BGP 产生默认路由的方式
拓扑图如上，基本配置略；
1、写默认路由并通告
ip route 0.0.0.0 0.0.0.0 null 0
写一条默认路由，指向空接口，并将其通告至BGP中，这样该路由器会将该默认路由通告给他的所有BGP邻居；
在R4中配置静态路由，并将其通告至BGP中，然后查看路由表情况；
R4(config)#ip route 0.0.0.0 0.0.0.0 null 0 R4(config)#router bgp 100 R4(config-router)#network 0.0.0.0 mask 0.0.0.0 //通告默认路由
在EBGP邻居R5上查看BGP路由表
R5#show ip route bgp Codes: L - local, 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, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is 45.1.1.1 to network 0.0.0.0 B* 0.0.0.0/0 [20/0] via 45.1.1.1, 00:03:01 1.0.0.0/32 is subnetted, 1 subnets B 1.1.1.1 [20/0] via 45.1.1.1, 00:02:33
然后在IBGP邻居R2和R3上查看BGP路由情况如下：
R3#show ip route bgp Codes: L - local, 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, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is 34.1.1.2 to network 0.0.0.0 B* 0.0.0.0/0 [200/0] via 34.1.1.2, 00:04:01 1.0.0.0/32 is subnetted, 1 subnets B 1.1.1.1 [200/0] via 23.1.1.1, 00:03:12 5.0.0.0/32 is subnetted, 1 subnets B 5.5.5.5 [200/0] via 34.1.1.2, 00:21:43 55.0.0.0/32 is subnetted, 1 subnets B 55.5.5.5 [200/0] via 34.1.1.2, 00:21:44 R2#show ip route bgp Codes: L - local, 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, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is 4.4.4.4 to network 0.0.0.0 B* 0.0.0.0/0 [200/0] via 4.4.4.4, 00:04:44 1.0.0.0/32 is subnetted, 1 subnets B 1.1.1.1 [20/0] via 12.1.1.1, 00:03:54 5.0.0.0/32 is subnetted, 1 subnets B 5.5.5.5 [200/0] via 4.4.4.4, 00:17:20 55.0.0.0/32 is subnetted, 1 subnets B 55.5.5.5 [200/0] via 4.4.4.4, 00:17:20
2、neighbor X.X.X.X default-originate
给邻居通告默认路由，在指定邻居路由器会收到一条默认路由，没有指定的，不会收到默认路由，在R4上配置给R5通告默认路由：
R4(config)#router bgp 100 R4(config-router)#neighbor 45.1.1.2 default-originate
R5上查看BGP路由情况：
R5#show ip bgp BGP table version is 47, 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, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 0.0.0.0 45.1.1.1 0 100 i *> 5.5.5.5/32 0.0.0.0 0 32768 i *> 55.5.5.5/32 0.0.0.0 0 32768 i
3、 default-information originate
对所有邻居都通告默认路由，但这种方式有一点特殊，下面通过配置分析来解释；
首先我们在R4上配置default-information originate命令后：
R4(config)#router bgp 100 R4(config-router)#default-information originate
在R5上查看是否收到默认路由：
R5#show ip bgp BGP table version is 55, 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, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 1.1.1.1/32 45.1.1.1 0 100 10 i *> 5.5.5.5/32 0.0.0.0 0 32768 i *> 55.5.5.5/32 0.0.0.0 0 32768 i
从上可知，并没有收到默认路由，现在我们在R4上配置一条默认路由并指向空接口，再次查看R5的路由情况
R4(config)#ip route 0.0.0.0 0.0.0.0 null 0
R5#clear ip bgp * soft R5#show ip bgp BGP table version is 55, 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, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 1.1.1.1/32 45.1.1.1 0 100 10 i *> 5.5.5.5/32 0.0.0.0 0 32768 i *> 55.5.5.5/32 0.0.0.0 0 32768 i
从输出可知，R5还是没有收到默认路由，现将所写的静态默认路由重分布至BGP中，再次查看EBGP邻居R5和IBGP邻居R2的BGP路由情况；
R4(config)#router bgp 100 R4(config-router)#redistribute static
R5#show ip bgp BGP table version is 62, 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, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 0.0.0.0 45.1.1.1 0 0 100 ? *> 1.1.1.1/32 45.1.1.1 0 100 10 i *> 5.5.5.5/32 0.0.0.0 0 32768 i *> 55.5.5.5/32 0.0.0.0 0 32768 i R2#show ip bgp BGP table version is 23, local router ID is 2.2.2.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *>i 0.0.0.0 4.4.4.4 0 100 0 ? *> 1.1.1.1/32 12.1.1.1 0 0 10 i *>i 5.5.5.5/32 4.4.4.4 0 100 0 50 i *>i 55.5.5.5/32 4.4.4.4 0 100 0 50 i
从上可知，R5、R2都收到了默认路由，下一跳是都是R4，证明此条默认路由是R4通告过来的，就此证明default-information originate是通告给所有BGP邻居。
现在将R4上的default-information originate命令no掉，但重分布静态默认路由的配置保持不会，并增加一条静态路由，再次在R4和R5上查看BGP路由情况；
R4(config)#router bgp 100 R4(config-router)#no default-information originate R4(config)#ip route 44.1.1.1 255.255.255.255 null 0 R4(config)#ip route 0.0.0.0 0.0.0.0 Null0 R4#show runn | se router bgp router bgp 100 bgp log-neighbor-changes redistribute static neighbor 2.2.2.2 remote-as 100 neighbor 2.2.2.2 update-source Loopback0 neighbor 2.2.2.2 next-hop-self neighbor 34.1.1.1 remote-as 100 neighbor 34.1.1.1 next-hop-self neighbor 45.1.1.2 remote-as 50
在R4和R5上查看BGP路由情况
R4# show ip bgp BGP table version is 15, local router ID is 4.4.4.4 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *>i 1.1.1.1/32 2.2.2.2 0 100 0 10 i *> 5.5.5.5/32 45.1.1.2 0 0 50 i *> 44.1.1.1/32 0.0.0.0 0 32768 ? *> 55.5.5.5/32 45.1.1.2 0 0 50 i R5#show ip bgp BGP table version is 64, 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, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 1.1.1.1/32 45.1.1.1 0 100 10 i *> 5.5.5.5/32 0.0.0.0 0 32768 i *> 44.1.1.1/32 45.1.1.1 0 0 100 ? *> 55.5.5.5/32 0.0.0.0 0 32768 i
从上面输出可知，并没有默认路由，而收到了44.1.1.1/32的路由，从此可得出的结论：在重分布静态路由至BGP中时，并不会重分布默认路由。
总结：在BGP中配置default-information originate时，必须先创建一条静态的默认路由，并且重分布静态至BGP中，才会产生默认路由，并通告给其它的BGP邻居。