任务 5：配置默认路由
步骤 1：在 R1 上配置静态路由，确保 R1 的路由表中获得本来自己没有的路由（非直连路由）
R1(config)#ip route 192.168.23.0 255.255.255.0 192.168.12.2 R1(config)#ip route 192.168.24.0 255.255.255.0 192.168.12.2 R1(config)#ip route 192.168.3.0 255.255.255.0 192.168.12.2 R1(config)#ip route 192.168.4.0 255.255.255.0 192.168.12.2
分析这个步骤中的配置，发现配置的所有静态路由的下一跳都是 192.168.12.2，所有完全可以用一条路由 （目标地址为所有 IP）来替代步骤 1 配置的多条静态路由
步骤 2：在 R1 上，使用 no ip routing 关闭路由功能，再用 ip routing 开启路由功能，这 样可以删除所有关于路由的配置
R1(config)#no ip routing R1(config)#ip routing
步骤 3：检查 R1 的路由表，确保现在确实只有直连网络路由
R1#show 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 C 192.168.12.0/24 is directly connected, FastEthernet0/0.12 C 192.168.1.0/24 is directly connected, Loopback0
步骤 4：在 R1 上，创建一条默认路由，下一跳是 192.168.12.2（R2）
R1(config)#ip route 0.0.0.0 0.0.0.0 192.168.12.2
步骤 5：检查 R1 的路由表，会看到一条“S*”的路由，这条就是默认路由
R1#show 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 192.168.12.2 to network 0.0.0.0 C 192.168.12.0/24 is directly connected, FastEthernet0/0.12 C 192.168.1.0/24 is directly connected, Loopback0 S* 0.0.0.0/0 [1/0] via 192.168.12.2
步骤 6：在 R3 和 R4 上也分别增加默认路由，分别以 Fa0/0.23 和 Fa0/0.24 为出站接口
R3(config)#ip route 0.0.0.0 0.0.0.0 fastEthernet 0/0.23 R3#show 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 0.0.0.0 to network 0.0.0.0 C 192.168.23.0/24 is directly connected, FastEthernet0/0.23 C 192.168.3.0/24 is directly connected, Loopback0 S* 0.0.0.0/0 is directly connected, FastEthernet0/0.23 R3#
R4(config)#ip route 0.0.0.0 0.0.0.0 fastEthernet 0/0.24 R4#show 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 0.0.0.0 to network 0.0.0.0 C 192.168.24.0/24 is directly connected, FastEthernet0/0.24 C 192.168.4.0/24 is directly connected, Loopback0 S* 0.0.0.0/0 is directly connected, FastEthernet0/0.24
步骤 7：到这一步为止，这个网络的任何两个接口之间都实现了互通，可以使用 ping 测试， 下面以 R1 的 Loopback0 到 R3 的 Loopback0 的测试为例：
R1#ping 192.168.3.1 source loopback 0 //指定源目的PING模拟器不支持，可直接ping 192.168.3.1 ping 192.168.4.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.3.1, timeout is 2 seconds: Packet sent with a source address of 192.168.1.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 20/31/40 ms R1#