3Com MSR 50 Series Configuration Manual page 921

4 Autonomous System Border Router (ASBR)
The router exchanging routing information with another AS is an ASBR, which
may not reside on the boundary of the AS. It can be an internal router or area
border router.
Figure 270 OSPF router types
Area 1
Internal
Router
Backbone area and virtual links
An AS has a unique area called backbone area, which is responsible for
distributing routing information between none-backbone areas. Routing
information between non-backbone areas must be forwarded by the backbone
area. Therefore, OSPF requires:
All non-backbone areas must maintain connectivity to the backbone area.
■
The backbone area itself must maintain connectivity.
■
In practice, due to physical limitations, the requirements may not be satisfied. In
this case, configuring OSPF virtual links is a solution.
A virtual link is established between two area border routers via a non-backbone
area and is configured on both ABRs to take effect. The area that provides the
non-backbone area internal route for the virtual link is a "transit area".
In the following figure, Area 2 has no direct physical link to the backbone area 0.
Configuring a virtual link between ABRs can connect Area 2 to the backbone area.
RIP
IS-IS
ASBR
ABR
Area 2
Introduction to OSPF
Area 4
Backbone Router
Area 3
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