Configuring an ABR to use a virtual link to the backbone

All OSPFv3 ABRs (area border routers) must have either a direct, physical or indirect, virtual link to the OSPFv3 backbone area (0.0.0.0 or 0). If an ABR does not have a physical link to the area backbone, it 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 note that virtual links can be linked in a series. If so, one end may not be physically connected to the backbone.

Because both ABRs in a virtual link connection are in the same OSPFv3 area, they use the same transit area ID. This setting is configured using area area-id virtual-link router-id in the router ospf3 context 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 router ID (in decimal or 32-bit dotted decimal format) 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 router ID (in decimal or 32-bit dotted decimal format) of the ABR that is directly connected to the backbone.

NOTE:

By default, the router ID is the lowest numbered IPv4 address or (user-configured) IPv4 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).