| To control the traffic types that go
in and out of the various types of areas, you need certain types of
OSPF routers, as shown in the main figure.
The router types are as follows:
--- As discussed previously, routers that
have all their interfaces within the same area are called internal
routers. Internal routers in the same area have identical
link-state databases and run a single copy of the routing
algorithm.
Backbone router --- Routers that are attached to the backbone
area of the OSPF network are called backbone routers. They have at
least one interface connected to area 0 (the backbone area). These
routers maintain OSPF routing information using the same procedures
and algorithms as internal routers.
Area Border Router (ABR) --- Routers with interfaces attached
to multiple areas. These routers maintain separate link-state
databases for each area to which they are connected, and route
traffic destined to or arriving from other areas. ABRs are exit
points for the area, which means routing information destined for
another area can only travel there via the local area's ABR. ABRs
summarize information about the attached areas from their link-state
databases and distribute the information into the backbone. The
backbone ABRs then forward the information to all other connected
areas. An area can have one or more ABR.
Autonomous System Boundary Router (ASBR) --- Routers that have
at least one interface connected to an external internetwork
(another autonomous system), such as a non-OSPF network. These
routers can import non-OSPF network information to the OSPF network,
and visa versa (this is referred to as redistribution).
A router can be more than one router type. For example, if a
router interconnects to area 0 and area 1, as well as to a non-OSPF
network, it would be both an ABR and ASBR.
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