3Com S7906E Configuration Manual page 638

Figure 1-2 Virtual link application 1
Another application of virtual links is to provide redundant links. If the backbone area cannot maintain
internal connectivity due to a physical link failure, configuring a virtual link can guarantee logical
connectivity in the backbone area, as shown below.
Figure 1-3 Virtual link application 2
The virtual link between the two ABRs acts as a point-to-point connection. Therefore, you can configure
interface parameters such as hello packet interval on the virtual link as they are configured on physical
interfaces.
The two ABRs on the virtual link exchange OSPF packets with each other directly, and the OSPF
routers in between simply convey these OSPF packets as normal IP packets.
Stub area
The ABR in a stub area does not distribute Type-5 LSAs into the area, so the routing table size and
amount of routing information in this area are reduced significantly.
You can configure the stub area as a totally stub area, where the ABR advertises neither the
destinations to other areas nor external routes.
Stub area configuration is optional, and not every area is eligible to be a stub area. In general, a stub
area resides on the border of the AS.
The ABR in a stub area generates a default route into the area.
Note the following when configuring a (totally) stub area:
The backbone area cannot be a (totally) stub area.
To configure an area as a stub area, the stub command must be configured on routers in the area.
To configure an area as a totally stub area, the stub command must be configured on routers in the
area, and the ABR of the area must be configured with the stub [ no-summary ] command.
1-5