Reference Guide

630 | Open Shortest Path First (OSPFv2 and OSPFv3)
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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. 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 34-1. Autonomous System Areas
Area Types
The backbone of the network is Area 0. It is also called Area 0.0.0.0 and is the core of any AS.