Cisco提供一种方式强IGP路由优先于EBGP路由。这个概念也就是"后门链路"(backdoor link).EBGP路由可以标记为后门链路，它将设置这些路由的管理距离与BGP本地或200相同，因为这个管理距离要高于IGP，所以首选IGP路由。
以下是具体配置:
//// r1 ////DIV>
int lo0
ip ad 1.1.1.1 255.255.255.0
int e1/0
ip ad 192.1.1.1 255.255.255.0
router os 1
network 0.0.0.0 255.255.255.255 a 0 //将所有接口宣告进OSPF
router bgp 200
no syn //关闭同步,同时也注定了全互连的拓扑
neighbor 192.1.1.2 remote-as 200
neighbor 192.1.1.2 update-source lo0 //以lo0接口IP作为更新源地址
//// r2 ////
int lo0
ip ad 2.2.2.2 255.255.255.0
int e1/0
ip ad 192.1.1.2 255.255.255.0
int e0/0
ip ad 193.1.1.2 255.255.255.0
int e2/0
ip ad 195.1.1.2 255.255.255.0
router os 1
network 0.0.0.0 255.255.255.255 a 0 //宣告所有接口进OSPF
router bgp 200
no syn
neighbor 192.1.1.1 remote-as 200
neighbor 192.1.1.1 update-source lo0
neighbor 192.1.1.1 next-hop-self
neighbor 195.1.1.5 remote-as 100
network 192.1.1.0 mask 255.255.255.0
//// r3 ////
int lo0
ip ad 3.3.3.3 255.255.255.0
int e0/0
ip ad 193.1.1.3 255.255.255.0
int e1/0
ip ad 194.1.1.3 255.255.255.0
int e3/0
ip ad 196.1.1.3 255.255.255.0
router os 1
netw 0.0.0.0 255.255.255.255 a 0
router bgp 300
no syn
neighbor 196.1.1.5 remote-as 100
neighbor 194.1.1.4 remote-as 300
neighbor 194.1.1.4 update-source lo0
neighbor 194.1.1.4 next-hop-self
network 194.1.1.0 mask 255.255.255.0
//// r4 ////
int lo0
ip ad 4.4.4.4 255.255.255.0
int e0/0
ip ad 194.1.1.3 255.255.255.0
router os 1
network 0.0.0.0 255.255.255.255 a 0
router bgp 300
no syn
neighbor 194.1.1.3 remote-as 300
neighbor 194.1.1.3 update-source lo0
network 192.1.1.0 mask 255.255.255.0 backdoor
//// r5 ////
int lo0
ip ad 5.5.5.5 255.255.255.0
int e2/0
ip ad 195.1.1.5 255.255.255.0
int e3/0
ip ad 196.1.1.5 255.255.255.0
router bgp 100
no syn
neighbor 195.1.1.2 remote-as 200
neighbor 196.1.1.3 remote-as 300
验证:
//后门链路作用前
r2#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 192.1.1.0 0.0.0.0 0 32768 i
*> 194.1.1.0 195.1.1.5 0 100 300 i
r2#
r2#
r2#
r2#sh ip ro bgp
B 194.1.1.0/24 [20/0] via 195.1.1.5, 00:00:49
r3#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 192.1.1.0 196.1.1.5 0 100 200 i
*> 194.1.1.0 0.0.0.0 0 32768 i
r3#sh ip ro bgp
B 192.1.1.0/24 [20/0] via 196.1.1.5, 00:02:28
r3#sh ip ro //后门链路作用前
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/21] via 193.1.1.2, 00:04:22, Ethernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 193.1.1.2, 00:04:22, Ethernet0/0
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/11] via 194.1.1.4, 00:04:22, Ethernet1/0
C 196.1.1.0/24 is directly connected, Serial3/0
C 193.1.1.0/24 is directly connected, Ethernet0/0
B 192.1.1.0/24 [20/0] via 196.1.1.5, 00:00:06
O 195.1.1.0/24 [110/74] via 193.1.1.2, 00:04:22, Ethernet0/0
C 194.1.1.0/24 is directly connected, Ethernet1/0
//后门链路作用后
r3#sh ip ro
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/21] via 193.1.1.2, 00:06:38, Ethernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 193.1.1.2, 00:06:38, Ethernet0/0
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/11] via 194.1.1.4, 00:06:38, Ethernet1/0
C 196.1.1.0/24 is directly connected, Serial3/0
C 193.1.1.0/24 is directly connected, Ethernet0/0
O 192.1.1.0/24 [110/20] via 193.1.1.2, 00:00:06, Ethernet0/0
O 195.1.1.0/24 [110/74] via 193.1.1.2, 00:06:38, Ethernet0/0
C 194.1.1.0/24 is directly connected, Ethernet1/0