隧道解决BGP路由黑洞
实验拓扑图：
说明：R1和R2之间建立EBGP邻居关系，R2与R4之间建立IBGP邻居关系同时建立隧道，R4与R5之间建立EBGP邻居关系。IP地址规划和接口如图所示；
要求：通过在R2和R4之间建立隧道使R5能够用源地址5.5.5.5ping通1.1.1.1
配置如下：
R1：
interface Loopback0
ip address 1.1.1.1 255.255.255.255
interface Serial1/2
ip address 12.12.12.1 255.255.255.0 //基本接口配置
router bgp 100
no synchronization
bgp log-neighbor-changes
network 1.1.1.1 mask 255.255.255.255 //通告自己的还回口地址，这里需要注意的是通告的地址必须是自己路由表中有的路由；网络号和掩码必须对应
neighbor 12.12.12.2 remote-as 200 //和R2建立EBGP邻居关系；这里直接用的接口地址建立邻居关系，如果用还回口建立邻居关系，必须修改为多跳。
R2:
interface Loopback0
ip address 2.2.2.2 255.255.255.255
interface Serial1/2
ip address 23.23.23.2 255.255.255.0
interface Serial1/3
ip address 12.12.12.2 255.255.255.0 //基本接口配置
router ospf 100
log-adjacency-changes
network 2.2.2.2 0.0.0.0 area 0
network 23.23.23.0 0.0.0.255 area 0 //IBGP路由协议，使内部能够互通
interface Tunnel0
ip address 10.1.1.1 255.255.255.0
tunnel source Serial1/2
tunnel destination 34.34.34.4 //创建隧道，指定隧道的IP地址，隧道的源地址，目的地址
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 10.1.1.2 remote-as 200 //和R4建立IBGP邻居关系，这里10.1.1.2为R4tunnel口的IP地址，通过这条命令从R4学过来的BGP路由的下一跳为10.1.1.2，这样就可以确保到5.5.5.5的数据包从tunnel口发送出去，这里是关键。
neighbor 10.1.1.2 next-hop-self //指定更新源为自己，不然学到的BGP路由不会放进路由表
neighbor 12.12.12.1 remote-as 100 //和R1建立EBGP邻居关系
R3：
interface Loopback0
ip address 3.3.3.3 255.255.255.255
interface Serial1/2
ip address 34.34.34.3 255.255.255.0
serial restart-delay 0
!
interface Serial1/3
ip address 23.23.23.3 255.255.255.0 //基本接口配置
router ospf 100
log-adjacency-changes
network 3.3.3.3 0.0.0.0 area 0
network 23.23.23.0 0.0.0.255 area 0
network 34.34.34.0 0.0.0.255 area 0 //内部路由协议配置
R4：
interface Loopback0
ip address 4.4.4.4 255.255.255.255
interface Serial1/2
ip address 45.45.45.4 255.255.255.0
interface Serial1/3
ip address 34.34.34.4 255.255.255.0 //基本接口配置
router ospf 100
log-adjacency-changes
network 4.4.4.4 0.0.0.0 area 0
network 34.34.34.0 0.0.0.255 area 0 //内部路由协议配置
interface Tunnel0
ip address 10.1.1.2 255.255.255.0
tunnel source Serial1/3
tunnel destination 23.23.23.2 //建立隧道
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 10.1.1.1 remote-as 200 //和R2建立IBGP邻居关系，注意用对方的tunnel口
neighbor 10.1.1.1 next-hop-self
neighbor 45.45.45.5 remote-as 300 //和R5建立EBGP邻居关系
R5：
interface Loopback0
ip address 5.5.5.5 255.255.255.255
interface Serial1/3
ip address 45.45.45.5 255.255.255.0 //基本接口配置
router bgp 300
no synchronization
bgp log-neighbor-changes
network 5.5.5.5 mask 255.255.255.255
neighbor 45.45.45.4 remote-as 200
测试：
R5#ping 1.1.1.1 source 5.5.5.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 5.5.5.5
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 116/164/260 ms
R5#