Intra area routes are routes that are originated and learned in the same area. OSPF uses LSA type 1 and type 2 to advertise intra area routes. OSPF uses the type 1 and type 2 LSA to build the topology map of the area and to determine the best routes within the area. In routing table the intra area routes are identified as O in the routing table.
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. 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.
HTH
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