OSPF Areas

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
Description
Backbone Area
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.
NSSA Area
The Not So Stubby Area (NSSA) is a type of stub area that can import external routes, with some limited exceptions.

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