OSPF概述：
OSPF(Open Shortest Path First開放式最短路徑優先）是一個內部網關協議(Interior Gateway Protocol，簡稱IGP），用於在單一自治系統（autonomous system,AS）內決策路由。是對鏈路狀態路由協議的一種實現，隸屬內部網關協議（IGP），故運作於自治系統內部。
Router-ID：
每一臺OSPF路由器只有一個Router-ID，Router-ID使用IP地址的形式來表示，確定Router-ID的方法爲：
★1 .手工指定Router-ID。
★2 .路由器上活動Loopback接口中IP地址最大的，也就是數字最大的，如C類地址優先於B類地址，一個非活動的接口的IP地址是不能被選爲Router-ID的。
★3 .如果沒有活動的Loopback接口，則選擇活動物理接口IP地址最大的。
注：如果一臺路由器收到一條鏈路狀態，無法到達該Router-ID的位置，就無法到達鏈路狀態中的目標網絡。
Router-ID只在OSPF啓動時計算，或者重置OSPF進程後計算。
使用設備爲思科2900，均在模擬環境中完成
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 10.1.1.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface fastEthernet 1/0
R1(config-if)#ip address 40.4.4.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface loopback 0
R1(config-if)#ip address 1.1.1.1 255.255.255.255
R1(config-if)#no shutdown
R1(config-if)#exit
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip address 10.1.1.2 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface fastEthernet 1/0
R2(config-if)#ip address 20.2.2.1 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface loopback 0
R2(config-if)#ip address 2.2.2.2 255.255.255.255
R2(config-if)#no shutdown
R2(config-if)#exit
R3(config)#interface fastEthernet 1/0
R3(config-if)#ip address 20.2.2.2 255.255.255.252
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface fastEthernet 0/0
R3(config-if)#ip address 30.3.3.1 255.255.255.252
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface loopback 0
R3(config-if)#ip address 3.3.3.3 255.255.255.255
R3(config-if)#no shutdown
R3(config-if)#exit
R4(config)#interface fastEthernet 0/0
R4(config-if)#ip address 30.3.3.2 255.255.255.252
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface fastEthernet 1/0
R4(config-if)#ip address 40.4.4.2 255.255.255.252
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface loopback 0
R4(config-if)#ip address 4.4.4.4 255.255.255.255
R4(config-if)#no shutdown
R4(config-if)#exit
R1(config)#router ospf 1
R1(config-router)#network 1.1.1.1 0.0.0.0 area 0
R1(config-router)#network 10.1.1.1 0.0.0.3 area 0
R1(config-router)#network 40.4.4.1 0.0.0.3 area 0
R1(config-router)#exit
R2(config)#router ospf 1
R2(config-router)#network 2.2.2.2 0.0.0.0 area 0
R2(config-router)#network 20.2.2.1 0.0.0.3 area 0
R2(config-router)#network 10.1.1.2 0.0.0.3 area 0
R2(config-router)#exit
R3(config)#router ospf 1
R3(config-router)#network 3.3.3.3 0.0.0.0 area 0
R3(config-router)#network 30.3.3.1 0.0.0.3 area 0
R3(config-router)#network 20.2.2.2 0.0.0.3 area 0
R3(config-router)#exit
R4(config)#router ospf 1
R4(config-router)#network 4.4.4.4 0.0.0.0 area 0
R4(config-router)#network 30.3.3.2 0.0.0.3 area 0
R4(config-router)#network 40.4.4.2 0.0.0.3 area 0
R4(config-router)#exit
R1(config)#do show ip route
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 10.1.1.2, 00:00:40, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 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 subnets
O 4.4.4.4 [110/2] via 40.4.4.2, 00:00:40, FastEthernet1/0
20.0.0.0/30 is subnetted, 1 subnets
O 20.2.2.0 [110/2] via 10.1.1.2, 00:00:42, FastEthernet0/0
40.0.0.0/30 is subnetted, 1 subnets
C 40.4.4.0 is directly connected, FastEthernet1/0
10.0.0.0/30 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, FastEthernet0/0
30.0.0.0/30 is subnetted, 1 subnets
O 30.3.3.0 [110/2] via 40.4.4.2, 00:00:44, FastEthernet1/0
R1#ping 2.2.2.2
Type 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 ms
R1#ping 3.3.3.3
Type 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 ms
R1#ping 4.4.4.4
Type 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 ms
R1#traceroute 3.3.3.3
Type 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 msec
R2(config)#interface fastEthernet 0/0
R2(config-if)#shutdown
R2(config-if)#exit
R1#traceroute 3.3.3.3
Type 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