Meaning – Inter-area routes are routes that originated in some other OSPF area and are advertised into your area. Inter-area routes are advertised with LSA type 3. The type 3 LSA is generated by Border routers. So for example let us consider a Border router that has interfaces in area 0 and in area 5. It will take the intra area routes for area 5 (learned from the type 1 and type 2 LSAs) and will generate type 3 LSAs for area 0 and advertise these type 3 LSAs into area 0. The Border router will also take the intra area routes for area 0 and will generate type 3 LSAs for area 5 and will advertise the type 3 LSAs to other OSPF routers in area 5. As John tells us in the routing table the inter-area routes are identified as O IA.
In OSPF routing an intra area route will always be preferred over an equivalent inter-area route.
Example of usage – “When a route crosses an OSPF Area Border Router (ABR), the route is known as an OSPF Inter-Area route. These routes are flagged as O IA in the show IP route command output.”
