OSPF (Open Shortest Path First) allows parts of a network to be grouped together into areas.

The topology within an area is hidden from the rest of the AS. Hiding this information enables a significant reduction in LSA traffic and reduces the computations needed to maintain the LSDB. Routing within the area is determined only by the topology of the area.

The three types of routers defined by OSPF are as follows:
  • Internal router (IR)—An internal router has all of its interfaces within the same area.

  • Area border router (ABR)—An ABR has interfaces in multiple areas. It is responsible for exchanging summary advertisements with other ABRs.

  • Autonomous system border router (ASBR)—An ASBR acts as a gateway between OSPF and other routing protocols, or other autonomous systems.