How to Solve the Problem that Default Routes Cannot Be Delivered to a Non-Backbone Area When a Neighbor in Full State Does Not Exist in the OSPF Backbone Area

In normal cases, the ABR automatically generates default Type 7 LSAs and advertises them to the NSSA only when at least a neighbor in Full state and an interface in Up state exist in the backbone area.

How to allow an NE20E-S to deliver default routes when a neighbor in Full state does not exist in the OSPF backbone area?

The backbone-peer-ignore command ignores the check of the neighbor status of the backbone area. Specifically, the ABR automatically generates default Type 7 LSAs to the NSSA when an interface in Up state exists in the backbone area, regardless of whether a neighbor in Full state exists.
Note that after the nssa default-route-advertise backbone-peer-ignore no-summary command is run, the ABR generates default Type 7 LSAs and default Type 3 LSAs when an interface in Up state exists in the backbone area, regardless of whether a neighbor in Full state exists. The default Type 3 LSAs take effect in priority over Type 7 LSAs.