Summary

This is an example of configuring OSPF virtual-link on Cisco routers. The virtual-link provides a virtual connection to the backbone area.

Related article

Please also read the following articles about the uses of virtual-link and the configuration and verification commands in Cisco.

N-Study
OSPF Virtual-Link : Virtual area 0 point-to-point link
https://www.n-study.com/en/ospf-detail/ospf-virtual-link
OSPF virtual link is a virtual area 0 point-to-point link. Virtual-link is explained in this section.
N-Study
Configuring and Verifying OSPF Virtual-link [Cisco]
https://www.n-study.com/en/ospf-detail/cisco-configuring-ospf-virtual-link
This section describes the configuration and verification commands for OSPF virtual-link on Cisco routers.

Network Diagram

図 OSPF バーチャルリンクの設定例 ネットワーク構成
Figure OSPF Virtual-link Configuration Example Network Diagram

Initial Configuration

Proceed from the completed configuration in the area shown in the network diagram.

OSPF Virtual-link Configuration Example configuration file download(Init)
OSPF Virtual-link Configuration Example configuration file download(comp)

Configuration and Verification

Verify the routing table before configuring the virtual-link. If you use the show ip route command on R1 in area 0, you will see the following

R1

R1#show ip route
-- omitted --

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 3 subnets
C       10.0.12.0 is directly connected, FastEthernet0/0
O IA    10.23.23.0 [110/20] via 10.0.12.2, 00:00:01, FastEthernet0/0
C       10.0.1.0 is directly connected, Loopback0

Before configuring virtual-link, it is not able to learn the route information for Area 34 that is not connected to Area 0. Naturally, communication with the Area 34 network is not possible.

Configure virtual-link to connect area 34 to the backbone area. The routers to configure virtual-link are R2 and R3. Area 23, to which R2 and R3, which configure virtual-link, commonly belong, is the transit area.

R2

router ospf 1
 area 23 virtual-link 3.3.3.3

R3

router ospf 1
 area 23 virtual-link 2.2.2.2

Verify that virtual-link is functioning properly. Look at the show ip ospf virtul-links command on R2.

R2

R2#show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 3.3.3.3 is up
  Run as demand circuit
  DoNotAge LSA allowed.
  Transit area 23, via interface FastEthernet0/1, 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 00:00:08
    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 msec

Figure Connecting Area 34 to Area 0 via Virtual-l
Figure Connecting Area 34 to Area 0 via Virtual-link

By establishing virtual-link between R2 and R3, Area 0 will be extended. Area 34 will also be connected to Area 0, and the routes in Area 34 can be learned successfully. Pinging from R1 to 10.34.4.0/24 in area 34 will return a response.

R1

R1#show ip route
-- omitted --

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 5 subnets
C       10.0.12.0 is directly connected, FastEthernet0/0
O IA    10.34.34.0 [110/30] via 10.0.12.2, 00:07:02, FastEthernet0/0
O IA    10.23.23.0 [110/20] via 10.0.12.2, 00:19:33, FastEthernet0/0
C       10.0.1.0 is directly connected, Loopback0
O IA    10.34.4.0 [110/31] via 10.0.12.2, 00:07:02, FastEthernet0/0
R1#ping 10.34.4.4 source 10.0.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.34.4.4, timeout is 2 seconds:
Packet sent with a source address of 10.0.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/84/96 ms

How the OSPF works