深入分析bgp选路规则(上)
一、拓扑图：
二、IBGP邻居学来的信息，不会传递给另外的IBGP邻居，需要再去手动指定邻居，BGP这条路径能不能通，要看这条路径否成为最佳路径和下一跳是否正确与可达：
1、配置各路由器的IP地址以及配置BGP协议。现在来看一下R3的路由表，看到R3能学到全网的路由：
3、再去R4上看一下R4的路由表发现没有学到10.1.255.0/30和130.130.1.0/24这两条路由：
8、但我在R4上traceroute 10.1.255.4之后，发现R4的路由表发生了变化10.1.255.0/30 不存在了。
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R4#traceroute 10.1.255.4
Type escape sequence to abort.
Tracing the route to 10.1.255.4
1 10.1.255.9 52 msec 32 msec 16 msec
2 * * *
3 * * *
4 * * *
R4#sh ip route
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
10.0.0.0/30 is subnetted, 3 subnets
B
10.1.255.4 [200/0] via 10.1.255.9, 00:00:24
C 10.1.255.12 is directly connected, Serial1/1
C 10.1.255.8 is directly connected, Serial1/0
150.150.0.0/24 is subnetted, 1 subnets
B
150.150.1.0 [20/0] via 10.1.255.14, 00:23:42
R4#ping 10.1.255.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.255.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/26/52 ms
R4# |
(超过8万字一篇发不了。待续，请接着看下篇)
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R4#sh ip route
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
10.0.0.0/30 is subnetted, 3 subnets
B
10.1.255.4 [200/0] via 10.1.255.9, 00:00:21
C 10.1.255.12 is directly connected, Serial1/1
C 10.1.255.8 is directly connected, Serial1/0
150.150.0.0/24 is subnetted, 1 subnets
B
150.150.1.0 [20/0] via 10.1.255.14, 00:02:27
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4
、终其原因是因为：IBGP对等体学习到的路由，是不会向其它的IBGP对等体通告的。拓扑中的R3从IBGP对等体R4学来的路由是不会向R2这个IBGP对等体通告，同理R3从R2学来的路由也不会向R4通告。所以就造成了R2和R4学不到完整的路由信息。
5、先在R2和R4上互指IBGP邻居，一定要互指，不然邻居不能完全建立起来：
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R2(config)#router bgp 64513
R2(config-router)#nei 10.1.255.10 remote 64513
R4
(config)#router bgp 64513
R4
(config-router)#nei 10.1.255.5 remote 64513
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6、相互指定完IBGP邻居之后，用clear ip bgp * 把路由表涮新一下。然后再来看一下R2和R4的路由表：
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R2#sh ip route
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
10.0.0.0/30 is subnetted, 4 subnets
C 10.1.255.4 is directly connected, Serial1/1
C 10.1.255.0 is directly connected, Serial1/0
B
10.1.255.12 [200/0] via 10.1.255.10, 00:00:47
B
10.1.255.8 [200/0] via 10.1.255.6, 00:00:56
130.130.0.0/24 is subnetted, 1 subnets
B
130.130.1.0 [20/0] via 10.1.255.1, 00:00:45
150.150.0.0/24 is subnetted, 1 subnets
B
150.150.1.0 [200/0] via 10.1.255.14, 00:00:24
R2#
R4
#sh ip route
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
10.0.0.0/30 is subnetted, 4 subnets
B
10.1.255.4 [200/0] via 10.1.255.5, 00:00:11
B
10.1.255.0 [200/0] via 10.1.255.5, 00:00:11
C 10.1.255.12 is directly connected, Serial1/1
C 10.1.255.8 is directly connected, Serial1/0
150.150.0.0/24 is subnetted, 1 subnets
B
150.150.1.0 [20/0] via 10.1.255.14, 00:05:26
R4#
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7
、通过互指邻居，发现R2能学到全网的路由但是R4却不能学到130.130.1.0/24这条路由，而且还发现到10.1.255.4与10.1.255.0 走的都是10.1.255.5。
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R4#ping 10.1.255.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.255.4, timeout is 2 seconds:
.....
(
发现不通)
Success rate is 0 percent (0/5)
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R3#sh ip route
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
10.0.0.0/30 is subnetted, 4 subnets
C 10.1.255.4 is directly connected, Serial1/0
B
10.1.255.0 [200/0] via 10.1.255.5, 00:07:05
B
10.1.255.12 [200/0] via 10.1.255.10, 00:06:59
C 10.1.255.8 is directly connected, Serial1/1
130.130.0.0/24 is subnetted, 1 subnets
B
130.130.1.0 [200/0] via 10.1.255.1, 00:06:50
150.150.0.0/24 is subnetted, 1 subnets
B
150.150.1.0 [200/0] via 10.1.255.14, 00:06:29
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2
、去看一下R2的路由表，发现R2没有学到10.1.255.12/24和150.150.1.0/24这两条路由：
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R2#sh ip route
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
10.0.0.0/30 is subnetted, 3 subnets
C 10.1.255.4 is directly connected, Serial1/1
C 10.1.255.0 is directly connected, Serial1/0
B
10.1.255.8 [200/0] via 10.1.255.6, 00:01:16
130.130.0.0/24 is subnetted, 1 subnets
B
130.130.1.0 [20/0] via 10.1.255.1, 00:00:46
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