Which OSPF area type allows external routes?

The Not So Stubby Area (NSSA) is the correct choice because it allows external routes while limiting certain other traffic. Specifically, NSSAs can import external routes from outside the OSPF domain into the OSPF network. This is particularly useful for areas that need to connect to external networks (like a company’s LAN connecting to the internet) but still want to retain some level of route summarization and control.

In NSSAs, Type 7 LSA (Link-State Advertisement) can be used to advertise these external routes, which will then be translated into the more common Type 5 LSAs by the area border routers when propagating them into other OSPF areas. This helps maintain a balance between connectivity and OSPF efficiency.

Other area types, such as stub areas and totally stubby areas, are designed to limit the types of routes they receive, making them less capable of handling external routes. A normal area, on the other hand, does not have restrictions on external routes but is not specialized for cases when some level of external route management is desired. Therefore, NSSAs effectively serve the purpose of maintaining external connectivity within OSPF's structural architecture while imposing some limitations they require for operational efficiency.