Reference Guide
Autonomous system areas
OSPF operates in a type of hierarchy. The largest entity within the hierarchy is the autonomous system (AS). The AS is a collection of
networks under a common administration that share a common routing strategy. OSPF is an intra-AS, interior gateway routing protocol that
receives routes from and sends routes to other AS.
You can divide an AS into several areas, which are groups of contiguous networks and attached hosts administratively grouped. Routers
with multiple interfaces can participate in multiple areas. These routers, called area border routers (ABRs), maintain separate databases for
each area. Areas are a logical grouping of OSPF routers that an integer or dotted-decimal number identies.
Areas allow you to further organize routers within the AS with one or more areas within the AS. Areas are valuable in that they allow
subnetworks to hide within the AS—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. An area number or the router’s IP address identies AS areas.
Areas, networks, and neighbors
The backbone of the network is Area 0, also called Area 0.0.0.0, the core of any AS. All other areas must connect to Area 0. An OSPF
backbone is responsible for distributing routing information between areas. It consists of all area border routers, networks not wholly
contained in any area and their attached routers.
The backbone is the only area with a default area number. You congure all other areas Area ID. If you congure two nonbackbone areas,
you must enable the B bit in OSPF. Routers, A, B, C, G, H, and I are the backbone, see Autonomous system areas.
• A stub area (SA) does not receive external route information, except for the default route. These areas do receive information from
interarea (IA) routes.
• A not-so-stubby area (NSSA) can import AS external route information and send it to the backbone as type-7 LSA.
• Stubby areas are also known as no summary areas.
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