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
--------------------------------------------------------------------------------------
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
---------------------------------------------------------------------------------
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
#
--------------------------------------------------------------------------------
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
----------------------------------------------------------------------------
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