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OSPF has certain restrictions when multiple areas
are configured. One area must be defined as Area 0, the backbone
area. It is called the backbone because all inter-area communication
must go through it. Thus, all areas should be physically connected
to Area 0 so that the routing information injected into this
backbone can be disseminated to other areas. The backbone area must
always be configured as Area 0. You cannot make any other area ID
function as the backbone.
There are situations, however, when a new area is
added after the OSPF internetwork has been designed, and it is not
possible to provide that new area with direct access to the
backbone. In these cases, a virtual link can be defined to provide
the needed connectivity to the backbone area, as shown in Figure .
The virtual link provides the disconnected area a logical path to
the backbone. All areas must connect directly to the backbone area
or through a transit area, as shown in Figure .
The virtual link has the following two
requirements:
When virtual links are used, they require special
processing during the SPF calculation. That is, the "real"
next-hop router must be determined so that the true cost to reach a
destination across the backbone can be calculated.
Virtual links serve the following purposes:
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They can link an area that does not have a
physical connection to the backbone. This linking could occur,
for example, when two organizations merge.
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They can patch the backbone if discontinuity
in Area 0 occurs. Discontinuity of the backbone might occur, for
example, if two companies merge their two separate OSPF networks
into a single one with a common Area 0. The only alternative for
the companies is to redesign the entire OSPF network and create
a unified backbone.
Another reason for creating a virtual link is to
add redundancy in cases when router failure might cause the backbone
to be split into two.
In Figure ,
the disconnected Area 0s are linked via a virtual link through the
common area, Area 3. If a common area does not already exist, one
can be created to become the transit area. Area 0 could become
partitioned, for example, if two OSPF networks were merged.
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