Backbone area

Every AS must have one (and only one) backbone area (identified as area 0 or The ABRs of all other areas in the same AS connect to the backbone area, either physically through an ABR or through a configured, virtual link. The backbone is a special type of normal area and serves as a transit area for carrying the inter-area-prefix-LSAs, AS-external-LSAs, and routed traffic between non-backbone areas, as well as the router-LSAs and network-LSAs and routed traffic internal to the area. ASBRs are allowed in backbone areas.