步驟一：分別爲R1、R2、R3配置接口地址，保證直連；
步驟二：啓用OSPF路由協議；
R2(config)#router os 100
R2(config-router)#router-id 2.2.2.2
R2(config-router)#net 202.101.12.2 0.0.0.0 a 0
R2(config-router)#net 202.101.23.2 0.0.0.0 a 1
（R1，R3同R2配置）
R2#sh ip route
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 202.101.12.1, 00:00:58, Serial1/3
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/65] via 202.101.23.3, 00:00:14, Serial1/2
（在OSPF中環回口在路由表中是32位主機路由，如果想讓之成爲你配置的，則需要將環回口類型改爲p-to-p）
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:34 202.101.12.1 Serial1/3
3.3.3.3 0 FULL/ - 00:00:29 202.101.23.3 Serial1/2