一、实验目的：验证EBGP、IBGP间路由传递下一跳问题
二、试验拓扑
三、各路由器主要配置
AR1、2、3、4都运行OSPF协议，实现路由互通
AR1：
interface GigabitEthernet0/0/0 ip address 12.0.0.1 255.255.255.0
bgp 3 peer 12.0.0.2 as-number 4 # ipv4-family unicast undo synchronization peer 12.0.0.2 enable # ospf 1 area 0.0.0.0 network 12.0.0.0 0.0.0.255
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AR2：
interface GigabitEthernet0/0/0 ip address 12.0.0.2 255.255.255.0 # interface GigabitEthernet0/0/1 ip address 23.0.0.2 255.255.255.0 # interface GigabitEthernet0/0/2 ip address 24.0.0.2 255.255.255.0
bgp 4 peer 12.0.0.1 as-number 3 peer 23.0.0.3 as-number 12 # ipv4-family unicast undo synchronization peer 12.0.0.1 enable peer 23.0.0.3 enable # ospf 1 area 0.0.0.0 network 12.0.0.0 0.0.0.255 network 23.0.0.0 0.0.0.255 network 24.0.0.0 0.0.0.255
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AR3：
bgp 12 peer 23.0.0.2 as-number 4 peer 24.0.0.4 as-number 12 # ipv4-family unicast undo synchronization peer 23.0.0.2 enable peer 24.0.0.4 enable # ospf 1 area 0.0.0.0 network 23.0.0.0 0.0.0.255 #
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AR4:
interface LoopBack1 ip address 4.4.4.4 255.255.255.255 # bgp 12 peer 23.0.0.3 as-number 12 # ipv4-family unicast undo synchronization import-route direct peer 23.0.0.3 enable # ospf 1 area 0.0.0.0 network 24.0.0.0 0.0.0.255
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四、结果与分析
在AR4上引入路由4.4.4.4到BGP，由于AR4与AR3 IBGP互联，通过IBGP把路由传到AR3，下一跳为：24.0.0.4
Destination/Mask Proto Pre Cost Flags NextHop Interface 4.4.4.4/32 IBGP 255 0 RD 24.0.0.4 GigabitEthernet0/0/1
4.4.4.4在AR3上通过EBGP传到AR2，EBGP间传递改变路由下一跳属性，AR2上故下一跳为：23.0.0.3
Destination/Mask Proto Pre Cost Flags NextHop Interface 4.4.4.4/32 EBGP 255 0 D 23.0.0.3 GigabitEthernet0/0/1
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可见，出现环路
<AR2>tracert 4.4.4.4 traceroute to 4.4.4.4(4.4.4.4), max hops: 30 ,packet length: 40,press CTRL_C t o break 1 23.0.0.3 50 ms 30 ms 30 ms 2 23.0.0.2 20 ms 20 ms 10 ms 3 23.0.0.3 30 ms 40 ms 40 ms 4 23.0.0.2 10 ms 30 ms 20 ms 5 23.0.0.3 40 ms 40 ms 40 ms 6 23.0.0.2 40 ms 30 ms 30 ms 7 23.0.0.3 30 ms 50 ms 30 ms 8 23.0.0.2 30 ms 30 ms 30 ms 9 23.0.0.3 40 ms 40 ms 50 ms 10 23.0.0.2 50 ms 40 ms 50 ms 11 23.0.0.3 70 ms 60 ms 50 ms 12 23.0.0.2 70 ms 70 ms 50 ms 13 23.0.0.3 90 ms 70 ms 80 ms 14 23.0.0.2 60 ms 60 ms 60 ms 15 23.0.0.3 80 ms 90 ms 90 ms 16 23.0.0.2 60 ms 80 ms 80 ms 17 23.0.0.3 100 ms 100 ms 90 ms 18 23.0.0.2 70 ms 110 ms 90 ms 19 23.0.0.3 110 ms 120 ms 110 ms 20 23.0.0.2 110 ms 100 ms 100 ms 21 23.0.0.3 120 ms 100 ms 120 ms 22 23.0.0.2 120 ms 110 ms 110 ms 23 23.0.0.3 130 ms 130 ms 120 ms 24 23.0.0.2 140 ms 120 ms 130 ms 25 23.0.0.3 140 ms 150 ms 130 ms 26 23.0.0.2 130 ms 130 ms 150 ms 27 23.0.0.3 150 ms 140 ms 140 ms 28 23.0.0.2 120 ms 120 ms 130 ms 29 23.0.0.3 140 ms 130 ms 140 ms 30 23.0.0.2 120 ms 130 ms 140 ms 可通过在AR3上配置 peer 23.0.0.3 next-hop-invariable 不改变下一跳属性 使到4.4.4.4 的路由下一跳为24.0.0.4.但模拟器上配置该命令未起作用，可能是模拟器BUG
