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
Area 0.0.0.0 Backbone
e2
Area 200.5.0.0
Router D
208.5.1.1
Area 192.5.1.0
Virtual Link
Router A
e8
Router E
206.5.1.1
Router B
Area Border
Router (ABR)
Router F
Router C
Autonomous System
Border Router (ASBR)
Area 195.5.0.0
Router G
RIP Router
Figure 8.1 OSPF operating in a network
Designated Routers in
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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