什么是路由黑洞？ 简单的说，它会默默的将数据包丢弃，使所有数据包有去无回，下面来看一个案例：
如图所示：
R1和R2建立EBGP邻居关系
R2和R5建立IBGP邻居关系
R5和R7建立EBGP邻居关系
R2、R3、R5之间运行RIPv2
首先看配置：
hostname r1
interface Loopback0
ip address 1.1.1.1 255.255.255.0
interface Serial1/0
ip address 192.168.12.1 255.255.255.0
serial restart-delay 0
interface Loopback0
ip address 1.1.1.1 255.255.255.0
interface Serial1/0
ip address 192.168.12.1 255.255.255.0
serial restart-delay 0
router bgp 100
no synchronization
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
network 192.168.12.0
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 ebgp-multihop 255
neighbor 2.2.2.2 update-source Loopback0
no auto-summary
!
ip route 2.2.2.0 255.255.255.0 192.168.12.2
no synchronization
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
network 192.168.12.0
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 ebgp-multihop 255
neighbor 2.2.2.2 update-source Loopback0
no auto-summary
!
ip route 2.2.2.0 255.255.255.0 192.168.12.2
hostname r2
interface Loopback0
ip address 2.2.2.2 255.255.255.0
!
interface Serial1/0
ip address 192.168.23.2 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.12.2 255.255.255.0
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.24.2 255.255.255.0
serial restart-delay 0
!
router rip
version 2
network 2.0.0.0
network 192.168.23.0
no auto-summary
!
router bgp 200
no synchronization
bgp log-neighbor-changes
network 192.168.12.0
network 192.168.23.0
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 ebgp-multihop 255
neighbor 1.1.1.1 update-source Loopback0
neighbor 5.5.5.5 remote-as 200
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 next-hop-self
no auto-summary
!
ip route 1.1.1.0 255.255.255.0 192.168.12.1
interface Loopback0
ip address 2.2.2.2 255.255.255.0
!
interface Serial1/0
ip address 192.168.23.2 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.12.2 255.255.255.0
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.24.2 255.255.255.0
serial restart-delay 0
!
router rip
version 2
network 2.0.0.0
network 192.168.23.0
no auto-summary
!
router bgp 200
no synchronization
bgp log-neighbor-changes
network 192.168.12.0
network 192.168.23.0
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 ebgp-multihop 255
neighbor 1.1.1.1 update-source Loopback0
neighbor 5.5.5.5 remote-as 200
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 next-hop-self
no auto-summary
!
ip route 1.1.1.0 255.255.255.0 192.168.12.1
hostname r3
interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
interface Serial1/0
ip address 192.168.35.3 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.23.3 255.255.255.0
serial restart-delay 0
router rip
version 2
network 3.0.0.0
network 192.168.23.0
network 192.168.35.0
no auto-summary
interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
interface Serial1/0
ip address 192.168.35.3 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.23.3 255.255.255.0
serial restart-delay 0
router rip
version 2
network 3.0.0.0
network 192.168.23.0
network 192.168.35.0
no auto-summary
hostname r5
interface Loopback0
ip address 5.5.5.5 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex half
!
interface Serial1/0
ip address 192.168.57.5 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.35.5 255.255.255.0
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.45.5 255.255.255.0
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router rip
version 2
network 5.0.0.0
network 192.168.35.0
no auto-summary
!
router bgp 200
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200
neighbor 3.3.3.3 remote-as 200
neighbor 7.7.7.7 remote-as 300
neighbor 7.7.7.7 ebgp-multihop 255
neighbor 7.7.7.7 update-source Loopback0
no auto-summary
!
ip route 7.7.7.0 255.255.255.0 192.168.57.7
interface Loopback0
ip address 5.5.5.5 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex half
!
interface Serial1/0
ip address 192.168.57.5 255.255.255.0
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.35.5 255.255.255.0
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.45.5 255.255.255.0
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router rip
version 2
network 5.0.0.0
network 192.168.35.0
no auto-summary
!
