Each AS has a backbone area that is responsible for distributing routing information between non-backbone areas. OSPF requires the following:
The backbone area itself must maintain connectivity to all non-backbone areas.
In practice, due to physical limitations, the requirements may not be satisfied. In this case, configuring OSPF virtual links is a solution.
After network partition, ABRs perform route summarization to reduce the number of LSAs advertised to other areas and minimize the effect of topology changes.