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.

Links:

https://networklessons.com/ospf/introduction-to-ospf

https://networklessons.com/ospf/basic-ospf-configuration

https://networklessons.com/ospf/ospf-multi-area-configuration

https://forum.networklessons.com/t/ospf-abr-type-3-lsa-filtering-on-cisco-ios/1050/62?u=lagapides