278 CHAPTER 28: OSPF CONFIGURATION
Figure 75 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 76 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.
(Totally) 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 in other areas nor the 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.
■ The stub command must be configured on routers in a (totally) stub area.
Area 0
Area 1
Area 2
ABRABR
Transit Area
Virtual Link
Area 0
Area 1
Virtual Link
R2
R1
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