Four different types of OSPF routers
exist, as shown in the figure:
- Internal router - 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) -
ABRs are routers with interfaces attached to multiple areas. They
maintain separate link-state databases for each area to which they
are connected, and they route traffic destined to or arriving from
other areas. ABRs are exit points for the area, which means that
routing information destined for another area can travel there only
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 ABRs.
- Autonomous System Boundary Router (ASBR)
- ASBRs are 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 vice 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 an ASBR.
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