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Advanced Configuration and Management Guide
You can further limit the broadcast area of flooding by defining an area range. The area range allows you to
assign an aggregate value to a range of IP addresses. This aggregate value becomes the address that is
advertised instead all of the individual addresses it represents being advertised. You can assign up to 32 ranges
in an OSPF area.
An OSPF router can be a member of multiple areas. Routers with membership in multiple areas are known as
Area Border Routers (ABRs). Each ABR maintains a separate topological database for each area the router is
in. Each topological database contains all of the LSA databases for each router within a given area. The routers
within the same area have identical topological databases. The ABR is responsible for forwarding routing
information or changes between its border areas.
An Autonomous System Boundary Router (ASBR) is a router that is running multiple protocols and serves as a
gateway to routers outside an area and those operating with different protocols. The ASBR is able to import and
translate different protocol routes into OSPF through a process known as redistribution. For more details on
redistribution and configuration examples, see
“Enable Route Redistribution” on page 8-28.
Area Border
Router (ABR)
Autonomous System
Border Router (ASBR)
RIP Router
Area 192.5.1.0
Area 200.5.0.0
Area 195.5.0.0
Area 0.0.0.0 Backbone
Virtual Link
Router A
Router C
e8
Router B
Router D
Router E
Router F
Router G
206.5.1.1
208.5.1.1
e2
Figure 8.1 OSPF operating in a network
Designated Routers in Multi-Access Networks
In a network that has multiple routers attached, OSPF elects one router to serve as the designated router (DR)
and another router on the segment to act as the backup designated router (BDR). This arrangement minimizes
the amount of repetitive information that is forwarded on the network by forwarding all messages to the designated
router and backup designated routers responsible for forwarding the updates throughout the network.
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