BGP选路之Local Preference

1、基本配置
R1:
sys
sysname R1
int loop 0
ip add 10.0.1.1 32
int loop 1
ip add 10.0.100.2 32
int g0/0/0
ip add 10.0.12.1 24
int g0/0/1
ip add 10.0.13.1 24
q

R2:
sys
sysname R2
int loop 0
ip add 10.0.2.2 32
int g0/0/0
ip add 10.0.12.2 24
int g0/0/1
ip add 10.0.24.2 24
q
ospf 1
 area 0.0.0.0
  network 10.0.24.0 0.0.0.255
  network 10.0.2.2 0.0.0.0

R3:
sys
sysname R3
int loop 0
ip add 10.0.3.3 32
int g0/0/1
ip add 10.0.13.3 24
int g0/0/0
ip add 10.0.34.3 24
q
ospf 1
 area 0.0.0.0
  network 10.0.34.0 0.0.0.255
  network 10.0.3.3 0.0.0.0


R4:
sys
sysname R4
int loop 0
ip add 10.0.4.4 32
int loop 1
ip add 10.0.100.4 32
int g0/0/1
ip add 10.0.24.4 24
int g0/0/0
ip add 10.0.34.4 24
q
ospf 1
 area 0.0.0.0
  network 10.0.24.0 0.0.0.255
  network 10.0.34.0 0.0.0.255
  network 10.0.4.4 0.0.0.0

 配置完成后，在R4上使用display ospf peer命令查看OSPF邻居关系。

r1:
bgp 100
 router-id 10.0.1.1
 peer 10.0.12.2 as-number 200
 peer 10.0.13.3 as-number 200
 #
 ipv4-family unicast
  undo synchronization
  network 10.0.1.1 255.255.255.255
  network 10.0.100.2 255.255.255.255
  peer 10.0.12.2 enable
  peer 10.0.13.3 enable
r2:
bgp 200
 router-id 10.0.2.2
 peer 10.0.3.3 as-number 200
 peer 10.0.3.3 connect-interface LoopBack0
 peer 10.0.4.4 as-number 200
 peer 10.0.4.4 connect-interface LoopBack0
 peer 10.0.12.1 as-number 100
 #
 ipv4-family unicast
  undo synchronization
  peer 10.0.3.3 enable
  peer 10.0.3.3 next-hop-local
  peer 10.0.4.4 enable
  peer 10.0.4.4 next-hop-local
  peer 10.0.12.1 enable
r3:
bgp 200
 router-id 10.0.3.3
 peer 10.0.2.2 as-number 200
 peer 10.0.2.2 connect-interface LoopBack0
 peer 10.0.4.4 as-number 200
 peer 10.0.4.4 connect-interface LoopBack0
 peer 10.0.13.1 as-number 100
 #
 ipv4-family unicast
  undo synchronization
  peer 10.0.2.2 enable
  peer 10.0.2.2 next-hop-local
  peer 10.0.4.4 enable
  peer 10.0.4.4 next-hop-local
  peer 10.0.13.1 enable
r4:
bgp 200
 peer 10.0.2.2 as-number 200
 peer 10.0.2.2 connect-interface LoopBack0
 peer 10.0.3.3 as-number 200
 peer 10.0.3.3 connect-interface LoopBack0
 #
 ipv4-family unicast
  undo synchronization
  peer 10.0.2.2 enable
  peer 10.0.3.3 enable

 在r4观察BGP路由信息的Local Preference属性

 可以看到，对于目的网络10.0.100.2/32，R4 选择了下一跳为10.0.2.2的路由作为最佳路由，这是由于R4在对下一跳为10.0.2.2与10.0.3.3的路由信息进行比较时，二者的Preferred Value属性、Local Preference属性、路由生成方式、AS_Path 属性、Origin 属性、MED属性、BGP对等体类型等都是相同的，于是R4最终选择了Router-ID较小的路由器R2发布的路由作为最佳路由。
为了使公司内部网络去往10.0.100.2/32的数据使用R3作为出口,可以在R3的BGP视图下使用default local-preference命令将R3的 BGP默认Local Preference值修改为200。

r2:
bgp 200
  peer 10.0.12.1 route-policy 1 import

#
route-policy 1 permit node 10
 if-match ip-prefix 1
 apply local-preference 400
#
route-policy 1 permit node 20
#
ip ip-prefix 1 index 10 permit 10.0.1.1 32