OSPF概述：

OSPF(Open Shortest Path First开放式最短路径优先）是一个内部网关协议(Interior Gateway Protocol，简称IGP），用于在单一自治系统（autonomous system,AS）内决策路由。是对链路状态路由协议的一种实现，隶属内部网关协议（IGP），故运作于自治系统内部。

Router-ID：

每一台OSPF路由器只有一个Router-ID，Router-ID使用IP地址的形式来表示，确定Router-ID的方法为：

★1 .手工指定Router-ID。★2 .路由器上活动Loopback接口中IP地址最大的，也就是数字最大的，如C类地址优先于B类地址，一个非活动的接口的IP地址是不能被选为Router-ID的。★3 .如果没有活动的Loopback接口，则选择活动物理接口IP地址最大的。注：如果一台路由器收到一条链路状态，无法到达该Router-ID的位置，就无法到达链路状态中的目标网络。Router-ID只在OSPF启动时计算，或者重置OSPF进程后计算。

使用设备为思科2900，均在模拟环境中完成

R1(config)#interface fastEthernet 0/0

R1(config-if)#ip address 10.1.1.1 255.255.255.252R1(config-if)#no shutdown R1(config-if)#exitR1(config)#interface fastEthernet 1/0R1(config-if)#ip address 40.4.4.1 255.255.255.252R1(config-if)#no shutdown R1(config-if)#exitR1(config)#interface loopback 0 R1(config-if)#ip address 1.1.1.1 255.255.255.255R1(config-if)#no shutdown R1(config-if)#exitR2(config)#interface fastEthernet 0/0R2(config-if)#ip address 10.1.1.2 255.255.255.252R2(config-if)#no shutdown R2(config-if)#exitR2(config)#interface fastEthernet 1/0R2(config-if)#ip address 20.2.2.1 255.255.255.252R2(config-if)#no shutdown R2(config-if)#exitR2(config)#interface loopback 0 R2(config-if)#ip address 2.2.2.2 255.255.255.255R2(config-if)#no shutdown R2(config-if)#exitR3(config)#interface fastEthernet 1/0R3(config-if)#ip address 20.2.2.2 255.255.255.252 R3(config-if)#no shutdown R3(config-if)#exitR3(config)#interface fastEthernet 0/0R3(config-if)#ip address 30.3.3.1 255.255.255.252R3(config-if)#no shutdown R3(config-if)#exitR3(config)#interface loopback 0 R3(config-if)#ip address 3.3.3.3 255.255.255.255R3(config-if)#no shutdown R3(config-if)#exitR4(config)#interface fastEthernet 0/0R4(config-if)#ip address 30.3.3.2 255.255.255.252R4(config-if)#no shutdown R4(config-if)#exitR4(config)#interface fastEthernet 1/0R4(config-if)#ip address 40.4.4.2 255.255.255.252R4(config-if)#no shutdown R4(config-if)#exitR4(config)#interface loopback 0R4(config-if)#ip address 4.4.4.4 255.255.255.255R4(config-if)#no shutdown R4(config-if)#exitR1(config)#router ospf 1R1(config-router)#network 1.1.1.1 0.0.0.0 area 0R1(config-router)#network 10.1.1.1 0.0.0.3 area 0R1(config-router)#network 40.4.4.1 0.0.0.3 area 0R1(config-router)#exitR2(config)#router ospf 1 R2(config-router)#network 2.2.2.2 0.0.0.0 area 0R2(config-router)#network 20.2.2.1 0.0.0.3 area 0R2(config-router)#network 10.1.1.2 0.0.0.3 area 0R2(config-router)#exitR3(config)#router ospf 1R3(config-router)#network 3.3.3.3 0.0.0.0 area 0R3(config-router)#network 30.3.3.1 0.0.0.3 area 0R3(config-router)#network 20.2.2.2 0.0.0.3 area 0R3(config-router)#exitR4(config)#router ospf 1R4(config-router)#network 4.4.4.4 0.0.0.0 area 0R4(config-router)#network 30.3.3.2 0.0.0.3 area 0R4(config-router)#network 40.4.4.2 0.0.0.3 area 0R4(config-router)#exitR1(config)#do show ip route      1.0.0.0/32 is subnetted, 1 subnetsC       1.1.1.1 is directly connected, Loopback0     2.0.0.0/32 is subnetted, 1 subnetsO       2.2.2.2 [110/2] via 10.1.1.2, 00:00:40, FastEthernet0/0     3.0.0.0/32 is subnetted, 1 subnetsO       3.3.3.3 [110/3] via 40.4.4.2, 00:00:40, FastEthernet1/0                [110/3] via 10.1.1.2, 00:00:40, FastEthernet0/0     4.0.0.0/32 is subnetted, 1 subnetsO       4.4.4.4 [110/2] via 40.4.4.2, 00:00:40, FastEthernet1/0     20.0.0.0/30 is subnetted, 1 subnetsO       20.2.2.0 [110/2] via 10.1.1.2, 00:00:42, FastEthernet0/0     40.0.0.0/30 is subnetted, 1 subnetsC       40.4.4.0 is directly connected, FastEthernet1/0     10.0.0.0/30 is subnetted, 1 subnetsC       10.1.1.0 is directly connected, FastEthernet0/0     30.0.0.0/30 is subnetted, 1 subnetsO       30.3.3.0 [110/2] via 40.4.4.2, 00:00:44, FastEthernet1/0R1#ping 2.2.2.2Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-trip min/avg/max = 28/36/60 msR1#ping 3.3.3.3Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-trip min/avg/max = 56/100/144 msR1#ping 4.4.4.4Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-trip min/avg/max = 32/48/88 msR1#traceroute 3.3.3.3Type escape sequence to abort.Tracing the route to 3.3.3.3  1 40.4.4.2 36 msec    10.1.1.2 124 msec    40.4.4.2 32 msec  2 20.2.2.2 96 msec    30.3.3.1 64 msec    20.2.2.2 60 msecR2(config)#interface fastEthernet 0/0R2(config-if)#shutdown R2(config-if)#exitR1#traceroute 3.3.3.3Type escape sequence to abort.Tracing the route to 3.3.3.3  1 40.4.4.2 76 msec 60 msec 80 msec  2 30.3.3.1 92 msec 72 msec 60 msec