Users Guide

Autonomous system areas
OSPF operates in a 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 (IGRP) 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 identies.
Areas allow you to further organize routers within the AS with one or more areas within the AS. Areas 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 identies 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 distributes routing information between areas. It consists of all area border routers and networks not wholly contained in any area
and their attached routers.
The backbone is the only area with a default area number. You congure all other areas Area ID. If you congure 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.
• Totally stubby areas are also known as no summary areas.
600
Layer 3