BGP确实有点难搞啊，不过没关系，慢慢折腾吧。折腾折腾着就折腾会了，哈哈。
首先看IBGP，拓扑图如下：
第一种情况用直连接口建立EBGP或者IBGP。
条件都一样：
1 底层可达，也就是保证能Ping通，因为BGP是建立在TCP的基础上的
2 命令（IBGP） ：R1(config-router)#neighbor 12.1.1.2 remote 100
R2(config-router)#neighbor 12.1.1.1 remote 100
2 命令（EBGP） ：R1(config-router)#neighbor 12.1.1.2 remote 200
R2(config-router)#neighbor 12.1.1.1 remote 100
IBGP：
R1#show ip bgp neighbors
BGP neighbor is 12.1.1.2, remote AS 100, internal link
BGP version 4, remote router ID 12.1.1.2
R2#show ip bgp neighbors
BGP neighbor is 12.1.1.1, remote AS 100, internal link
BGP version 4, remote router ID 12.1.1.1
BGP state = Established, up for 00:01:05
EBGP：
R1#show ip bgp neighbors
BGP neighbor is 12.1.1.2, remote AS 200, external link
BGP version 4, remote router ID 12.1.1.2
BGP state = Established, up for 00:00:03
R2#show ip bgp neighbors
BGP neighbor is 12.1.1.1, remote AS 100, external link
BGP version 4, remote router ID 12.1.1.1
BGP state = Established, up for 00:00:34
OK第一种情况的BGP邻居建立了。
第二种情况是用环回接口建立IBGP邻居。
R1：
C 1.0.0.0/8 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
S 2.2.2.2 [1/0] via 12.1.1.2
C 12.0.0.0/8 is directly connected, Serial0/0
R2：
1.0.0.0/32 is subnetted, 1 subnets
S 1.1.1.1 [1/0] via 12.1.1.1
C 2.0.0.0/8 is directly connected, Loopback0
C 12.0.0.0/8 is directly connected, Serial0/0
R1(config)#router bgp 100
R1(config-router)#neighbor 2.2.2.2 remote 100
R2(config)#router bgp 100
R2(config-router)#nei 1.1.1.1 remote 100
R2#show ip bgp neighbors
BGP neighbor is 1.1.1.1, remote AS 100, internal link
BGP version 4, remote router ID 0.0.0.0
BGP state = Active
发现邻居却不能建立。原因是：BGP的更新源和neighbor不一致。
拿R1来说，neighbor是2.2.2.2，更新源却是12.1.1.2.
所以加如下命令：
R1(config)#router bgp 100
R1(config-router)#neighbor 2.2.2.2 update-source loopback 0
R2(config-router)#neighbor 1.1.1.1 update-source loopback 0
再查看：
R2#show ip bgp neighbors
BGP neighbor is 1.1.1.1, remote AS 100, internal link
BGP version 4, remote router ID 1.1.1.1
R1#show ip bgp neighbors
BGP neighbor is 2.2.2.2, remote AS 100, internal link
BGP version 4, remote router ID 2.2.2.2
BGP state = Established, up for 00:00:48
邻居建立了。
所以为条件：
1 底层可达，也就是保证能Ping通，因为BGP是建立在TCP的基础上的
2 命令R1(config-router)#neighbor 12.1.1.2 remote 100
R2(config-router)#neighbor 12.1.1.1 remote 100
3命令R1(config-router)#neighbor 2.2.2.2 update-source loopback 0
R2(config-router)#neighbor 1.1.1.1 update-source loopback 0
第三种情况是用环回接口建立EBGP邻居。
R1(config)#router bgp 100
R1(config-router)#neighbor 2.2.2.2 remote 200
R1(config-router)#neighbor 2.2.2.2 update-source lo 0
R2(config)#router bgp 200
R2(config-router)#nei 1.1.1.1 remote 100
R2(config-router)#nei 1.1.1.1 update-source lo 0
按照IBGP的命令配置完后，查看邻居：
R2#show ip bgp neighbors
BGP neighbor is 1.1.1.1, remote AS 100, external link
BGP version 4, remote router ID 0.0.0.0
BGP state = Idle
发现却没有建立，因为要建立环回接口的EBPG邻居必须要设置条数
命令如下：R1(config-router)#neighbor 2.2.2.2 ebgp-multihop 2
R2(config-router)#neighbor 1.1.1.1 ebgp-multihop 2
R1#show ip bgp neighbors
BGP neighbor is 2.2.2.2, remote AS 200, external link
BGP version 4, remote router ID 2.2.2.2
BGP state = Established, up for 00:00:05
邻居起来了。
所以条件为：
1 底层可达，也就是保证能Ping通，因为BGP是建立在TCP的基础上的
2 命令
R1(config-router)#neighbor 12.1.1.2 remote 200
R2(config-router)#neighbor 12.1.1.1 remote 100
3命令
R1(config-router)#neighbor 2.2.2.2 update-source loopback 0
R2(config-router)#neighbor 1.1.1.1 update-source loopback 0
R1(config-router)#neighbor 2.2.2.2 ebgp-multihop 2
R2(config-router)#neighbor 1.1.1.1 ebgp-multihop 2
