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In the main figure, area 3 does not have a
direct physical connection to the backbone (area 0), which is an
OSPF requirement because the backbone is a collection point for LSAs.
ABRs forward summary LSAs to the backbone, which in turn forwards
the traffic to all areas. All interarea traffic transits the
backbone.
To provide connectivity to the backbone, a virtual link must be
configured between R2 and R1. Area 1 will be the transit area and R1
will be the entry point into area 0. R2 will have a logical
connection to the backbone through the transit area.
Both sides of the virtual link must be configured, as follows:
area 1 virtual-link 10.3.10.5
--- With this command,
area 1 is defined to be the transit area and the router ID of
the other side of the virtual link is configured.
R1:
area 1 virtual-link 10.7.20.123
--- With this
command, area 1 is defined to be the transit area and the router
ID of the other side of the virtual link is configured.
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