R1(config)#router ospf 10               //启用OSPF路由协议，进程ID为10
R1(config-router)#network 10.1.1.0 0.0.0.255 area 0   //指明直连网段10.1.1.0，注意后面跟的是子网掩码的反码
R1(config-router)#network 10.2.2.0 0.0.0.31 area 0
R1(config-router)#network 10.3.3.0 0.0.0.255 area 0
R1(config-router)#exit

R2(config)#router ospf 20
R2(config-router)#network 10.3.3.0 0.0.0.255 area 0
R2(config-router)#network 172.16.3.0 0.0.0.255 area 0
R2(config-router)#exit


R3(config)#router ospf 30
R3(config-router)#network 172.16.3.0 0.0.0.255 area 0
R3(config-router)#network 172.16.1.0 0.0.0.255 area 
R3(config-router)#network 172.16.1.0 0.0.0.255 area 0
R3(config-router)#network 172.16.2.0 0.0.0.31 area 0
R3(config-router)#exit

注：子网掩码的反码：就是用255.255.255.255减去子网掩码所得到的数，例如：255.255.255.224的掩码反码为：255.255.255.255-255.255.255.224=0.0.0.31，就和上面的10.2.2.0/27和172.16.2.0/27网段计算出来的一样。

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

     172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
O       172.16.1.1/32 [110/129] via 10.3.3.2, 00:00:02, Serial1/1
O       172.16.3.0/24 [110/128] via 10.3.3.2, 00:00:02, Serial1/1
O       172.16.2.1/32 [110/129] via 10.3.3.2, 00:00:02, Serial1/1
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.3.3.0/24 is directly connected, Serial1/1
C       10.2.2.0/27 is directly connected, Loopback1
C       10.1.1.0/24 is directly connected, Loopback0

R1#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
172.16.3.2        0   FULL/  -        00:00:36    10.3.3.2        Serial1/1

R1#show ip ospf database 

            OSPF Router with ID (10.2.2.1) (Process ID 10)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.2.2.1        10.2.2.1        37          0x80000004 0x003032 4
172.16.2.1      172.16.2.1      27          0x80000002 0x00330E 4
172.16.3.2      172.16.3.2      28          0x80000003 0x001EEF 4