Configuring an ABR to use a virtual link to the backbone

All ABRs must have either a direct, physical or indirect, virtual link to the OSPF backbone area (0.0.0.0 or 0.) If an ABR does not have a physical link to the area backbone, the ABR can use a virtual link to provide a logical connection to another ABR having a direct physical connection to the area backbone. Both ABRs must belong to the same area, and this area becomes a transit area for traffic to and from the indirectly connected ABR.

NOTE:

A backbone area can be purely virtual with no physical backbone links. Also, virtual links can be "daisy chained." If so, the virtual link may not have one end physically connected to the backbone.

Because both ABRs in a virtual link connection are in the same OSPF area, they use the same transit area ID. This setting is automatically determined by the ABRs and should match the area ID value configured on both ABRs in the virtual link.

The ABRs in a virtual link connection also identify each other with a neighbor router setting:
  • On the ABR having the direct connection to the backbone area, the neighbor router is the IP address of the router interface needing a logical connection to the backbone.

  • On the opposite ABR (the one needing a logical connection to the backbone), the neighbor router is the IP address of the ABR that is directly connected to the backbone.

NOTE:

By default, the router ID is the lowest numbered IP address or (user-configured) loopback interface configured on the device.

When you establish an area virtual link, you must configure it on both of the ABRs (both ends of the virtual link.)