BGP路由黑洞

AR1配置：
bgp 10
 router-id 10.0.1.1
 peer 10.0.12.2 as-number 20 
 ipv4-family unicast
  undo synchronization
  network 10.0.1.1 255.255.255.255 
  network 10.0.12.0 255.255.255.0 
  peer 10.0.12.2 enable

配置next-hop-local原因是:IBGP从EBGP学习到路由下一跳不变，会导致下一跳不可达，所以IBGP配置local，告知下一跳是自己。

AR2:
bgp 20
router-id 10.0.2.2
 peer 10.0.4.4 as-number 20 
 peer 10.0.4.4 connect-interface LoopBack0
 peer 10.0.12.1 as-number 10 
 ipv4-family unicast
  undo synchronization
  peer 10.0.4.4 enable
  peer 10.0.4.4 next-hop-local 
  peer 10.0.12.1 enable

AR4:
bgp 20
 router-id 10.0.4.4
 peer 10.0.2.2 as-number 20 
 peer 10.0.2.2 connect-interface LoopBack0
 peer 10.0.45.5 as-number 30 
 ipv4-family unicast
  undo synchronization
  peer 10.0.2.2 enable
  peer 10.0.2.2 next-hop-local 
  peer 10.0.45.5 enable

AR5:
bgp 30
 router-id 10.0.5.5
 peer 10.0.45.4 as-number 20 
 ipv4-family unicast
  undo synchronization
  network 10.0.5.5 255.255.255.255 
  network 10.0.45.0 255.255.255.0 
  peer 10.0.45.4 enable    

AR2，AR3，AR4配置OSPF 1

ospf 1 router-id 10.0.0.2 
 area 0.0.0.0 
  network 10.0.2.2 0.0.0.0 
  network 10.0.23.0 0.0.0.255 

ospf 1 router-id 10.0.0.3
 area 0.0.0.0 
  network 10.0.3.3 0.0.0.0 
  network 10.0.23.0 0.0.0.255 
  network 10.0.34.0 0.0.0.255

ospf 1 router-id 10.0.0.4
 area 0.0.0.0 
  network 10.0.4.4 0.0.0.0 
  network 10.0.34.0 0.0.0.255

根据BGP第四条选路原则，当IBGP学到的路由传递给BGP邻居之前通过IGP必须知道该路由，也就是说，AR2学习到的AR1的的路由传递给AR4时，改路由经过AR3，AR3必须知道AR1的路由；同理也必须知道AR5的路由，可以通过IBGP全互联和OSPF 引入BGP解决此问题；
办法1 ospf引入bgp(如果是公网bgp，不建议这么搞，除非你路由器和运维能力很牛逼)：

AR2和AR4分别：
bgp 20
import-route bgp

办法2 IBGP全互联：

AR2：
bgp 20
 router-id 10.0.2.2
 peer 10.0.3.3 as-number 20 
 peer 10.0.3.3 connect-interface LoopBack0
 peer 10.0.4.4 as-number 20 
 peer 10.0.4.4 connect-interface LoopBack0
 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 
AR3：
bgp 20
 router-id 10.0.3.3
 peer 10.0.2.2 as-number 20 
 peer 10.0.2.2 connect-interface LoopBack0
 peer 10.0.4.4 as-number 20 
 peer 10.0.4.4 connect-interface LoopBack0
 ipv4-family unicast
  undo synchronization
  peer 10.0.2.2 enable
  peer 10.0.4.4 enable
  AR4：bgp 20
 router-id 10.0.4.4
 peer 10.0.2.2 as-number 20 
 peer 10.0.2.2 connect-interface LoopBack0
 peer 10.0.3.3 as-number 20 
 peer 10.0.3.3 connect-interface LoopBack0
 ipv4-family unicast
  undo synchronization
  peer 10.0.2.2 enable
  peer 10.0.2.2 next-hop-local 
  peer 10.0.3.3 enable
  peer 10.0.3.3 next-hop-local

BGP路由黑洞

原文：https://blog.51cto.com/19940820/2536316