1 What are OSPF virtual links?

OSPF virtual links are special types of interfaces that allow two routers to exchange OSPF information as if they were directly connected, even if they are not. OSPF virtual links use the existing network topology to create a tunnel between the routers, and they use area 0 as the transit area. OSPF virtual links can be useful for scenarios such as connecting a discontiguous area, extending an area across a non-OSPF network, or creating a backup path for an area.

2 How to configure OSPF virtual links?

To configure an OSPF virtual link, you need to identify the two routers that will form the endpoints of the link, and the area that they will use as the transit area. The transit area must be area 0, or an area that is directly connected to area 0. You also need to assign a cost to the virtual link, which will affect the routing decisions. The cost should reflect the distance and bandwidth of the underlying path. To configure an OSPF virtual link, you need to use the following command on both routers:

router ospf process-id
network network-address wildcard-mask area area-id
area transit-area-id virtual-link router-id        

The process-id is the identifier of the OSPF process on the router, the network-address and wildcard-mask specify the interface that belongs to the area, the area-id is the area number of the endpoint router, the transit-area-id is the area number of the transit area, and the router-id is the OSPF router ID of the other endpoint router.

5 How to verify and troubleshoot OSPF virtual links?

To verify and troubleshoot OSPF virtual links, you can use various commands and tools on the routers. Some of the most useful commands are:

- show ip ospf virtual-links : This command displays information about the status, cost, and neighbors of the virtual links.

- show ip ospf neighbor : This command displays information about the OSPF adjacencies, including the virtual links. You can use this command to check if the virtual link is established and synchronized.

- show ip route ospf : This command displays the OSPF routing table, including the routes learned through the virtual links. You can use this command to check if the routing is correct and consistent.

- ping and traceroute : These commands can help you test the connectivity and latency of the virtual link and the transit area. You can use these commands to identify and isolate any problems along the path.