OSPF router types
Interior routers
This type of OSPF router belongs to only one area. Interior routers flood type-1 LSAs to all routers in the same
area and maintain identical LSDBs. In the following example, the routers R1, R3, R4, and R6 are all interior
routers because all of their links are to other routers in the same area.
Figure 39: Example of interior routers
Area border routers (ABRs)
This type of OSPF router has membership in multiple areas . ABRs are used to connect the various areas in an
AS to the backbone area for that AS. Multiple ABRs can be used to connect a given area to the backbone, and a
given ABR can belong to multiple areas other than the backbone.
An ABR maintains a separate LSDB for each area to which it belongs. (All routers within the same area have
identical LSDBs.) The ABR is responsible for flooding summary LSAs between its border areas. You can reduce
summary LSA flooding by configuring area ranges. An area range enables you to assign an aggregate address to
a range of IP addresses. This aggregate address is advertised instead of all the individual addresses it
represents. You can assign up to eight ranges in an OSPF area. In the following example, routers R2 and R5 are
ABRs because they both have membership in more than one area.
Figure 40: Example of deploying ABRs to connect areas to the backbone
Autonomous system boundary router (ASBR)
This type of OSPF router runs multiple interior gateway protocols and serves as a gateway to other autonomous
systems operating with interior gateway protocols. The ASBR imports and translates different protocol routes into
OSPF through redistribution. ASBRs can be used in backbone areas, normal areas, and NSSAs, but not in stub
areas. For more details on redistribution and configuration examples, see
on page
205.
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