Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
Standard areas and backbone areas accept all link-state advertisements (LSAs). Every router in a standard area contains the same Open Shortest Path First (OSPF) database. If the standard area's ID number is 0, the area is a backbone area. The backbone area must be contiguous, and all OSPF areas must connect to the backbone area. If a direct connection to the backbone area is not possible, you must create a virtual link to connect to the backbone area through a nonbackbone area.
Stub areas, totally stubby areas, and not-so-stubby areas (NSSAs) flood only certain types of LSAs. For example, none of these areas floods Type 5, which are LSAs that originate OSPF autonomous system boundary routers (ASBRs). Instead, stub areas and totally stubby areas are injected with a single default route from an ABR. Routers inside a stub area or a totally stubby area will send all packets destined for another area to the area border router (ABR). In addition, a totally stubby area does not accept Type 3, 4, or 5 summary LSAs, which advertise inter-area routes. These LSAs are replaced by a default route at the ABR. As a result, routing tables are kept small within the totally stubby area.
An NSSA floods Type 7 LSAs within its own area, but does not accept or flood Type 5 LSAs. Therefore, an NSSA does not accept all LSAs. Similar to Type 5 LSAs, a Type 7 LSA is an external LSA that originates from an ASBR. However, Type 7 LSAs are only flooded to an NSSA.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 11, OSPF Stub Area Types, pp. 437-438 Cisco: What Are OSPF Areas and Virtual Links?: Normal, Stub, Totally Stub and NSSA Area Differences