OSPF operates within a single autonomous system (AS).
Networks within this single AS, however, can be divided into a number
of areas. By default, Area 0 is created. Area 0 can either function
alone or act as the OSPF backbone for a larger number of areas.
Each OSPF area is named using a 32-bit identifier which in most
cases is written in the same dotted-decimal notation as an IP4 address.
For example, Area 0 is usually written as 0.0.0.0.
The topology of an area is maintained in its own link state database
and is hidden from other areas, which reduces the amount of traffic
routing required by OSPF. The topology is then shared in a summarized
form between areas by a connecting router.
OSPF Area Type
The backbone area (Area 0) is the core of
an OSPF network. All other areas are connected to it and all traffic
between areas must traverse it. All routing between areas is distributed
through the backbone area. While all other OSPF areas must connect
to the backbone area, this connection doesn’t need to be direct
and can be made through a virtual link.
Normal OSPF Area
In a normal OSPF area there are no restrictions;
the area can carry all types of routes.
Stub OSPF Area
A stub area does not receive routes from
other autonomous systems. Routing from the stub area is performed
through the default route to the backbone area.
The Not So Stubby Area (NSSA) is a type
of stub area that can import external routes, with some limited