虚连接（Virtual-link）：由于网络的拓扑结构复杂，有时无法满足每个区域必须和骨干区域直接相连的要求，为解决此问题，OSPF提出了虚链路的概念。
虚连接是设置在两个路由器之间，这两个路由器都有一个端口与同一个非主干区域相连。虚连接被认为是属于主干区域的，在OSPF路由协议看来，虚连接两端的两个路由器被一个点对点的链路连接在一起。在OSPF路由协议中，通过虚连接的路由信息是作为域内路由来看待的。
命令分析
Router(config-router)#area transit area-id virtual-link router-id
1， 在OSPF的路由选择配置模式下，
2， Transit area-id ：转换区域的区域号
3， Router-id ：虚连接到目标的路由ID
4， 两个ABR上都需要配置。
注意
如果其中一台路由器配置了虚连接，而另外一台没有配置，则会提示这样的信息。
*Mar 1 01:48:10.907: %OSPF-4-ERRRCV:Received invalid packet: mismatch area ID,from backbone area must be virtual-link but not found from 2.2.2.2,Serial0/1
如果配置成功则会提示这样的信息
*Mar 1 00:41:32.795: %OSPF-5-ADJCHG: Process 3, Nbr 4.4.4.4 on OSPF_VL2 from LOADING to FULL, Loading Done
area area-id virtual-link router-id
Defines a virtual link between ABRs.
如果上述方式还不容易理解，你也可以将virtual-link可以想象成：将R2与R3合并成一台路由器，如下图：
r1#sh ip ro
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
O IA 193.1.1.0/24 [110/20] via 192.1.1.2, 00:00:57, Ethernet0/0
C 192.1.1.0/24 is directly connected, Ethernet0/0
O IA 194.1.1.0/24 [110/74] via 192.1.1.2, 00:00:24, Ethernet0/0
r2#sh ip ro
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
C 193.1.1.0/24 is directly connected, Ethernet1/0
C 192.1.1.0/24 is directly connected, Ethernet0/0
C 194.1.1.0/24 is directly connected, Serial2/0
r3#sh ip ro
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
C 193.1.1.0/24 is directly connected, Ethernet1/0
O IA 192.1.1.0/24 [110/20] via 193.1.1.2, 00:01:06, Ethernet1/0
O IA 194.1.1.0/24 [110/74] via 193.1.1.2, 00:00:37, Ethernet1/0
r4#sh ip ro
4.0.0.0/24 is subnetted, 1 subnets
C 4.4.4.0 is directly connected, Loopback0
O IA 193.1.1.0/24 [110/74] via 194.1.1.2, 00:00:17, Serial2/0
O IA 192.1.1.0/24 [110/74] via 194.1.1.2, 00:00:17, Serial2/0
C 194.1.1.0/24 is directly connected, Serial2/0
Vani(config)#do sh ip osp v
Virtual Link OSPF_VL0 to router 3.3.3.3 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface Ethernet0/1, Cost of using 10
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:06
Adjacency State FULL (Hello suppressed)
Index 2/3, retransmission queue length 0, number of retransmission 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 mse
show ip ospf virtual-links
Displays information about OSPF virtual links.
可以使用上述命令检验virtual-links 配置
