BGP路由黑洞

IP地址如圖所示；
AR1和AR2配置EBGP，AR4和AR5也配置EBGP，AR2和AR4配置IBGP;
根據BGP選路原則4：（一條從IBGP學習到的路由發給BGP鄰居以前，經過IGP必須知道該路由，即IGP與BGP同步）。在華爲路由器上，默認是將BGP於IGP的同步檢查關閉的，緣由是爲了實現IBGP路由的正常通告。但關閉同步會出現「路由黑洞」。所以有兩種解決方案：1.將BGP路由引入IGP，2.IBGP路由器必須全互聯。
BGP配置

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緣由是:IBGP從EBGP學習到路由下一跳不變，會致使下一跳不可達，因此IBGP配置local，告知下一跳是本身。

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

根據BGP第四條選路原則，當IBGP學到的路由傳遞給BGP鄰居以前經過IGP必須知道該路由，也就是說，AR2學習到的AR1的的路由傳遞給AR4時，改路由通過AR3，AR3必須知道AR1的路由；同理也必須知道AR5的路由，能夠經過IBGP全互聯和OSPF 引入BGP解決此問題；
辦法1 ospf引入bgp(若是是公網bgp，不建議這麼搞，除非你路由器和運維能力很牛逼)：

AR2和AR4分別：
bgp 20
import-route bgp

辦法2 IBGP全互聯：

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