Large network scale results in huge LSDB, consuming much storage space and affecting network performance. You can configure OSPF to divide the AS to different areas to solve such a problem.
The stub area is a special area defined by OSPF, which has only one ABR. When an area meets certain requirements, you can configure it as a stub area. In this case, OSPF routes can be quickly aggregated and effectively spread.
External routes cannot be imported to the stub area. To import external routes to the OSPF routing table and still keep other stub area features, the network administrator can configure the area as the NSSA area.
This describes how to configure a virtual link. Through the configuration, you can establish a logical channel between non-adjacent areas to satisfy the requirements of OSPF for areas.