Routing – Other Uses of OSPF Virtual-Link

ospfrouting

I am currently taking a CCNP-Route class. In our proposal paper we have been asked to implement an OSPF virtual-link. However, none of the OSPF areas are discontiguous from area 0. I questioned my professor on the matter and he insisted that there were other uses for virtual-links other than discontiguous areas. I am having trouble finding other use-cases. What other uses do virtual-links have?

I have included the image of our topology for reference.
Network Topology

Best Answer

You should get familiar with the Cisco documentation, and how to search for it. For example, OSPF Design Guide, Virtual Links:

Virtual Links - Virtual links are used for two purposes:

  • Linking an area that does not have a physical connection to the backbone.
  • Patching the backbone in case discontinuity of area 0 occurs.

You can also look at the RFCs related to the subject. For example, RFC 2328, OSPF Version 2:

  1. Virtual Links

The single backbone area (Area ID = 0.0.0.0) cannot be disconnected, or some areas of the Autonomous System will become unreachable. To establish/maintain connectivity of the backbone, virtual links can be configured through non-backbone areas. Virtual links serve to connect physically separate components of the backbone. The two endpoints of a virtual link are area border routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other area border router), and the non-backbone area the two routers have in common (called the Transit area). Virtual links cannot be configured through stub areas (see Section 3.6).

The virtual link is treated as if it were an unnumbered point-to- point network belonging to the backbone and joining the two area border routers. An attempt is made to establish an adjacency over the virtual link. When this adjacency is established, the virtual link will be included in backbone router-LSAs, and OSPF packets pertaining to the backbone area will flow over the adjacency. Such an adjacency has been referred to in this document as a "virtual adjacency".

In each endpoint router, the cost and viability of the virtual link is discovered by examining the routing table entry for the other endpoint router. (The entry's associated area must be the configured Transit area). This is called the virtual link's corresponding routing table entry. The InterfaceUp event occurs for a virtual link when its corresponding routing table entry becomes reachable. Conversely, the InterfaceDown event occurs when its routing table entry becomes unreachable. In other words, the virtual link's viability is determined by the existence of an intra-area path, through the Transit area, between the two endpoints. Note that a virtual link whose underlying path has cost greater than hexadecimal 0xffff (the maximum size of an interface cost in a router-LSA) should be considered inoperational (i.e., treated the same as if the path did not exist).

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