My understanding: The NSSA can receive route information from the external AS but cannot receive external AS route info from internal areas so to perform this operation they must be running BGP and OSPF.
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You have a misunderstanding; there is nothing in OSPF which has any requirement for BGP, or any other routing protocol. The OSPF concept of AS doesn't mean that it must be a BGP AS; it means that it is a network outside the scope of the OSPF areas connected to the area 0 for the OSPF area.
The outside AS can be as simple as connected routes of a router which are not natively injected into OSPF. Using redistribution to get the connected routes into OSPF makes them external routes (routes from a different AS) inside OSPF.
In fact, you could have two OSPF ASes which can redistribute routes from one to the other using redistribution. As far as OSPF is concerned redistribution from any source into OSPF is injecting routes from a different AS.
A stub area which has routes redistributed into is would need to be NSSA.
answered Mar 7, 2016 at 2:59
