The main difference between an OSPF NSSA area and a stub area is?

An NSSA (Not-So-Stubby Area) is designed to allow the importation of external routes, specifically from an Autonomous System Boundary Router (ASBR). This characteristic differentiates it significantly from a stub area, which does not permit external routes to be introduced. In an NSSA, external routes can be injected into the routing table, but they are treated differently than they would be in a normal area.

This means that while NSSA areas can utilize the benefits of summarized routing and reduced overhead associated with stub areas, they still allow for a controlled method of bringing in external routing information. Having this capability enables better flexibility and connectivity for complex networking scenarios where both OSPF and external routing protocols are in play.

The other options highlight characteristics that do not align with the definitions and functionality of NSSA and stub areas. Stub areas prevent external routes altogether, ensuring that their routing table remains simpler and focused solely on internal OSPF routes. The constraints on the use of ABRs (Area Border Routers) and the number of ASBRs in each type of area also support this fundamental difference in design purpose and operational capacity between these two OSPF area types.