ospf虚电路
1.配置各台路由器的IP地址，并且使用ping命令确认各路由器的直连口的互通性
2.首先配置R2，R3，R4的ospf协议，配置过程中注意区域号
3.完成配置后，查看R1 的路由表
R1#sho ip route
Gateway of lastresort is not set
172.16.0.0/16 is variably subnetted, 6subnets, 3 masks
C 172.16.1.0/24 is directly connected,Loopback0
L 172.16.1.1/32 is directly connected,Loopback0
C 172.16.255.0/30 is directly connected,Serial1/0
L 172.16.255.1/32 is directly connected,Serial1/0
O IA 172.16.255.4/30 [110/128] via172.16.255.2, 00:00:49, Serial1/0 R1仅学到area2的路由
4.查看R2的路由
R2#sho ip route
Gateway of lastresort is not set
172.16.0.0/16 is variably subnetted, 6subnets, 3 masks
O 172.16.1.1/32 [110/65] via172.16.255.1, 00:02:08, Serial1/0 R2仅学到R1所在area 0的路由
C 172.16.255.0/30 is directly connected,Serial1/0
L 172.16.255.2/32 is directly connected,Serial1/0
C 172.16.255.4/30 is directly connected,Serial1/1
L 172.16.255.5/32 is directly connected,Serial1/1
5.查看R3的路由
R3#sho ip route
Gateway of lastresort is not set
172.16.0.0/16 is variably subnetted, 5subnets, 3 masks
O IA 172.16.16.1/32 [110/65] via 172.16.255.10,00:04:49, Serial1/1 R3仅学到area 1的路由
C 172.16.255.0/24 is directly connected,Serial1/1
is directly connected,Serial1/0
L 172.16.255.6/32 is directly connected,Serial1/0
L 172.16.255.9/32 is directly connected,Serial1/1
6.查看R4的路由
R4#sho ip route
Gateway of lastresort is not set
172.16.0.0/16 is variably subnetted, 5subnets, 3 masks
C 172.16.16.0/24 is directly connected,Loopback0
L 172.16.16.1/32 is directly connected,Loopback0
C 172.16.255.0/24 is directly connected,Serial1/0
O IA 172.16.255.4/30 [110/192] via 172.16.255.9,00:02:59, Serial1/0 R4仅学到area 2 的路由
L 172.16.255.10/32 is directly connected,Serial1/0
7.通过以上查看，发现任何一台路由器都无法学习到完整的网络路由。产生此问题的主要原因是：骨干区域被area2分割，造成整个网络变成两个ospf的自治系统，所以相互之间都无法学习完整的路由
8.这种情况下，可以通过配置虚电路来解决骨干区域被分割的问题，在R2 与R3的路由上实施如下配置：
R2(config)#routerospf 1
R2(config-router)#area2 virtual-link 172.16.255.9 创建虚电路，命令中的area 2 指出一条虚电路存在于area 2中，Vritual-link 172.16.255.9指出创建虚电路的对端R3 的路由器的router id。
R3(config)#routerospf 1
R3(config-router)#area2 virtual-link 172.16.255.5 配置到R2 路由器的虚电路
9，查看任意一台路由器，确认路由学习情况，
R1#sho iproute
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6subnets, 3 masks
C 172.16.1.0/24 is directly connected,Loopback0
L 172.16.1.1/32 is directly connected,Loopback0
O IA 172.16.255.0/24 [110/192] via172.16.255.2, 00:30:21, Serial1/0
O 172.16.255.8/30 [110/128] via172.16.255.2, 00:14:19, Serial1/0
C 172.16.255.0/30 is directly connected,Serial1/0
L 172.16.255.1/32 is directly connected,Serial1/0
O IA 172.16.16.1/32 [110/193] via 172.16.255.2,00:30:21, Serial1/0 此时R1 已经学习到完整的网络路由。
10.此时，无法学习完整的路由问题已经被解决。Ospf Vritual-link 不仅可以解决普通区域与骨干区域非直连问题，还可以解决骨干被分割的问题，但是，此类问题一般是由网络迁移或是本身设计问题所造成的，ospf的虚电路仅仅是一种 网络过渡的解决方案。