the difference between totally stubby and NSSA is that the totally stubby area doesn't allow any summary routes from other areas, and no external routes. Instead it receives a default route from the area border router. The NSSA is basically the same as a stubby area in that it doesn't allow summary routes, but in the case where you have to redistribute connected routes or routes from another routing protocol, you can use an NSSA, which will allow external routes as Type 7 LSAs.
