LSA Types:
Type 1: Router LSA
Type 2: Network LSA
Type 3: Network summary LSA
Type 4: ASBR summary LSA
Type 5: AS external LSA
Type 6: Group membership LSA
Type 7: NSSA external LSA
Type 8: External attributes LSA
Type 9: Opaque LSA (link-local scope)
Type 10: Opaque LSA (area-local scope)
Type 11:Opaque LSA (AS scope)
Type 6 for Multicast OSPF.
1,2,3,4,5 and 7 are commonly used.
Type 8 to 11 not deployed.
Stub Areas
The purpose of having stubs in OSPF is to filter the LSAs flooding the area.
Backbone area and non-stub area:
Accepts Type 1,2,3,4,5 but drops Type 7
Stub area:
Accepts Type 1,2,3 but drops 4,5 and 7
Totally Stub area:
Accepts Type 1,2 but drops 3*,4,5 and 7.
*Exception: Type 3 with a single default route is accepted.
Not-so-stubby-area (NSSA)
Accepts Type 1,2,3,4 and 7 but drops 5.
Type 7 LSA only floods in NSSA, router receives Hello packets from neighbour router and observe the option, the N bit if set to 1 is a NSSA capable router if set to 0 is NSSA incapable. N bit must match to form adjacency. P bit if set will make the ABR in NSSA translate type 7 to type 5 and propagate type 5 LSA to other areas. If P bit is not set then ABR of NSSA will drop type 7 LSA and the advertisement only contain within NSSA. N/P bit coexists.
To form adjacency the E-bit and N-bit must match.
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