356 | Open Shortest Path First (OSPFv2)
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Autonomous System (AS) Areas
OSPF operates in a type of hierarchy. The largest entity within the hierarchy is the AS, which is a
collection of networks under a common administration that share a common routing strategy (Figure 20-1).
OSPF is an intra-AS (interior gateway) routing protocol, although it is capable of receiving routes from
and sending routes to other ASs.
You can divide an AS into a number of areas, which are groups of contiguous networks and attached hosts.
Routers with multiple interfaces can participate in multiple areas. These routers, area border routers
(ABRs), maintain separate databases for each area. Areas are a logical grouping of OSPF routers identified
by an integer or dotted-decimal number.
Areas allow you to further organize your routers within the AS. One or more areas are required within the
AS. Areas are valuable in that they allow sub-networks to “hide” within the AS, thus minimizing the size
of the routing tables on all routers. An area within the AS may not see the details of another area's
topology. AS areas are known by their area number or the router’s IP address.
Figure 20-1. Autonomous System Areas
Router A
Router D
Router B
Router C
Router E
Router F
Router
Router H
Router I
Router J
Router L
Router K
Router M
Area 100
Area 200
Area 300
Area 0
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