Default OSPF area type
2. Stub Area
External link (type 5) LSAs are replaced with
a single default route
3. Totally Stubby Area
Type 3, 4, and 5 LSAs are replaced with a
default route
4. Not-So-Stubby Area (NSSA)
A stub area containing an ASBR; type 5 LSAs
are converted to type 7 within the area
