（CCNP)BGP改变metric和本地优先级影响路径选择

r1:

inter f0/0

ip add 192.168.1.1 255.255.255.0

no sh

inter e1/0

ip add 192.168.6.2 255.255.255.0

no sh

router bgp 1 //启用bgp路由，as为1

nei 192.168.1.2 remoet-as 1 //定义ibgp邻居

nei 192.168.6.2 remote-as 1 //定义ibgp邻居

net 192.168.1.0 //通告路由到bgp

net 192.168.6.0 //通告路由到bgp

r2:

inter e1/0

ip add 192.168.1.2 255.255.255.0

no sh

inter f0/0

ip add 192.168.2.1 255.255.255.0

no sh

router bgp 1 //启用bgp路由，as为1

nei 192.168.1.1 remote-as 1 //定义ibgp邻居

nei 192.168.2.2 remote-as 2 //定义ebgp邻居

nei 192.168.2.2 route-map set-med out //引用名为set-med的route-map，作用为当通告给邻居路由的时候改变metric

nei 192.168.2.2 route-map set-lo in //引用名为set-lo的route-map，作用为当邻居通告路由进入as1时改变本地优先级

net 192.168.1.0 //通告路由到bgp

net 192.168.2.0 //通告路由到bgp

access-list 1 permit 192.168.1.0 //这是改变metric引用的acl

access-list 2 permit 192.168.2.0 //这是改变metric引用的acl

access-list 3 permit 172.16.1.0 //这是改变本地优先级引用的acl

route-map set-med permit 10 //改变metric的route-map,序列号为10

match ip add 1 //同acl1匹配

set metric 100 //如果匹配就将metric改为100

ex

route-map set-med permit 20 //改变metric的route-map,序列号为20

match ip add 2 //同acl2匹配

set metric 200 //如果匹配就将metric改为200

ex

route-map set-lo permit 10 //改变本地优先级的route-map,序列号为10

match ip add 3 //引用acl3

set local-preference 100 //如果匹配就将本地优先级改为100

route-map set-lo permit 20 //没有match的route-map同所有的条目匹配

ex

end

clear ip bgp 192.168.2.2 soft //软重置邻居192.168.2.2，注意这条命令是在特权模式下使用的

r3:

inter e1/0

ip add 192.168.6.1 255.255.255.0 //r3的配置同r2差不多，不再多说

no sh

inter f0/0

ip add 192.168.5.2 255.255.255.0

ex

router bgp 1

nei 192.168.6.2 remote-as 1

nei 192.168.5.1 remote-as 2

net 192.168.6.0

net 192.168.5.0

nei 192.168.5.1 route-map set-med out

nei 192.168.5.1 route-map set-lo in

ex

access-list 1 permit 192.168.1.0

access-list 2 permit 192.168.6.0

access-list 3 permit 172.16.1.0

route-map set-med permit 10

match ip add 1

set metric 200

ex

route-map set-med permit 20

match ip add 2

set metric 100

route-map set-lo permit 10

match ip add 3

set local-preference 200

ex

route-map set-lo permit 20

ex

end

clear ip bgp 192.168.5.1 soft

r4:

inter e1/0

ip add 192.168.2.2 255.255.255.0

no sh

inter f0/0

ip add 192.168.3.1 255.255.255.0

no sh

router bgp 2

nei 192.168.2.1 remote-as 1

nei 192.168.3.2 remote-as 2

net 192.168.2.0

net 192.168.3.0

r5:

inter e1/0

ip add 192.168.5.1 255.255.255.0

no sh

inter f0/0

ip add 192.168.4.2 255.255.255.0

no sh

router bgp 2

nei 192.168.5.2 remote-as 1

nei 192.168.4.1 remote-as 2

net 192.168.5.0

net 192.168.4.0

r6:

