在进行ccnp课程学习的时候经常会遇见各种问题，而问题最多的莫过于bgp协议的backdoor实验，backdoor再加上帧中继那就是很多人的恶梦了，那么我们今天就来做一个backdoor的实验，backdoor在CCIE考试的实验考试的时候也是用到了的，它的主要目的就是把ebgp学习的路由的AD值从20变为200，从而改变路径的选择。

1、实验拓扑如下：

3、具体的命令
r1：
interface Serial0/0
ip address 123.1.1.1 255.255.255.0
encapsulation frame-relay
serial restart-delay 0
frame-relay map ip 123.1.1.3 103 broadcast
frame-relay map ip 123.1.1.1 103 broadcast
frame-relay map ip 123.1.1.2 102 broadcast
no frame-relay inverse-arp
!
router bgp 1
no synchronization
bgp router-id 1.1.1.1
bgp log-neighbor-changes
neighbor 123.1.1.2 remote-as 2
neighbor 123.1.1.3 remote-as 3
no auto-summary
 
r2：
interface Loopback0
ip address 192.168.1.1 255.255.255.0
ip ospf network point-to-point
!
interface Serial0/0
ip address 123.1.1.2 255.255.255.0
encapsulation frame-relay
serial restart-delay 0
frame-relay map ip 123.1.1.3 203 broadcast
frame-relay map ip 123.1.1.1 201 broadcast
frame-relay map ip 123.1.1.2 201 broadcast
no frame-relay inverse-arp
!
 
router ospf 23
router-id 2.2.2.2
log-adjacency-changes
network 2.2.2.2 0.0.0.0 area 0
network 192.168.1.1 0.0.0.0 area 0
network 192.168.23.2 0.0.0.0 area 0
!
router bgp 2
no synchronization
bgp router-id 2.2.2.2
bgp log-neighbor-changes
network 2.2.2.0 mask 255.255.255.0
network 192.168.1.0
network 192.168.2.0 backdoor
neighbor 123.1.1.1 remote-as 1
neighbor 123.1.1.3 remote-as 3
no auto-summary
!
 
R3：
interface Loopback0
ip address 192.168.2.2 255.255.255.0
!
interface Serial0/0
ip address 123.1.1.3 255.255.255.0
encapsulation frame-relay
serial restart-delay 0
frame-relay map ip 123.1.1.3 302 broadcast
frame-relay map ip 123.1.1.1 301 broadcast
frame-relay map ip 123.1.1.2 302 broadcast
no frame-relay inverse-arp
!
 
router ospf 23
router-id 3.3.3.3
log-adjacency-changes
network 192.168.23.3 0.0.0.0 area 0
!
router bgp 3
no synchronization
bgp router-id 3.3.3.3
bgp log-neighbor-changes
network 192.168.1.0 backdoor
network 192.168.2.0
neighbor 123.1.1.1 remote-as 1
neighbor 123.1.1.2 remote-as 2
no auto-summary

4、实验结果
R2(config-router)#do sh ip rou
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route
 
Gateway of last resort is not set
 
C    192.168.23.0/24 is directly connected, Serial0/1
     123.0.0.0/24 is subnetted, 1 subnets
C       123.1.1.0 is directly connected, Serial0/0
C    192.168.1.0/24 is directly connected, Loopback0
B    192.168.2.0/24 [200/0] via 123.1.1.3, 00:09:13
 
R3(config-router)#do sh ip rou
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route
 
Gateway of last resort is not set
 
C    192.168.23.0/24 is directly connected, Serial0/1
     123.0.0.0/24 is subnetted, 1 subnets
C       123.1.1.0 is directly connected, Serial0/0
O    192.168.1.0/24 [110/65] via 192.168.23.2, 00:08:47, Serial0/1
C    192.168.2.0/24 is directly connected, Loopback0