OSPF area types

OSPF is built upon a hierarchy of network areas. All areas for a given OSPF domain reside in the same AS. An AS is defined as a number of contiguous networks, all of which share the same interior gateway routing protocol.

An AS can be divided into multiple areas. Each area represents a collection of contiguous networks and hosts, and the topology of a given area is not known by the internal routers in any other area. Areas define the boundaries to which types 1 and 2 LSAs are broadcast, which limits the amount of LSA flooding that occurs within the AS and also helps to control the size of the LSDBs maintained in OSPF routers. An area is represented in OSPF by either an IP address or a number. Area types include:
  • Backbone

  • Normal

  • Not-so-stubby (NSSA)

  • Stub

All areas in an AS must connect with the backbone through one or more ABRs. If a normal area is not directly connected to the backbone area, it must be configured with a virtual link to an ABR that is directly connected to the backbone. The remaining area types do not allow virtual link connections to the backbone area.

Example of an AS with multiple areas and external routes