inter e1/0

ip add 192.168.3.2 255.255.255.0

no sh

inter e1/1

ip add 192.168.4.1 255.255.255.0

no sh

inter loop 1

ip add 172.16.1.1 255.255.255.0

router bgp 2

nei 192.168.3.1 remote-as 2

nei 192.168.4.2 remote-as 2

r6在用route-map改变metric前的bgp路径，可以清楚的看到前往192.168.1.0和192.168.6.0的metric值为0，而且下一跳都为

192.168.2.1

R6(config-if)#do show ip bgp

BGP table version is 14, local router ID is 192.168.1.1

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

*> 172.16.1.0/24 0.0.0.0 0 32768 i

* i192.168.1.0 192.168.5.2 0 100 0 1 i

*>i 192.168.2.1 0 100 0 1 i

*>i192.168.2.0 192.168.3.1 0 100 0 i

*> 192.168.3.0 0.0.0.0 0 32768 i

* i 192.168.3.1 0 100 0 i

*> 192.168.4.0 0.0.0.0 0 32768 i

* i 192.168.4.2 0 100 0 i

*>i192.168.5.0 192.168.4.2 0 100 0 i

* i192.168.6.0 192.168.5.2 0 100 0 1 i

*>i 192.168.2.1 0 100 0 1 i

用route-map改变metric后，可以看到ibgp邻居192.168.3.1通告的前往192.168.1.0和192.168.6.0的metric分别变为100和200，而

ibgp邻居192.168.4.2通告的前往192.168.1.0和192.168.6.0的metric分别为200和100.med值越小越好这导致r6前往192.168.1.0通过

r2，而前往192.168.6.0通r3.

R6#show ip bgp

BGP table version is 18, local router ID is 192.168.1.1

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

*> 172.16.1.0/24 0.0.0.0 0 32768 i

* i192.168.1.0 192.168.5.2 200 100 0 1 i

*>i 192.168.2.1 100 100 0 1 i

*>i192.168.2.0 192.168.3.1 0 100 0 i

*> 192.168.3.0 0.0.0.0 0 32768 i

* i 192.168.3.1 0 100 0 i

*> 192.168.4.0 0.0.0.0 0 32768 i

* i 192.168.4.2 0 100 0 i

*>i192.168.5.0 192.168.4.2 0 100 0 i

*>i192.168.6.0 192.168.5.2 100 100 0 1 i

* i 192.168.2.1 200 100 0 1 i

r1上前往172.16.1.0是经过r2的

R1#show ip bgp

BGP table version is 9, local router ID is 192.168.6.2

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

* i172.16.1.0/24 192.168.5.1 0 100 0 2 i

*>i 192.168.2.2 0 100 0 2 i

* i192.168.1.0 192.168.1.2 0 100 0 i

*> 0.0.0.0 0 32768 i

*>i192.168.2.0 192.168.1.2 0 100 0 i

* i192.168.3.0 192.168.5.1 0 100 0 2 i

*>i 192.168.2.2 0 100 0 2 i

*>i192.168.4.0 192.168.2.2 0 100 0 2 i

* i 192.168.5.1 0 100 0 2 i

*>i192.168.5.0 192.168.6.1 0 100 0 i

* i192.168.6.0 192.168.6.1 0 100 0 i

*> 0.0.0.0 0 32768 i

本地优先级值越大越好，修改优先级后通过r3前往172.16.1.0的本地优先级为200比r2通告的优先级高，所以通过r3前往172.16.1.0

R1#show ip bgp

BGP table version is 10, local router ID is 192.168.6.2

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

*>i172.16.1.0/24 192.168.5.1 0 200 0 2 i

* i 192.168.2.2 0 100 0 2 i

* i192.168.1.0 192.168.1.2 0 100 0 i

*> 0.0.0.0 0 32768 i

*>i192.168.2.0 192.168.1.2 0 100 0 i

* i192.168.3.0 192.168.5.1 0 100 0 2 i

*>i 192.168.2.2 0 100 0 2 i

*>i192.168.4.0 192.168.2.2 0 100 0 2 i

* i 192.168.5.1 0 100 0 2 i

*>i192.168.5.0 192.168.6.1 0 100 0 i

* i192.168.6.0 192.168.6.1 0 100 0 i

*> 0.0.0.0 0 32768 i

本文出自 “话巴山夜雨时” 博客，请务必保留此出处http://ccnpnetwork.blog.51cto.com/1648283/1046986