An inter-area route reaches a destination in an OSPF area that the router does not connect to directly. It differs from an intra-area route, which reaches a destination within the router’s own area, and an external route, which reaches outside the OSPF autonomous system. section 18.1.2
Area 2 destination
An ABR connected to areas 0 and 1 that learns a route to area 2 has learned an inter-area route. In a multi-area OSPF design, traffic between non-backbone areas passes through area 0. section 18.1.2
Figure 18.2 A multi-area OSPF network consisting of four areas. Area 0 is the backbone area to which other areas must connect. Areas 1, 2, and 3 are non-backbone areas, and traffic between them must pass through area 0.
Use the multi-area topology to trace how area membership determines whether a destination is intra-area or inter-area. section 18.1.2
