背景：
世界和平时，某大国和其余国家和平相处，网络畅通无阻
[R1]dis bgp routing-table
Total Number of Routes: 9
BGP Local router ID is 1.1.1.1
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.1.1/32 0.0.0.0 0 0 i
*> 10.2.2.2/32 0.0.0.0 0 0 i
*> 10.3.3.3/32 0.0.0.0 0 0 i
*> 20.1.1.1/32 12.12.12.2 0 0 20i
*> 20.2.2.2/32 12.12.12.2 0 0 20i
*> 20.3.3.3/32 12.12.12.2 0 0 20i
*> 40.1.1.1/32 12.12.12.2 0 20 40i
*> 40.2.2.2/32 12.12.12.2 0 20 40i
*> 40.3.3.3/32 12.12.12.2 0 20 40i
后来：
大国与鬼子之间产生矛盾，鬼子的网络上充满了对大国的反动言论！所以大国政府希望“和谐”了鬼子网络言论，使大国人民无法访问鬼子们的信息
[R2]dis cu | be bgp
bgp 20
network 20.1.1.1 255.255.255.255
network 20.2.2.2 255.255.255.255
network 20.3.3.3 255.255.255.255
undo synchronization
peer 12.12.12.1 as-number 10
peer 24.24.24.4 as-number 40
peer 24.24.24.4 route-policy as40 import
#
route-policy as40 permit node 10
if-match as-path 1
#
ip as-path 1 deny 40$（拒绝as40始发路由）
ip as-path 1 permit .*(允许其他as的路由)
#
Return
[R2]dis bgp routing-table
Total Number of Routes: 6
BGP Local router ID is 2.2.2.2
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.1.1/32 12.12.12.1 0 0 10i
*> 10.2.2.2/32 12.12.12.1 0 0 10i
*> 10.3.3.3/32 12.12.12.1 0 0 10i
*> 20.1.1.1/32 0.0.0.0 0 0 i
*> 20.2.2.2/32 0.0.0.0 0 0 i
*> 20.3.3.3/32 0.0.0.0 0 0 i
从大国的bgp路由表可见，鬼子的网络信息以及被“和谐”了
后来大国希望与鬼子和平商讨，以促进世界和平发展，要求可以接受鬼子们的网络信息，但是必须进过as30的检查
[R2]dis cu | be bgp
bgp 20
network 20.1.1.1 255.255.255.255
network 20.2.2.2 255.255.255.255
network 20.3.3.3 255.255.255.255
undo synchronization
peer 12.12.12.1 as-number 10
peer 24.24.24.4 as-number 40
peer 23.23.23.3 as-number 30
peer 24.24.24.4 route-policy as40 import
peer 23.23.23.3 route-policy as40 import
#
route-policy as40 permit node 10
if-match as-path 1
#
ip as-path 1 deny 40$
ip as-path 1 permit .*
ip as-path 1 permit _40 30$
ip as-path 1 permit _40 .+30$
#
Return
[R2]dis bgp routing-table
Total Number of Routes: 18
BGP Local router ID is 2.2.2.2
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.1.1/32 12.12.12.1 0 0 10i
*> 10.2.2.2/32 12.12.12.1 0 0 10i
*> 10.3.3.3/32 12.12.12.1 0 0 10i
*> 20.1.1.1/32 0.0.0.0 0 0 i
*> 20.2.2.2/32 0.0.0.0 0 0 i
*> 20.3.3.3/32 0.0.0.0 0 0 i
*> 30.1.1.1/32 23.23.23.3 0 0 30i
* 24.24.24.4 0 40 30i
*> 30.2.2.2/32 23.23.23.3 0 0 30i
* 24.24.24.4 0 40 30i
*> 30.3.3.3/32 23.23.23.3 0 0 30i
* 24.24.24.4 0 40 30i
*> 40.1.1.1/32 23.23.23.3 0 30 40i
*> 40.2.2.2/32 23.23.23.3 0 30 40i
*> 40.3.3.3/32 23.23.23.3 0 30 40i
可见来自as40的路由，都进过as30
As30网络设备性能有差异，能以接受大量路由表，所以仅仅接受大国和鬼子的路由！
[R3]dis cu | be bgp
bgp 30
network 30.1.1.1 255.255.255.255
network 30.2.2.2 255.255.255.255
network 30.3.3.3 255.255.255.255
undo synchronization
peer 34.34.34.4 as-number 40
peer 23.23.23.2 as-number 20
peer 34.34.34.4 route-policy 1 import
peer 23.23.23.2 route-policy 1 import
#
route-policy 1 permit node 10
if-match as-path 1
#
ip as-path 1 permit 20$（允许as20）
ip as-path 1 permit 40$（允许as40）
ip as-path 1 deny .$（拒绝所有）
#
load xml-configuration
#
user-interface con 0
user-interface vty 0 4
#
return
[R3]dis bgp routing-table
Total Number of Routes: 12
BGP Local router ID is 30.3.3.3
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 20.1.1.1/32 23.23.23.2 0 0 20i
* 34.34.34.4 0 40 20i
*> 20.2.2.2/32 23.23.23.2 0 0 20i
* 34.34.34.4 0 40 20i
*> 20.3.3.3/32 23.23.23.2 0 0 20i
* 34.34.34.4 0 40 20i
*> 30.1.1.1/32 0.0.0.0 0 0 i
*> 30.2.2.2/32 0.0.0.0 0 0 i
*> 30.3.3.3/32 0.0.0.0 0 0 i
*> 40.1.1.1/32 34.34.34.4 0 0 40i
*> 40.2.2.2/32 34.34.34.4 0 0 40i
*> 40.3.3.3/32 34.34.34.4 0 0 40i
