OSPF Review and Terminology

ASBR router
- This router functions as redistributing external routes from some other routing domain to OSPF domain.

ABR router
- This router function as connecting area 0 and other area

Stub Area 
- allow injection of external routes in a limited fashion in stub area
- this area does not allow redistribution

NSSA
- block type 5 and type 4 LSA
- allow type 3 LSA
- Type 7 only exists on NSSA area


What is Type 3
- The Type 3 LSA is originated by ABRs into one area to describe links in another area

What is Type 4
- The Type 4 LSA is originated by an ABR but it is used to describe ASBR in one area to routers in another areas

What is Type 5
- OSPF uses the Type5 external link LSAs to advertise external route originated from an ASBR.


What is Type 7
- The Type 7 that is generated by an NSSA ASBR. The 5 LSAs are not allowed in NSSA areas, so the NSSA ASBR generates a type 7 LSA ins tea. This type 7 LSA gets translated back into a type 5 by the NSSA ABR