router bgp 200
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200
neighbor 3.3.3.3 remote-as 200
neighbor 7.7.7.7 remote-as 300
neighbor 7.7.7.7 ebgp-multihop 255
neighbor 7.7.7.7 update-source Loopback0
no auto-summary
!
ip route 7.7.7.0 255.255.255.0 192.168.57.7
interface Serial1/1
ip address 192.168.57.7 255.255.255.0
serial restart-delay 0
!
interface Serial1/2
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router bgp 300
no synchronization
bgp log-neighbor-changes
neighbor 5.5.5.5 remote-as 200
neighbor 5.5.5.5 ebgp-multihop 255
no auto-summary
!
ip route 5.5.5.0 255.255.255.0 192.168.57.5
ip address 192.168.57.7 255.255.255.0
serial restart-delay 0
!
interface Serial1/2
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router bgp 300
no synchronization
bgp log-neighbor-changes
neighbor 5.5.5.5 remote-as 200
neighbor 5.5.5.5 ebgp-multihop 255
no auto-summary
!
ip route 5.5.5.0 255.255.255.0 192.168.57.5
现在查看R1的路由表
r7#sh ip route
B 1.1.1.0 [20/0] via 5.5.5.5, 00:02:54 //为节约篇幅未完整显示
B 1.1.1.0 [20/0] via 5.5.5.5, 00:02:54 //为节约篇幅未完整显示
可见R7学到了R1的路由，从表面上看这个实验很完美，达了目的， 然而这时问题出现了，作个测试，在R7上PING R1
r7#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 0-byte ICMP Echos to 7.7.7.7, timeout:
Sending 5, 0-byte ICMP Echos to 7.7.7.7, timeout:
.....
这究竟是怎么回事呢？原来，我们在R5上关闭了同步，这时它会将一条并没有
优化的路由传送给R7，当R7要发向R1发包时，它看到R5是它的下一跳，于是将包发给R5，然后R5又查看它的路由表，发现到R1的下一跳是R2，并继续查找，发现在通过R3可以达到R2，于是它将数据送给R3，这时问题出现了，因为R3没有运行BGP，它不知道R1怎么走，于是它将数据包丢弃，从而造成路由黑洞。由此可见，BGP与IGP同步的重要性，什么是同步？在上一篇我已经提到了，不再多阐述。(同理R1无法访问R7，但有R7的路由条目)
（优化的条件：１、下一跳可达，题中满足；２、同步，题中不满足）
既然问题出现了，那到底该怎么去解决呢？
首先，如果在R5上开启同步，这样的情况就不会发生了，因为R5从R2收到关于R1的路由条目后，它会的查找它的IGP路由表，看是否有这样一条路由可以到达R1，如果没有，它就不会将这条路由传递给R7，此时可避免黑洞问题。但R7学到R1的路由才是我们真正的目的，那该怎么做呢？我们可以将BGP的路由重发布到RIP中，这样IGP和BGP就可以完成同步，但是这样做并不好，在实验环境中我们当然可以这样做，但试想现在INTERNET中有多达20几万条路由条目，如果重发布到我们的IGP路由器中，很显然大多数路由器是无法支撑的。
解决方法一：Full Mesh
分别在R2和R3，R3和R5上运行BGP ，这样R3就可以学到到达R1的路由，这时你需要在R1、R2、R3之间分别建立邻居关系，当路由器很多的时候，全互联要求建立n*(n-1)/2个邻居关系，这显然是很麻烦。（略）
解决方法二：Foute-Reflector
路由反射器，我们可以将R3做成一个路由反射器，使它能将从R1学的路由条目反射给R5，正常情况下为了防止环路，从IBGP学到的路由不会再传给其它IBGP邻居。 现在只需要在路由反射客户和路由反射器间建立邻居关系，邻居关系减少到n-1条。
路由反射器（RR）的条件：
1、如果路由是从非客户的IBGP学到的只反射给客户
2、如果路由是从客户学到的，将它反射给发起该路由的客户以外的所有非客户及客户
3、如果路由是从EBGP对等体学到的，将它反射给所有客户和非客户
做法：清除R2与R5的邻居关系，只在R2与R3、R3与R5之间建立IBGP关系
然后在R3的路由配置模式下：
neighbor 2.2.2.2 route-reflector-client
