9、发现只要去traceroute 跟踪一下通过10.1.255.5学到的地址，它就会变成走10.1.255.9,只要路由表显示通过10.1.255.9 的话。到目的网段就能通。但过一会就又会变成走10.1.255.5了。因为我们之前手动地指定了它的邻居，所以当用路径跟踪这条路由后会发现自己的邻居路由器的地址是10.1.255.9，进而把10.1.255.9当作最佳路径来转发了。
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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 36 msec 56 msec 20 msec
2
R4#sh ip bgp
BGP table version is 245, local router ID is 10.1.255.13
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
* i10.1.255.0/30 10.1.255.5 0 100 0 i
* i10.1.255.4/30 10.1.255.5 0 100 0 i
*>i 10.1.255.9 0 100 0 i (成为最佳路径了)
*> 10.1.255.8/30 0.0.0.0 0 32768 i
* i 10.1.255.9 0 100 0 i
*> 10.1.255.12/30 0.0.0.0 0 32768 i
* 10.1.255.14 0 0 64514 i
* i130.130.1.0/24 10.1.255.1 0 100 0 64512 i
*> 150.150.1.0/24 10.1.255.14 0 0 64514 i
R4#
R4#sh ip bgp 10.1.255.4
BGP routing table entry for 10.1.255.4/30, version 317
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x840
Advertised to update-groups:
2
Local
10.1.255.5 (inaccessible) from 10.1.255.5 (10.1.255.5) (发现选这条就是inaccessiable)
Origin IGP, metric 0, localpref 100, valid, internal
Local
10.1.255.9 from 10.1.255.9 (10.1.255.9) (选这条就可以通了)
Origin IGP, metric 0, localpref 100, valid, internal, best
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10、过一会之后，我们再看一下bgp表：
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R4#sh ip bgp
BGP table version is 219, local router ID is 10.1.255.13
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
*>i10.1.255.0/30 10.1.255.5 0 100 0 i
*>i10.1.255.4/30 10.1.255.5 0 100 0 i
* i 10.1.255.9 0 100 0 i(没有成为最佳路径了,这样就不通)
*> 10.1.255.8/30 0.0.0.0 0 32768 i
* i 10.1.255.9 0 100 0 i
*> 10.1.255.12/30 0.0.0.0 0 32768 i
* 10.1.255.14 0 0 64514 i
* i130.130.1.0/24 10.1.255.1 0 100 0 64512 i
*> 150.150.1.0/24 10.1.255.14 0 0 64514 i
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11
、再来看一下130.130.1.0/24在BGP表中的情况：
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R4#sh ip bgp 130.130.1.0
BGP routing table entry for 130.130.1.0/24, version 0
Paths: (1 available, no best path)
Not advertised to any peer
64512
10.1.255.1 (inaccessible) from 10.1.255.5 (10.1.255.5)(只有一条路径,显示下一跳不可达。)
Origin IGP, metric 0, localpref 100, valid, internal
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12
、再来看一下到10.1.255.0/30的bgp表：
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R4#sh ip bgp 10.1.255.0
BGP routing table entry for 10.1.255.0/30, version 374
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
Local
10.1.255.5 from 10.1.255.5 (10.1.255.5)
Origin IGP, metric 0, localpref 100, valid, internal, best(虽然是最佳的，但是下一跳是
错误的)
R4#sh ip bgp 10.1.255.5
BGP routing table entry for 10.1.255.4/30, version 1053
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x840
Advertised to update-groups:
2
Local
10.1.255.5 (inaccessible) from 10.1.255.5 (10.1.255.5)(在这里10.1.255.5并不可达。)
Origin IGP, metric 0, localpref 100, valid, internal
Local
10.1.255.9 from 10.1.255.9 (10.1.255.9)
Origin IGP, metric 0, localpref 100, valid, internal, best
R4#
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13、通过上图分析，发现其实这些路由都存在于BGP表当中，能不能ping 通就看BGP有没有把它当作最佳路径和下一跳是否正确与可达。
三、造成以上的原因是：当两条路由度量值(metric)都相等时，而且都来自于相同的自治系统的iBGP的邻居，那么会优先选择router-id较小的iBGP邻居的路由。从之前的图中我们可以看到R4àR3与 R4àR2 (我们手动指定的邻居产生的) metric都为0。
1、通过上述论述，我们需要在R2上指定一个较高的router-id，以让R4放弃从R2而选择R3这条路径,那么我们要把R2的router-id指定成比10.1.255.5 要高。
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R2(config)#router bgp 64513
R2(config-router)#bgp router-id 172.1.255.5(指成172.1.255.5的话比10.1.255.9要高)
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2、现在再来看一下R4上的路由表和BGP表，现在能全部学到了：
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10.0.0.0/30 is subnetted, 4 subnets
B 10.1.255.4 [200/0] via 10.1.255.9, 00:03:21
B 10.1.255.0 [200/0] via 10.1.255.5, 00:02:28
C 10.1.255.12 is directly connected, Serial1/1
C 10.1.255.8 is directly connected, Serial1/0
130.130.0.0/24 is subnetted, 1 subnets
B 130.130.1.0 [200/0] via 10.1.255.1, 00:02:23
150.150.0.0/24 is subnetted, 1 subnets
B 150.150.1.0 [20/0] via 10.1.255.14, 03:23:35
R4#sh ip bgp
BGP table version is 1083, local router ID is 10.1.255.13
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
*>i10.1.255.0/30 10.1.255.5 0 100 0 i
* i10.1.255.4/30 10.1.255.5 0 100 0 i
*>i 10.1.255.9 0 100 0 i
*> 10.1.255.8/30 0.0.0.0 0 32768 i
* i 10.1.255.9 0 100 0 i
*> 10.1.255.12/30 0.0.0.0 0 32768 i
* 10.1.255.14 0 0 64514 i
*>i130.130.1.0/24 10.1.255.1 0 100 0 64512 i
*> 150.150.1.0/24 10.1.255.14 0 0 64514 i
R4#
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3、从上面看出学到的10.1.255.4/30 是通过10.1.255.9/30 这个地址，这个地址是可达的。而10.1.255.0/30是通过10.1.255.5/30，之前是不可达。现在我们通过改router-id之后就变成可达的了：
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R4#sh ip bgp 10.1.255.0
BGP routing table entry for 10.1.255.0/30, version 1082
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to update-groups:
2
Local
10.1.255.5 from 10.1.255.5 (192.16.255.5)
Origin IGP, metric 0, localpref 100, valid, internal, best
R4#ping 10.1.255.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.255.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/43/72 ms
R4#
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四、总结：IBGP邻居学来的信息，不会传递给另外的IBGP邻居，需要再手动去指定邻居，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。当某个路由器到达一个网络有两条路径，而且两条路由度量值(metric)都相等时，并且都来自于相同的自治系统的iBGP的邻居，那么会优先选择router-id较小的iBGP邻居的路由。
本文转自wxs-163 51CTO博客，原文链接:http://blog.51cto.com/supercisco/268203
