How to configure OSPF Virtual Link

Hello David

OSPF, by design, must have a contiguous backbone Area 0, and have all other areas directly connected to that backbone via ABRs. This is part of the fundamental design of the protocol. The primary reason for this is that the backbone area is used to efficiently distribute routing information throughout the network. It acts as a sort of hub, where routing information from non-backbone areas is advertised within the backbone, and then redistributed to other non-backbone areas. If you were to create a topology with a discontinuous backbone area, as depicted below, you may run into the following problems:

  • Routing Loops: If the backbone area is not contiguous, like the diagram above, it could lead to routing loops. This is because OSPF routers use the backbone area to share routing information. If the backbone area is discontiguous, some routers may not receive all the necessary routing information, which can lead to incorrect routing decisions and potential routing loops.
  • Incomplete Routing Tables: Another problem could be incomplete routing tables. If the backbone area is discontiguous, some routers might not have complete information about the network topology. This could lead to inefficient routing or even dropped packets if routers don’t know how to reach certain parts of the network.
  • Network Segmentation: In worst-case scenarios, a discontiguous backbone could lead to network segmentation, where parts of the network become unreachable from others.

To more fully understand the problems involved, try creating a topology like the one above, and don’t create a virtual link. See how routes are advertised to all the routers and see if routing is performed correctly.

I hope this has been helpful!