OSPF why is a backbone area 0 necessary
OSPF by definition requires a backbone area 0 in order to operate correctly. This is done by definition and by design.
OSPF is a link-state routing protocol, which means that each router participating in OSPF must have a complete topology of the whole network including all of the participating OSPF routers. If an OSPF area grows too large, it will use many resources (CPU, memory, etc.) on the routers and it will slow down in its convergence time.
In order to make OSPF scalable to much larger networks, the concept of areas was introduced. As OSPF was being developed, much research went into its multi-area design. Based on the tests that engineers performed, they found that they can ensure a satisfactory convergence time only if all non-backbone areas are directly connected to area 0.
In addition, OSPF will not directly propagate routes from one non-backbone area into another, nor will it propagate routes learned from one backbone area back into another discontiguous backbone area.
It is for this reason that one of the prerequisites of the creation of non-backbone OSPF areas is that they must be directly connected to the backbone area 0.
Links
https://forum.networklessons.com/t/ospf-multi-area-configuration/1698/14?u=lagapides