3com S7906E Manuel De Montage

 

1-5 

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. 

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: 

z 

The backbone area cannot be a (totally) stub area. 

z 

To configure an area as a stub area, the stub command must be configured on routers in the area. 

z 

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.