Bgp
的本地优先属性
Router#config t
Router(config)#host R2
R2(config)#int s1/1
R2(config-if)#ip addr 202.110.101.1 255.255.255.0
R2(config-if)#encap hdlc
R2(config-if)#clock rate 56000
R2(config-if)#no shut
R2(config)#int s1/2
R2(config-if)#ip addr 202.110.104.2 255.255.255.0
R2(config-if)#encap hdlc
R2(config-if)#no shut
R2(config-if)#exi
R2(config)#router bgp 64500
R2(config-router)#neighbor 202.110.101.2 remote-as 64500
R2(config-router)#neighbor 202.110.104.1 remote-as 64500
R2(config-router)#net 202.110.101.0
R2(config-router)#net 202.110.104.0
Router(config)#host R3
R3(config)#int s1/1
R3(config-if)#ip addr 202.110.101.2 255.255.255.0
R3(config-if)#encap hdlc
R3(config-if)#no shut
R3(config-if)#int s1/2
R3(config-if)#ip addr 202.110.102.1 255.255.255.0
R3(config-if)#encap hdlc
R3(config-if)#clock rate 56000
R3(config-if)#no shut
R3(config-if)#exit
R3(config)#
R3(config)#router bgp 64500
R3(config-router)#neighbor 202.110.101.1 remote-as 64500
R3(config-router)#neighbor 202.110.102.2 remote-as 64501
R3(config-router)#net 202.110.101.0
R3(config-router)#net 202.110.102.0
config)#
R4(config)#int lo0
R4(config-if)#ip addr 192.168.10.1 255.255.255.0
R4(config-if)#exit
R4(config)#int s1/2
R4(config-if)#ip addr 202.110.102.2 255.255.255.0
R4(config-if)#encap hdlc
R4(config-if)#no shut
R4(config-if)#exit
R4(config)#int s1/1
R4(config-if)#ip addr 202.110.103.1 255.255.255.0
R4(config-if)#encap hdlc
R4(config-if)#clock rate 56000
R4(config-if)#no shut
R4(config-if)#exit
R4(config)#router bgp 64501
R4(config-router)#neighbor 202.110.102.1 remote-as 64500
R4(config-router)#neighbor 202.110.103.2 remote-as 64500
R4(config-router)#net 192.168.10.0
R4(config-router)#net 202.110.102.0
R4(config-router)#net 202.110.103.0
Router(config)#host R5
R5(config)#int s1/1
R5(config-if)#ip addr 202.110.103.2 255.255.255.0
R5(config-if)#encap hdlc
R5(config-if)#no shut
R5(config-if)#exit
R5(config)#int
R5(config)#int s1/2
R5(config-if)#ip addr 202.110.104.1 255.255.255.0
R5(config-if)#encap hdlc
R5(config-if)#clock rate 56000
R5(config-if)#no shut
R5(config-if)#exit
R5(config)#router bgp 64500
R5(config-router)#neighbor 202.110.103.1 remote-as 64501
R5(config-router)#neighbor 202.110.104.2 remote
R5(config-router)#neighbor 202.110.104.2 remote-as 64500
R5(config-router)#net 202.110.103.0
R5(config-router)#net 202.110.104.0 ^
R5(config-router)#net 202.110.104.0
R5(config-router)#exit
R2
上测试
R2#show ip ro
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 202.110.101.0/24 is directly connected, Serial1/1
B
192.168.10.0/24 [200/0] via 202.110.102.2, 00:00:20
B 202.110.102.0/24 [200/0] via 202.110.101.2, 00:18:46
B 202.110.103.0/24 [200/0] via 202.110.104.1, 00:13:38
C 202.110.104.0/24 is directly connected, Serial1/2
由
R2
上的路由，可以看出默认情况下学习
R4
上的路由走
R3,
下面我们将人为通过修改路由器的本地优先属性，使
R2
上学习到
R4
的路由走
R5
，如果成功，那么今天的目的就基本达到了。本地优先属性的修改语法是：
（
config-router
）
#bgp default local-preference
值
//
该值默认范围是
0-----4294967295
，默认是
100
，且属性值越高越好。
我们让
R3
为默认的
100
，人为将
R5
的属性值改为
120
，看试验效果。
R5(config)#router bgp 64500
R5(config-router)#bgp default local-preference 120
R5(config-router)#net 202.110.103.0
R5(config-router)#net 202.110.104.0
在
R2
上测试，
R2#clear ip ro *
R2#show ip ro
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 202.110.101.0/24 is directly connected, Serial1/1
B 192.168.10.0/24 [200/0] via 202.110.102.2, 00:00:01
当重新学习路由表后，该路由没有变，不是我们想要的试验结果。
B 202.110.102.0/24 [200/0] via 202.110.101.2, 00:00:01
B 202.110.103.0/24 [200/0] via 202.110.104.1, 00:00:01
C 202.110.104.0/24 is directly connected, Serial1/2
R2#clear ip bgp *
*Apr 30 20:22:31.027: %BGP-5-ADJCHANGE: neighbor 202.110.101.2 Down User reset
*Apr 30 20:22:31.031: %BGP-5-ADJCHANGE: neighbor 202.110.104.1 Down User reset
提示重新启动。
R2#show ip bgp
BGP table version is 8, local router ID is 202.110.104.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i192.168.10.0 202.110.102.2 0 100 0 64501 i
*>i 202.110.103.1 0 120 0 64501 I
>
表示最好路由，优先属性变成了
120
，
r>i202.110.101.0 202.110.101.2 0 100 0 i
* i202.110.102.0 202.110.101.2 0 100 0 i
*>i 202.110.103.1 0 120 0 64501 i
* i202.110.103.0 202.110.102.2 0 100 0 64501 i
*>i 202.110.104.1 0 120 0 i
r>i202.110.104.0 202.110.104.1 0 120 0 i
有这张
bgp
表可以看出明显存在下一跳问题，也可以按照以前的修改下一跳，克服保留下一跳的问题。
分别在
R3
和
R5
上做，因为理论上
R2
的下一跳应该在这两台路由器上。
R3(config)#router bgp 64500
R3(config-router)#neighbor 202.110.101.1 next-hop-self
R5(config)#router bgp 64500
R5(config-router)#neighbor 202.110.104.2 next-hop-self
R2#show ip bgp
BGP table version is 12, local router ID is 202.110.104.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* i192.168.10.0 202.110.101.2 0 100 0 64501 i
*>i 202.110.104.1 0 120 0 64501 i
*> 202.110.101.0 0.0.0.0 0 32768 i
* i 202.110.101.2 0 100 0 i
* i202.110.102.0 202.110.101.2 0 100 0 i
*>i 202.110.104.1 0 120 0 64501 i
* i202.110.103.0 202.110.101.2 0 100 0 64501 i
*>i 202.110.104.1 0 120 0 i
*> 202.110.104.0 0.0.0.0 0 32768 i
* i 202.110.104.1 0 120 0 i
R2#
C 202.110.101.0/24 is directly connected, Serial1/1
B 192.168.10.0/24 [200/0] via 202.110.103.1, 00:01:02
经过的路径也改了过来。
B 202.110.102.0/24 [200/0] via 202.110.103.1, 00:01:02
B 202.110.103.0/24 [200/0] via 202.110.104.1, 00:01:02
C 202.110.104.0/24 is directly connected, Serial1/2
本文转自shenleigang 51CTO博客，原文链接：
http://blog.51cto.com/shenleigang/154602
，如需转载请自行联系原作者
