BGP-MED

Posted 2020-10-21

BGP-MED属性：
MED（Multi-Exit Discriminator）属性用于判断流量进入AS时的最佳路由，当一个运行BGP的设备通过不同的EBGP对等体得到目的地址相同但下一跳不同的多条路由时，在其它条件相同的情况下，将优先选择MED值较小者作为最佳路由。

MED属性仅在相邻两个AS之间传递，收到此属性的AS一方不会再将其通告给任何其他第三方AS。MED属性可以手动配置，如果路由没有配置MED属性，BGP选路时将该路由的MED值按缺省值0来处理。

如图：实现10的网络走上，60的网络走下面效果！

R1的配置

interface GigabitEthernet0/0/0
ip address 12.12.12.1 255.255.255.0

interface GigabitEthernet0/0/1
ip address 13.13.13.1 255.255.255.0

interface NULL0

interface LoopBack0
ip address 1.1.1.1 255.255.255.0

interface LoopBack100
ip address 100.1.1.1 255.255.255.0

bgp 100
peer 2.2.2.2 as-number 200
peer 2.2.2.2 ebgp-max-hop 255
peer 2.2.2.2 connect-interface LoopBack0
peer 3.3.3.3 as-number 200
peer 3.3.3.3 ebgp-max-hop 255
peer 3.3.3.3 connect-interface LoopBack0
group as200 internal

ipv4-family unicast
undo synchronization
network 100.1.1.0 255.255.255.0
peer 2.2.2.2 enable
peer 3.3.3.3 enable
peer as200 enable

ip route-static 2.2.2.0 255.255.255.0 12.12.12.2
ip route-static 3.3.3.0 255.255.255.0 13.13.13.3

user-interface con 0
authentication-mode password
user-interface vty 0 4
user-interface vty 16 20

wlan ac

return
<R1>

R2的配置

bgp 200
peer 1.1.1.1 as-number 100
peer 1.1.1.1 ebgp-max-hop 255
peer 1.1.1.1 connect-interface LoopBack0
peer 4.4.4.4 as-number 200
peer 4.4.4.4 connect-interface LoopBack0
peer 5.5.5.5 as-number 200
#
ipv4-family unicast
undo synchronization
peer 1.1.1.1 enable
peer 1.1.1.1 route-policy 10 export
peer 4.4.4.4 enable
peer 4.4.4.4 next-hop-local
peer 5.5.5.5 enable
peer 5.5.5.5 next-hop-local

route-policy 10 permit node 10
if-match acl 2000
apply cost 100

acl number 2000
rule 5 permit source 10.10.10.0 0.0.0.255
rule 10 deny

R3的配置

bgp 200
peer 1.1.1.1 as-number 100
peer 1.1.1.1 ebgp-max-hop 255
peer 1.1.1.1 connect-interface LoopBack0
peer 4.4.4.4 as-number 200
peer 4.4.4.4 connect-interface LoopBack0
peer 5.5.5.5 as-number 200
#
ipv4-family unicast
undo synchronization
peer 1.1.1.1 enable
peer 1.1.1.1 route-policy 10 export
peer 4.4.4.4 enable
peer 4.4.4.4 next-hop-local
peer 5.5.5.5 enable
peer 5.5.5.5 next-hop-local

acl number 2000
rule 5 permit source 60.1.1.0 0.0.0.255
rule 10 permit source 60.1.2.0 0.0.0.255
rule 15 permit source 60.1.3.0 0.0.0.255
rule 20 deny

route-policy 10 permit node 10
if-match acl 2000
apply cost 100

R1的路由表