利用Loopback建立BGP邻居
如何确保路由器的源地址是稳定、一致不变？一般情况下我们使用Loopback地址。
在路由器上配置了Loopback地址后，当路由器之间的链路Down掉了，那他们的邻居关系依然保持，因为源地址没变。
一般要求路由器与邻居之间至少有一条路径是可用的。这样邻居关系就不会出现问题。
实验目的：通过使用Loopback口建立邻居关系，实现172.16.1.0&192.168.1.0网段互相访问
========AS 123中先运行一个IGP协议（这里以EIGRP为例）========
R2(config)#router eigrp 90
R2(config-router)#no au
R2(config-router)#net 12.1.1 .2 0.0.0.0
R2(config-router)#net 2.2.2 .2 0.0.0.0
R1(config)#router ei 90
R1(config-router)#no au
R1(config-router)#net 12.1.1 .1 0.0.0.0
R1(config-router)#net 13.1.1 .1 0.0.0.0
R1(config-router)#net 1.1.1 .1 0.0.0.0
R3(config)#router eigrp 90
R3(config-router)#no au
R3(config-router)#net 13.1.1 .3 0.0.0.0
R3(config-router)#net 3.3.3 .3 0.0.0.0
=====R1、R2利用loopback建立IBGP邻居关系=============
R2(config)#router bgp 123
R2(config-router)#bgp router-id 2.2.2 .2
R2(config-router)#neighbor 1.1.1 .1 remote-as 123
R2(config-router)#neighbor 1.1.1 .1 update-source loopback 0
//在BGP建邻居过程中，neighbor命令所指定的邻居的IP地址，也必须是邻居用来和本路由器建立TCP连接的地址。如果不匹配，两端建立不起邻居关系。可以在路由器上用这个命令指定使用哪个接口来和邻居建立联系，这个接口就是自已的更新源
R1(config)#router bgp 123
R1(config-router)#bgp router-id 1.1.1 .1
R1(config-router)#nei 2.2.2 .2 remote-as 123
R1(config-router)#neighbor 2.2.2 .2 update-source loopback 0
=====R1、R3利用loopback建立IBGP邻居关系=============
R3(config)#router bgp 123
R3(config-router)#bgp router-id 3.3.3 .3
R3(config-router)#neighbor 1.1.1 .1 remote-as 123
R3(config-router)#neighbor 1.1.1 .1 update-source loopback 0
R1(config)#router bgp 123
R1(config-router)#neighbor 3.3.3 .3 remote-as 123
R1(config-router)#neighbor 3.3.3 .3 update-source loopback 0
=====R2、R3利用loopback建立IBGP邻居关系=============
R2(config)#router bgp 123
R2(config-router)#neighbor 3.3.3 .3 remote-as 123
R2(config-router)#neighbor 3.3.3 .3 update-source loopback 0
R3(config)#router bgp 123
R3(config-router)#neighbor 2.2.2 .2 remote-as 123
R3(config-router)#neighbor 2.2.2 .2 update-source loopback 0
=======R2、R4建立EBGP邻居关系=========
R4(config)#router bgp 4
R4(config-router)#bgp router-id 4.4.4 .4
R4(config-router)#neighbor 2.2.2 .2 remote-as 123
R4(config-router)#neighbor 2.2.2 .2 update-source loopback 0
R4(config-router)#neighbor 2.2.2 .2 ebgp-multihop 2
//在EBGP中使用Loopback端口，需要使用命令Neighbor ip ebgp-multihop x，X为跳数。这个命令只能使用在EBGP情况下。BGP中Next-hop（下一跳）不是指下一个路由器，而是指到达目标网络的下一个AS。
R4(config)#ip route 2.2.2 .0 255.255.255.0 24.1.1.2
//要有正确的去往对方loopback口的路由才能正常的建立邻居关系
R2(config-router)#neighbor 4.4.4 .4 remote-as 4
R2(config-router)#neighbor 4.4.4 .4 update-source loopback 0
R2(config-router)#neighbor 4.4.4 .4 ebgp-multihop 2
R2(config)#ip route 4.4.4 .0 255.255.255.0 24.1.1.4
=======R3、R5建立EBGP邻居关系=========
R5(config)#router bgp 5
R5(config-router)#bgp router-id 5.5.5 .5
R5(config-router)#neighbor 3.3.3 .3 remote-as 123
R5(config-router)#neighbor 3.3.3 .3 update-source loopback 0
R5(config-router)#neighbor 3.3.3 .3 ebgp-multihop 2
R5(config)#ip route 3.3.3 .0 255.255.255.0 35.1.1.3
R3(config-router)#neighbor 5.5.5 .5 remote-as 5
R3(config-router)#neighbor 5.5.5 .5 update-source loopback 0
R3(config-router)#neighbor 5.5.5 .5 ebgp-multihop 2
R3(config)#ip route 5.5.5 .0 255.255.255.0 35.1.1.5
===========检查各路由器的邻居关系=============
R4#sh ip bgp sum
BGP router identifier 4.4.4 .4, local AS number 4
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
R2(config)#do sh ip bgp sum
BGP router identifier 2.2.2 .2, local AS number 123
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
R1#sh ip bgp sum
BGP router identifier 1.1.1 .1, local AS number 123
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
R3#sh ip bgp sum
BGP router identifier 3.3.3 .3, local AS number 123
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
R5#sh ip bgp sum
BGP router identifier 5.5.5 .5, local AS number 5
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
======在R4，R5上各起一个网段，实现互访==========
R4(config)#inter lo 1
R4(config-if)#ip add 172.16.1.1 255.255.255.0
R5(config)#inter lo 1
R5(config-if)#ip add 192.168.1.1 255.255.255.0
R4(config-if)#router bgp 4
R4(config-router)#network 172.16.1.0 mask 255.255.255.0
R5(config-if)#router bgp 5
R5(config-router)#network 192.168.1.0 mask 255.255.255.0
R2(config)#router bgp 123
R2(config-router)#neighbor 1.1.1 .1 next-hop-self
R2(config-router)#neighbor 3.3.3 .3 next-hop-self
R3(config)#router bgp 123
R3(config-router)#neighbor 1.1.1 .1 next-hop-self
R3(config-router)#neighbor 2.2.2 .2 next-hop-self
=========测试172.16.1.0&192.168.1.0网络互访=====
R4#p 192.168.1.1 sou 172.16.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 528/948/1204 ms
R5#ping 172.16.1.1 source 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 776/1082/1200 ms
