如图所示：
R2位于AS 202 ,R3位于 AS 10101
概念：bgp后门是将特定的EBGP路由的管理距离从20修改为200（为IBGP管理距离）
R1配置：
router eigrp 100
network 1.1.1.0 0.0.0.255
network 12.1.1.0 0.0.0.255
network 192.168.1.0
no auto-summary
R2配置：
router eigrp 100
network 2.2.2.0 0.0.0.255
network 12.1.1.0 0.0.0.255
network 23.1.1.0 0.0.0.255
no auto-summary
router bgp 202
no synchronization
bgp log-neighbor-changes
neighbor 23.1.1.2 remote-as 10101
no auto-summary
R3配置：
router eigrp 100
network 3.3.3.0 0.0.0.255
network 23.1.1.0 0.0.0.255
network 192.168.1.0
no auto-summary
router bgp 10101
no synchronization
bgp log-neighbor-changes
network 192.168.1.0
neighbor 23.1.1.1 remote-as 202
no auto-summary
在R2上观察192.168.1.0/24的路由
r2#show ip route
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 12.1.1.1, 00:04:23, Serial1/0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, Serial1/1
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, Serial1/0
B 192.168.1.0/24 [20/0] via 23.1.1.2, 00:02:21
发现显示的是B的路由 因为EBGP的管理距离是20小于EIGRP的90
使用命令：
r3(config-router)#network 192.168.1.0 mask 255.255.255.0 backdoor
这个时侯EBGP的管理距离变为200
再R2上观察192.168.1.0/24的路由
r2#show ip route
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/2297856] via 12.1.1.1, 00:02:49, Serial1/0
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
C 23.1.1.0 is directly connected, Serial1/1
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, Serial1/0
D 192.168.1.0/24 [90/2172416] via 23.1.1.2, 00:01:46, Serial1/1
[90/2172416] via 12.1.1.1, 00:01:46, Serial1/0