neighbor 5.5.5.5 route-reflector-client //将R2和R5作为RR的客户
此时R5上能收到关于R1的路由,它也会传给R7
解决方法三 ：Confederation
联邦是将整个大的AS区域再划分成多个小的AS区域,比如现在有AS200就相当于中国，而整个中国显然可以再分为若干个省分，现在AS65012和AS65003 就是划分出来的“小AS”，意思这里的AS号是私有的，在出AS200时它将自动被去掉（64512--65535可用），这样划分后，R3和R5就为联邦EBGP邻居了，这时它关于R 1的路由条目就可以传给R5了。
R2：
router bgp 65012
//指定的联邦AS号
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200 //R2对外宣称自己的AS号为200，它会告诉R1它的AS号为200，因为65012是私有的
network 192.168.12.0
network 192.168.23.0
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 ebgp-multihop 255
neighbor 1.1.1.1 update-source Loopback0
neighbor 3.3.3.3 remote-as 65012
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 next-hop-self
no auto-summary
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200 //R2对外宣称自己的AS号为200，它会告诉R1它的AS号为200，因为65012是私有的
network 192.168.12.0
network 192.168.23.0
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 ebgp-multihop 255
neighbor 1.1.1.1 update-source Loopback0
neighbor 3.3.3.3 remote-as 65012
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 next-hop-self
no auto-summary
R3：
r3#sh run | b r b
router bgp 65012
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200 //对R5稳定自己的AS号为200，它不会对R2宣称，因为它们属于同一个联邦AS
neighbor 2.2.2.2 remote-as 65012
neighbor 2.2.2.2 update-source Loopback0
neighbor 5.5.5.5 remote-as 65003
neighbor 5.5.5.5 ebgp-multihop 255
neighbor 5.5.5.5 update-source Loopback0
no auto-summary
!
router bgp 65012
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200 //对R5稳定自己的AS号为200，它不会对R2宣称，因为它们属于同一个联邦AS
neighbor 2.2.2.2 remote-as 65012
neighbor 2.2.2.2 update-source Loopback0
neighbor 5.5.5.5 remote-as 65003
neighbor 5.5.5.5 ebgp-multihop 255
neighbor 5.5.5.5 update-source Loopback0
no auto-summary
!
R5：
router bgp 65003
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200 //对R3和R7宣称自己的AS号为200
neighbor 3.3.3.3 remote-as 65012
neighbor 3.3.3.3 ebgp-multihop 255
neighbor 7.7.7.7 remote-as 300
neighbor 7.7.7.7 ebgp-multihop 255
neighbor 7.7.7.7 update-source Loopback0
no auto-summary
!
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200 //对R3和R7宣称自己的AS号为200
neighbor 3.3.3.3 remote-as 65012
neighbor 3.3.3.3 ebgp-multihop 255
neighbor 7.7.7.7 remote-as 300
neighbor 7.7.7.7 ebgp-multihop 255
neighbor 7.7.7.7 update-source Loopback0
no auto-summary
!
