边界网关协议BGP实现自治系统AS的路由可达
EBGP：运行于不同 AS 之间的 BGP 称为 EBGP。为了防止 AS 间产生环路，当 BGP 设备接收 EBGP对等体发送的路由时，会将带有本地 AS 号的路由丢弃。
IBGP：运行于同一 AS 内部的 BGP 称为 IBGP。为了防止 AS 内产生环路， BGP 设备不将从 IBGP 对等体学到的路由通告给其他 IBGP 对等体缺省需要与所有 IBGP对等体建立全连接才能实现 AS 内部各 IBGP 设备间的路由互通。
使用BGP在AS内转发路由条目时，要保证本AS内的IGP路由表已经有这条路由
网络拓扑图：
按照拓扑图配置路由器接口IP地址：
以R2为例子配置：
[R2]int g0/0/0
[R2-GigabitEthernet0/0/0]ip add 59.74.112.1 24
[R2-GigabitEthernet0/0/0]int g0/0/1
[R2-GigabitEthernet0/0/1]ip add 172.16.30.1 24
[R2-GigabitEthernet0/0/1]int g0/0/2
[R2-GigabitEthernet0/0/2]ip add 172.16.10.1 24
[R2-GigabitEthernet0/0/2]quit
[R2]
R1、R3、R4同理
在 Router2、Router3 和 RouterD4间配置 IBGP 连接。
[R2]bgp 65009
[R2-bgp]router-id 2.2.2.2
[R2-bgp]peer 172.16.30.2 as-number 65009
[R2-bgp]peer 172.16.10.2 as-number 65009
[R3]bgp 65009
[R3-bgp]router-id 3.3.3.3
[R3-bgp]peer 172.16.30.1 as-number 65009
[R3-bgp]peer 172.16.20.2 as-number 65009
[R3-bgp]quit
[R4]bgp 65009
[R4-bgp]router-id 4.4.4.4
[R4-bgp]peer 172.16.10.1 as-number 65009
[R4-bgp]peer 172.16.20.1 as-number 65009
[R4-bgp]quit
[R4]
在 Router1 和 Router2 之间配置 EBGP 连接。
配置R1：
[R1]bgp 65008
[R1-bgp]router-id 1.1.1.1
[R1-bgp]peer 59.74.112.1 as-number 65009
配置R2：
[R2-bgp]peer 59.74.112.2 as-number 65008
显示R2的BGP配置结果：
[R2-bgp]dis bgp peerBGP local router ID : 2.2.2.2Local AS number : 65009Total number of peers : 3 Peers in established state : 3Peer V AS MsgRcvd MsgSent OutQ Up/Down State Pre
fRcv59.74.112.2 4 65008 3 3 0 00:01:06 Established 0172.16.10.2 4 65009 5 6 0 00:03:31 Established 0172.16.30.2 4 65009 6 7 0 00:04:35 Established 0
[R2-bgp]
测试BGP协议：
配置R1发布路由172.16.60.0/24并查看路由表信息
[R1-bgp]ipv4-family unicast
[R1-bgp-af-ipv4]net 172.16.60.0 24
[R1-bgp-af-ipv4]quit
[R1-bgp]dis bgp routing-table BGP Local router ID is 1.1.1.1 Status codes: * - valid, > - best, d - damped,h - history, i - internal, s - suppressed, S - StaleOrigin : i - IGP, e - EGP, ? - incompleteTotal Number of Routes: 1Network NextHop MED LocPrf PrefVal Path/Ogn*> 172.16.60.0/24 0.0.0.0 0 0 i
[R1-bgp]并查看R2的路由表有没有更新：
[R2-bgp]dis bgp routing-table BGP Local router ID is 2.2.2.2 Status codes: * - valid, > - best, d - damped,h - history, i - internal, s - suppressed, S - StaleOrigin : i - IGP, e - EGP, ? - incompleteTotal Number of Routes: 1Network NextHop MED LocPrf PrefVal Path/Ogn*> 172.16.60.0/24 59.74.112.2 0 0 65008i
[R2-bgp]
而且发现AS65009内的所有路由器都已经收到这一条路由更新：
[R3]bgp 65009
[R3-bgp]dis bgp routing-table BGP Local router ID is 3.3.3.3 Status codes: * - valid, > - best, d - damped,h - history, i - internal, s - suppressed, S - StaleOrigin : i - IGP, e - EGP, ? - incompleteTotal Number of Routes: 1Network NextHop MED LocPrf PrefVal Path/Ogni 172.16.60.0/24 59.74.112.2 0 100 0 65008i
[R3-bgp]
[R4]bgp 65009
[R4-bgp]dis bgp routing-table BGP Local router ID is 4.4.4.4 Status codes: * - valid, > - best, d - damped,h - history, i - internal, s - suppressed, S - StaleOrigin : i - IGP, e - EGP, ? - incompleteTotal Number of Routes: 1Network NextHop MED LocPrf PrefVal Path/Ogni 172.16.60.0/24 59.74.112.2 0 100 0 65008i
[R4-bgp]
R3和R4 虽然学到了 AS65008 中的 172.16.60.0/24 的路由，但因为下一跳 59.74.112.2 不可达，所以不是有效路由。
所以配置BGP引入直连路由
配置AS边界路由器R2
[R2-bgp]ipv4-family unicast
[R2-bgp-af-ipv4]import-route direct
此时查看R1的路由表发现更新了到达AS 65009各个路由器的路径：
[R1-bgp]dis bgp routing-table BGP Local router ID is 1.1.1.1 Status codes: * - valid, > - best, d - damped,h - history, i - internal, s - suppressed, S - StaleOrigin : i - IGP, e - EGP, ? - incompleteTotal Number of Routes: 1Network NextHop MED LocPrf PrefVal Path/Ogn*> 172.16.60.0/24 0.0.0.0 0 0 i
[R1-bgp]dis bgp routing-tableBGP Local router ID is 1.1.1.1 Status codes: * - valid, > - best, d - damped,h - history, i - internal, s - suppressed, S - StaleOrigin : i - IGP, e - EGP, ? - incompleteTotal Number of Routes: 4Network NextHop MED LocPrf PrefVal Path/Ogn59.74.112.0/24 59.74.112.1 0 0 65009?*> 172.16.10.0/24 59.74.112.1 0 0 65009?*> 172.16.30.0/24 59.74.112.1 0 0 65009?*> 172.16.60.0/24 0.0.0.0 0 0 i
[R1-bgp]
查看R3、R4的BGP路由表也是一样的结果
