OSPF NSSA For Beginners (ABR As Default Gateway for External and Internal Addresses 3.2)

1. What is NSSA?
Answer: It is a Stub Area that allows ASBR to import external destinations into OSPF.

2. How does NSSA work?
Answer: Let's describe a scenario as follows::
- ASBR learns a new external address, creates a Type 7 LSA, and floods it into NSSA.
- Internal routers receive this LSA and create a new entry for this address in their routing tables.
- ABR receives a Type 7 LSA, converts it into an External LSA, and floods it into other areas. ABR indicates in the External LSA that it is the originating. That is, ABR behaves like an ASBR for routers in other areas.
- Routers in other areas think ABR is the ASBR for this external address specified in Type 5 LSA.

3. How do you compare Stub Area and NSSA?
Answer: Commons: They don't support Type 4, 5 LSA. Internal routes use ABR as defult gateway.
Differences: There are 3 differences:
1) Stub Area cannot import external destinations.
2) Stub Area routers use ABR to access outside world. NSSA routers access external addresses in two ways: a) For addresses announced by NSSA ASBR, use ASBR as next hop. b) For other external addresses, use ABR as the next hop.
3) NSSA uses Type 7 LSA
4) In NSSA, ABR plays dual roles. It is an ABR and ASBR. It creates Type 5 LSA for external addresses announced by ASBR and floods them into other areas.

4. What does ABR do to make routers in other areas think it is an ASBR?
Answer: The short answer is E bit. Let's assume a ABR connects area 0, 1. Area 1 is NSSA.
1) When configuring ABR as an NSSA router in area 1, ABR changes its area 0 Router LSAs by setting its E bit to 1. It means that ABR is an ASBR for area 0. Then ABR re-floods its Router LSA in area 0.
2). When area 1's ASBR learns a new external address, it floods a Type 7 LSA in area 1.
3) When ABR receives this LSA, it converts it to External LSA and floods the LSA into area 0.
4) When area 0 routers receive External LSA from ABR, they see ABR as the ASBR originating the LSA.

5. How does a host in a non-NSSA area send packets to external destinations discovered by NSSA's ASBR?
Answer. It is done in 3 steps:
1) Host uses the non-NSSA router as the default gateway and forwards packet to it.
2) The router in non-NSSA area forwards packets to next hop ABR. This external address is specified in a Type 5 LSA that was originated by ABR. As fasr as these routers (in non-NSSA area) concers, ABR is their ASBR for these external addresses.
3) ABR forwards packets to NSSA's ASBR.
4) ASBR forwards packets to the next hop in the alien AS>

This article is the FAQ of an OSPF simulation: No-So-Stub-Area. To play this interactive simulation, see external links listed below.