这时查看R3和R5的邻居关系
r3#sh ip bgp su
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2 4 65012 10 9 1 0 0 00:06:26 2
5.5.5.5 4 65003 1 1 0 0 0 never Active
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2 4 65012 10 9 1 0 0 00:06:26 2
5.5.5.5 4 65003 1 1 0 0 0 never Active
发现在它们始终处于Active状态，不能完成邻居的建立，这又是为什么呢？
分析一下：
R5和R3都打了这条命令，bgp confederation identifier 200
又因为他们处于不同的联邦AS，所以它们都会宣称自己的AS号为200，然而我们看到它们的Neighbor却不是这样通告的
R3上：neighbor 5.5.5.5 remote-as 65003
R5上：neighbor 3.3.3.3 remote-as 65012
所以这时会报一个错误：AS号错误，如下
所以这时会报一个错误：AS号错误，如下
r5#
*Oct 19 12:20:10.311: %SYS-5-CONFIG_I: Configured from console by console
r5#
*Oct 19 12:20:19.155: %BGP-3-NOTIFICATION: sent to neighbor 3.3.3.3 2/2 (peer in wrong AS) 2 bytes 00C8
r5# FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 00C8 00B4 0303 0303 1002 0601 0400 0100 0102 0280 0002 0202 00
*Oct 19 12:20:10.311: %SYS-5-CONFIG_I: Configured from console by console
r5#
*Oct 19 12:20:19.155: %BGP-3-NOTIFICATION: sent to neighbor 3.3.3.3 2/2 (peer in wrong AS) 2 bytes 00C8
r5# FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 00C8 00B4 0303 0303 1002 0601 0400 0100 0102 0280 0002 0202 00
解决方法：
R3上：bgp confederation peer 65003 // // 不对65003宣称自己的AS为200
R5上：bgp confederation peer 65012 // 不对65012宣称自己的AS为200
验证
r5#sh ip bgp
BGP table version is 13, local router ID is 5.5.5.5
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
BGP table version is 13, local router ID is 5.5.5.5
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
*> 1.1.1.0/24 2.2.2.2 0 100 0 (65012) 100 i
r> 7.7.7.0/24 7.7.7.7 0 0 300 i
*> 192.168.12.0 2.2.2.2 0 100 0 (65012) i
r> 192.168.23.0 3.3.3.3 0 100 0 (65012) i
*> 192.168.35.0 0.0.0.0 0 32768 i
* 3.3.3.3 0 100 0 (65012) i //括号中的是私有AS号，在出R5时会被去掉，在看7上可以看到效果
*> 192.168.57.0 0.0.0.0 0 32768 i
* 7.7.7.7 0 0 300 i
*> 1.1.1.0/24 2.2.2.2 0 100 0 (65012) 100 i
r> 7.7.7.0/24 7.7.7.7 0 0 300 i
*> 192.168.12.0 2.2.2.2 0 100 0 (65012) i
r> 192.168.23.0 3.3.3.3 0 100 0 (65012) i
*> 192.168.35.0 0.0.0.0 0 32768 i
* 3.3.3.3 0 100 0 (65012) i //括号中的是私有AS号，在出R5时会被去掉，在看7上可以看到效果
*> 192.168.57.0 0.0.0.0 0 32768 i
* 7.7.7.7 0 0 300 i
r7#sh ip bgp
BGP table version is 12, local router ID is 7.7.7.7
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
BGP table version is 12, local router ID is 7.7.7.7
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
*> 1.1.1.0/24 5.5.5.5 0 200 100 i
*> 7.7.7.0/24 0.0.0.0 0 32768 i
*> 192.168.12.0 5.5.5.5 0 200 i
*> 192.168.23.0 5.5.5.5 0 200 i
*> 192.168.35.0 5.5.5.5 0 0 200 i
* 192.168.57.0 5.5.5.5 0 0 200 i
*> 0.0.0.0 0 32768 i
*> 1.1.1.0/24 5.5.5.5 0 200 100 i
*> 7.7.7.0/24 0.0.0.0 0 32768 i
*> 192.168.12.0 5.5.5.5 0 200 i
*> 192.168.23.0 5.5.5.5 0 200 i
*> 192.168.35.0 5.5.5.5 0 0 200 i
* 192.168.57.0 5.5.5.5 0 0 200 i
*> 0.0.0.0 0 32768 i
实验完毕！
注：由于开了过多的设备，CPU实在难以支撑，没办法拷下所有配置过程，只作了总结性讲解，请见谅！ 有需要的我们可以再作交流。
本文转自hexianguo 51CTO博客，原文链接：http://blog.51cto.com/xghe110/106619，如需转载请自行联系原作者
