一、实验拓扑图：


二、实验目的：
       学习使用BGP Local-AS属性，我们对这个属性进行简要的介绍。如拓扑图所示，在figure1中，ISP-A和ISP-B没有通信需求，网络运营没有状况，现在由于业务需求A和B之间需要合并，也就是说B需要并入ISP-A的自治系统内，而B有庞大的客户群体（这里禁用ISP-C代替），所以不希望B的客户更改配置。BGP Local-AS属性就是允许B在并入AS100的同时，对它的客户依然保持AS200的信息。
三、具体实验内容：
       步骤：

－实验基本配置（包括Figure1 d的BGP的配置）；

－配置figure2，在ISP-B上配置BGP Local-AS属性；

－结论

实验基本配置（包括BGP的配置）：
ISP-A#sh run
Building configuration...

Current configuration : 1244 bytes
!
version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ISP-A
interface Serial1/0

ip address 10.0.0.1 255.255.255.0

serial restart-delay 0
!
router bgp 100

no synchronization

bgp log-neighbor-changes

neighbor 10.0.0.2 remote-as 300

no auto-summary
!
end
ISP-C#sh run
Building configuration...

Current configuration : 1376 bytes
!
version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ISP-C
!
interface Loopback0

ip address 192.168.9.1 255.255.255.0
!

interface Serial1/0

ip address 192.168.1.2 255.255.255.252

serial restart-delay 0
!
interface Serial1/1

ip address 10.0.0.2 255.255.255.0

serial restart-delay 0
!
router bgp 300

no synchronization

bgp log-neighbor-changes

network 192.168.9.0

neighbor 10.0.0.1 remote-as 100

neighbor 192.168.1.1 remote-as 200

no auto-summary
!

end


ISP-B#sh run
Building configuration...

Current configuration : 1333 bytes
!
version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ISP-B
interface Loopback0

ip address 192.168.4.1 255.255.255.0
!
interface Serial1/1

ip address 192.168.1.1 255.255.255.252

serial restart-delay 0
router bgp 200

no synchronization

bgp log-neighbor-changes

network 192.168.4.0

neighbor 192.168.1.2 remote-as 300

no auto-summary
!

end



配置figure2，在ISP-B上配置BGP Local-AS属性:
先将A-B的链路做通：

ISP-B(config)#int s 1/0 ISP-B(config-if)#ip add 20.0.0.1 255.255.255.0 ISP-B(config-if)#no shu ISP-A(config)#int s 1/1 ISP-A(config-if)#ip add 20.0.0.2 255.255.255.0 ISP-A(config-if)#no shu

然后就是重点：

ISP-B(config)#no router bgp 200 ISP-B(config)#router bgp 100 ISP-B(config-router)#nei 20.0.0.2 remote 100 ISP-B(config-router)#nei 192.168.1.2 remote 300 ISP-B(config-router)#nei 192.168.1.2 local-as 200 ISP-B(config-router)#net 192.168.4.0 mask 255.255.255.0

同时在A上更改：

ISP-A(config)#router bgp 100 ISP-A(config-router)#nei 20.0.0.1 remote 100

现在在C上查看BGP信息

ISP-B#sh ip bgp sum BGP router identifier 192.168.4.1, local AS number 100 BGP table version is 6, main routing table version 6 3 network entries using 303 bytes of memory 4 path entries using 192 bytes of memory 4 BGP path attribute entries using 240 bytes of memory 2 BGP AS-PATH entries using 48 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 783 total bytes of memory BGP activity 4/1 prefixes, 5/1 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 20.0.0.2 4 100 19 19 6 0 0 00:13:19 2 192.168.1.2 4 300 30 29 6 0 0 00:16:20 1

再到B上看：

ISP-C#sh ip bgp sum BGP router identifier 192.168.9.1, local AS number 300 BGP table version is 5, main routing table version 5 3 network entries using 303 bytes of memory 5 path entries using 240 bytes of memory 5 BGP path attribute entries using 300 bytes of memory 2 BGP AS-PATH entries using 48 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 891 total bytes of memory BGP activity 6/3 prefixes, 8/3 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.0.0.1 4 100 50 55 5 0 0 00:15:18 2 192.168.1.1 4 200 54 55 5 0 0 00:15:18 2

两端的显示是不一样的。
结论和问题：
1．
这样可以免去在客户端配置的麻烦，对于客户端是透明的。
问题：

ISP-B#sh ip bgp BGP table version is 6, local router ID is 192.168.4.1 Network Next Hop Metric LocPrf Weight Path *>i1.1.1.0/24 20.0.0.2 0 100 0 i *> 192.168.4.0 0.0.0.0 0 32768 i * i192.168.9.0 10.0.0.2 0 100 0 300 i *> 192.168.1.2 0 0 200 300 i

这两条链路，BGP依据什么原则选择了下面那条AS路径较长的？

老兄,这么多洋文,看不懂,找内行的解析