BGP的同步.docx

BGP的同步基本原理：在BGP协议设计同步这个功能的作用是用来防止“路由黑洞”，所谓的路由黑洞是路由层面不具备去往该目的网段的路由，而数据到达时候全部都丢弃掉的这种现象被称作为路由黑洞。所谓的同步指的是iBGP与IGP的一种同步关系，为什么需要把BGP学习来的路由需要同步到IGP当中？运行BGP协议的两个路由器是使用单播的方式来进行对等体的建立，既然是单播那么就是以“空投”的形式来完成数据的传递，中间可能有其他的路由器，但是对于运行BGP的路由器来讲根本就不能判定是否跟对等体之间还有其他的路由器。一旦中间有其他的路由器那么势必会有路由黑洞。在同步开启的时候，同步判定原则如下：当通过iBGP对等体接受的路由信息，查看自身的IGP路由表，如果IGP的路由表中没有该路由信息，那么此条目不会被判定为可达的路由。如果IGP的路由表中有该路由信息，那么此路由被判定为可达的路由，可以安装到BGP表中或者继续传递下去。在现在IOS版本12.0之后同步会自动的被关闭掉。具体的原因我们到实验中来分析。BGP同步的命令为：synchronization实验拓扑：实验基本配置：R1：hostname R1!interface Loopback0 ip address !interface Serial0/0 ip address !router bgp 1 no synchronization-默认同步是关闭的 bgp router-id network mask neighbor remote-as 234 no auto-summary!ip http serverR2：hostname R2!interface Loopback0 ip address !interface Serial0/0 ip address !interface Serial0/1 ip address router ospf 1 router-id passive-interface Serial0/0 network 55 area 0!router bgp 234 no synchronization bgp router-id neighbor remote-as 1 neighbor remote-as 234 no auto-summary!R3：hostname R3!interface Loopback0 ip address !interface Serial0/0 ip address serial restart-delay 0!router ospf 1 router-id network 55 area 0R4：hostname R4!interface Loopback0 ip address !interface Serial0/0 ip address !interface Serial0/1 ip address !router ospf 1 router-id passive-interface Serial0/1 network 55 area 0!router bgp 234 no synchronization bgp router-id neighbor remote-as 234 neighbor remote-as 5 no auto-summaryR5：hostname R5!interface Loopback0 ip address !interface Serial0/0 ip address !router bgp 5 no synchronization bgp router-id network mask neighbor remote-as 234 no auto-summary实验中的对等体有：R1-R2，R2-R4，R4-R5来看一下BGP路由传递的情况：R1# show ip bgp BGP table version is 4, local router ID is Status codes: s suppressed, d damped, h history, * valid, best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path* /24 0 32768 i* /24 0 234 5 iR5#show ip bgp BGP table version is 4, local router ID is Status codes: s suppressed, d damped, h history, * valid, best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path* /24 0 234 1 i* /24 0 32768 i可以看到R1与R5都相互学习到对方的路由信息，那么我们来做一下测试：R1#ping source Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds:Packet sent with a source address of .Success rate is 0 percent (0/5)此时已经出现了“路由黑洞”，一步一步的来追查信息。R1#show ip route Routing entry for /24 Known via bgp 1, distance 20, metric 0 Tag 234, type external Last update from 00:14:06 ago Routing Descriptor Blocks: * , from , 00:14:06 ago Route metric is 0, traffic share count is 1 AS Hops 2 Route tag 234R2#show ip route Routing entry for /24 Known via bgp 234, distance 200, metric 0 Tag 5, type internal Last update from 00:14:26 ago Routing Descriptor Blocks: * , from , 00:14:26 ago Route metric is 0, traffic share count is 1 AS Hops 1 Route tag 5R3#show ip route % Network not in table可以看到R3没有去往的路由信息，所有的数据全部都丢弃在R3，原因是R2与R4传递路由的时候是单播传递，R3直接转发BGP的更新数据分组，并没有读取里面的更新信息。由于默认情况下同步是关闭的，所以没有办法防止路由黑洞。接下来在R4开启路由的同步功能。R4(config)#router bgp 234R4(config-router)#synchronization来查看一下R4学习BGP路由的情况。R4#show ip bgpBGP table version is 6, local router ID is Status codes: s suppressed, d damped, h history, * valid, best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path* i/24 0 100 0 1 i* /24 0 0 5 i可以看到R4并没有认为为最优路由，原因是当通过iBGP对等体学习的路由在R4自身的IGP表中并没有这个条目，所以同步原则判定失败，R4没有安装进自己的BGP转发表，那么R5就不可能接受到该路由条目信息。解决办法可以在R2把R1传递过来的路由信息重发布到IGP中，那么R4的IGP路由表就有了该路由信息，再来看一下结果输出。R2(config)#router ospf 1R2(config-router)#redistribute bgp 234 subnets此时来查看R2的OSPF数据库：R2#show ip ospf database external OSPF Router with ID () (Process ID 1) Type-5 AS External Link States LS age: 53 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: (External Network Number ) Advertising Router: LS Seq Number: 80000001 Checksum: 0xDACC Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 1 Forward Address: External Route Tag: 1那么R4的IGP可以计算出该路由条目，那么此时查看一下R4的IGP路由。R4#show ip route Routing entry for /24 Known via ospf 1, distance 110, metric 1 Tag 1, type extern 2, forward metric 128 Last update from on Serial0/0, 00:01:57 ago Routing Descriptor Blocks: * , from , 00:01:57 ago, via Serial0/0 Route metric is 1, traffic share count is 1 Route tag 1IGP表中已经有了这个/24的路由条目，那么来查看一下BGP的转发表：R4#show ip bgp BGP table version is 8, local router ID is Status codes: s suppressed, d damped, h history, * valid, best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Pathri/24 0 100 0 1 i* /24 0 0 5 i这个条目已经被当做最优的路由安装进BGP的转发表了，其中路由的前缀为r，原因为R4通过iBGP学习到/24、也通过OSPF学习到/24这个条目，通过多种不同的协议学习到相同的路由比较AD值，iBGP的AD值为200，OSPF的AD值为110。所以选择OSPF的路