OSPFv2 and OSPFv3: multi-area, adjacencies, network types, area types (stub, totally stubby, NSSA), summarization and filtering

OSPF scales by dividing the network into areas. Area 0 is the backbone, and every other area must connect to it (directly or through a virtual link). Routers inside an area share an identical LSDB. An ABR (area border router) has interfaces in area 0 and another area and passes summarized information between them as type 3 summary LSAs. An ASBR (autonomous system boundary router) redistributes external routes, which appear as type 5 LSAs. Inside an area, type 1 (router) and type 2 (network, from the DR) LSAs describe the topology. Areas limit the scope of SPF recalculation and LSA flooding.