What is a stub area in OSPF?

A stub area in OSPF (Open Shortest Path First) is designed to optimize routing by limiting certain types of route advertisements to enhance efficiency within the area. Specifically, a stub area does not receive external link-state advertisements (LSAs), which originate from outside the OSPF area. This means that, while routers in a stub area can communicate with each other and know about other routes within their area, they cannot learn about external routes from other OSPF areas.

This characteristic helps reduce the size of the routing table and minimizes the amount of routing information exchanged. By restricting external LSAs, stub areas help improve OSPF performance by lowering the overhead associated with OSPF routing updates. This is particularly beneficial in scenarios where external routes are not necessary for the internal communication needs of the stub area.

In summary, the defining feature of a stub area is its limitation on receiving external LSAs, which helps in optimizing efficiency and reducing routing overhead. The other options do not accurately define a stub area in the context of OSPF.