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In practice, the requirements may not be satisfied due to lack of physical links. OSPF virtual links can
solve this problem.
A virtual link is established between two ABRs through a non-backbone area and is configured on both
ABRs to take effect. The non-backbone area is called a transit area.
In the following figure, Area 2 has no direct physical link to the backbone area 0. You can configure a
virtual link between the two ABRs to connect Area 2 to the backbone area.
Figure 18
Virtual link application 1
Virtual links can also be used to provide redundant links. If the backbone area cannot maintain internal
connectivity due to the failure of a physical link, you can configure a virtual link to replace the failed
physical link, as shown in
Figure 19
Virtual link application 2
The virtual link between the two ABRs acts as a point-to-point connection. You can configure interface
parameters such as hello interval on the virtual link as they are configured on a physical interface.
The two ABRs on the virtual link unicast OSPF packets to each other, and the OSPF routers in between
convey these OSPF packets as normal IP packets.
Stub area
A stub area does not distribute Type-5 LSAs, so the routing table size and amount of routing information
in this area are reduced significantly. The ABR generates a default route into the area.
You can configure the stub area as a totally stub area, where the ABR advertises neither inter-area routes
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.
When you configure a totally stub area, follow these guidelines:
•
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.
Area 0
Area 1
Virtual link
R2
R1
