How are the default routes configured in one internal router transmitted to other internal router in an area ? Is it by Type 5 LSA ? Then it should be transmitted to all through the AS as Type 5.Correct me if I am wrong.
As soon as an OSPF router originates a type-5 LSA that carries the default route it becomes an ASBR, even if it does not originate any other external routes.
If you try this out on a router, you will see that this router will now set the E-bit in its router-LSA (type-1 LSA), signifying that it considers itself to be an ASBR.