OSPF backbone area 0
OSPF requires that a backbone area exist for all OSPF topologies. The backbone area is always defined as Area 0. All areas that are not Area 0 are considered non-backbone areas. All non-backbone areas must have an ABR that is also connected to Area 0. To find out why, take a look at OSPF why is a backbone area 0 necessary.
Area 0 must exist on at least one interface of an ABR router.