CCNP知识：边界网关协议（BGP）改变权重影响路径选择

拓扑图如下：

 

下面是4个路由器的具体的配置命令：

r1属于as1，r2属于as3，r3属于as2，r4属于as4.
r1:
in e1/1
ip add 192.168.1.1 255.255.255.0
no sh
in e1/2
ip add 192.168.2.1 255.255.255.0
no sh
in loop 1
ip add 1.1.1.1 255.0.0.0
ip route 3.0.0.0 255.0.0.0 192.168.1.2
router bgp 1
nei 3.3.3.3 remote-as 2
nei 3.3.3.3 update-source loop 1
nei 3.3.3.3 ebgp-mutlihop 2
nei 3.3.3.3 route-map setweight in
nei 192.168.2.2 remote-as 3
ip prefix-list gor3 permit 10.0.0.0/8
http://www.luyouqiwang.com/14470/
route-map setweight permit 10
match ip add prefix-list gor3
set weight 200
route-map setweight permit 20
r2:
in e1/2
ip add 192.168.2.2 255.255.255.0
no sh
in e1/3
ip add 192.168.4.1 255.255.255.0
no sh
router bgp 3
nei 192.168.2.1 remote-as 1
nei 192.168.4.2 remote-as 4
r3:
in e1/1
ip add 192.168.1.2 255.255.255.0
no sh
in e1/0
ip add 192.168.3.1 255.255.255.0
no sh
in loop 1
ip add 3.3.3.3 255.0.0.0
ip route 1.0.0.0 255.0.0.0 192.168.1.1
router bgp 2
nei 1.1.1.1 remote-as 1
nei 1.1.1.1 update-source loop 1
nei 1.1.1.1 ebgp-mutlihop 2
nei 192.168.3.2 remote-as 4
r4:
in e1/1
ip add 192.168.3.2 255.255.255.0
no sh
in e1/2
ip add 192.168.4.2 255.255.255.0
in loop 1
ip add 10.143.132.1 255.0.0.0
router bgp 4
nei 192.168.3.1 remote-as 2
nei 192.168.4.1 remote-as 3
net 10.0.0.0
记住想要使这种改变生效，必须得重置邻居关系，在这里我使用软重置。切忌在生产环境中使用硬重置，这会导致所有邻居关系中断

并重新计算路由，这在有数十万条路由条目的路由器上绝对会是个灾难，没有一个小时以上的时间路由绝对不会收敛。
我们先看看在重置以前选择的最佳路由是经过哪里。
R1#show ip bgp
BGP table version is 5, local router ID is 1.1.1.1
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
*  10.0.0.0         3.3.3.3                                0 2 4 i
*>                  192.168.2.2                            0 3 4 i
有些东西我得解释一下，第一列的*>代表去往10.0.0.0网段选择的最佳路径是经过下一跳为192.168.2.2的r2路由器。我们看到

weight都为0，并且as path经过的数量都一样，都为2个。而路由表也体现了这一点。
R1#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    1.0.0.0/8 is directly connected, Loopback1
S    3.0.0.0/8 [1/0] via 192.168.1.2
B    10.0.0.0/8 [20/0] via 192.168.2.2, 00:00:49
C    192.168.1.0/24 is directly connected, Ethernet1/1
C    192.168.2.0/24 is directly connected, Ethernet1/2
注意，我软重置邻居关系。这条命令是在特权模式下打的，而不是在配置模式下。
clear ip bgp * soft

 

 

现在再看看选择的最佳路由变了。
R1#clear ip bgp * soft
R1#show ip bgp
BGP table version is 6, local router ID is 1.1.1.1
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
*> 10.0.0.0         3.3.3.3                              200 2 4 i
*                   192.168.2.2                            0 3 4 i
现在选择的最佳路由为下一跳为3.3.3.3的r3路由器，而这条路由的weight值也变为我在路由映射表中设置的200了。
最后再看看路由表。
R1#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    1.0.0.0/8 is directly connected, Loopback1
S    3.0.0.0/8 [1/0] via 192.168.1.2
B    10.0.0.0/8 [20/0] via 3.3.3.3, 00:02:55
C    192.168.1.0/24 is directly connected, Ethernet1/1
C    192.168.2.0/24 is directly connected, Ethernet1/2

呵呵，可能有点晕。不过这些都是CCNP认证考试的必须熟悉和掌握的基础知识了，多花点时间和精力在上面，一定会有很多收获的。