1>display bgp routing-table 
 *>   3.3.3.3/32         13.1.1.3        500                   0      300i

<R2>display bgp routing-table
 *>i  3.3.3.3/32         1.1.1.1         500        100        0      300i
 *                       23.1.1.3        1000                  0      300i

9.MED值比较条件

在AS-PATH中，默认第一个AS号相同才进行比较,R1修改AS号。
[R1]ip ip-prefix dsrw.com permit 3.3.3.3 32
[R1]route-policy dsrw permit node 10
[R1-route-policy]if-match ip-prefix dsrw.com
[R1-route-policy]apply as-path 20 additive 
[R1-route-policy]quit
[R1]route-policy dsrw permit node 20
[R1]bgp 12
[R1-bgp]peer 13.1.1.3 route-policy dsrw import 

10.MED值比较条件

在AS-PATH中，默认第一个AS号相同才进行比较,R2修改AS号。
[R2]ip ip-prefix dsrw.com permit 3.3.3.3 32
[R2]route-policy dsrw permit node 10
[R2-route-policy]if-match ip-prefix dsrw.com
[R2-route-policy]apply as-path 10 additive 
[R2-route-policy]quit
[R2]route-policy dsrw permit node 20
[R2]bgp 12
[R2-bgp]peer 23.1.1.3 route-policy dsrw import 

11.R2查看BGP路由信息，AS-PATH不一致时，默认不比较MED值。

[R2]display bgp routing-table 
 *>   3.3.3.3/32         23.1.1.3        1000                  0      10 300i
 * i                     1.1.1.1         500        100        0      20 300i

12.使用以下命令要求BGP比较，AS不一致时，开始比较MED值

[R2]bgp 12
[R2-bgp]compare-different-as-med 
[R2]display bgp routing-table 
 *>i  3.3.3.3/32         1.1.1.1         500        100        0      20 300i
 *                       23.1.1.3        1000                  0      10 300i