How OSPF Determines the Cost(metric)

In OSPF, a single AS can be divided into smaller groups called areas.An area is a set of networks and hosts within an AS that have been administratively grouped together.

Areas are logical groupings of hosts and networks, including their routers having interfaces connected to any of the included networks. Each area maintains a separate link state database whose information may be summarized towards the rest of the network by the connecting router.

Backbone area
Stub area
Totally stubby area
Not-so-stubby area
The backbone area (also known as area 0.0.0.0) forms the core of an OSPF network. All other areas are connected to it, and inter-area routing happens via routers connected to the backbone area and to their own associated areas.

A stub area is an area which does not receive external routes except the default route, but does receive inter-area routes.

A totally stubby area (TSA), which is a nonstandard but useful extension by Cisco , is similar to a stub area, however this area does not allow summary routes in addition to the external routes, that is, inter-area (IA) routes are not summarized into totally stubby areas.

A not-so-stubby area (NSSA) is a type of stub area that can import autonomous system (AS) external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas

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