How to Configure OSPF Not-So-Stubby Area (NSSA)

OSPF Stub Areas

1. Stub Area - No Type 4 ASBR or 5 AS-external LSAs allowed.
2. Totally Stub Area - No Type 3 Summary, 4 ASBR or 5 External LSAs allowed except the default summary route.
3. NSSA (Not-So-Stubby Area) - No Type 5 AS-external LSAs allowed,
but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse.
4. NSSA Totally Stub - No Type 3 Summary LSA, 4 ASBR or 5 External LSAs except the default summary route,
but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed.

NSSA:
This type of area allows the flexibility of importing a few external routes into the area while still trying to retain the stub characteristic.
Assume that one of the routers in the stub area is connected to an external AS running a different routing protocol, it now becomes the ASBR,
and hence the area can no more be called a stub area. However, if the area is configured as a NSSA,
then the ASBR generates a NSSA external link-state advertisement (LSA) (Type-7) which can be flooded throughout the NSSA area.
These Type-7 LSAs are converted into Type-5 LSAs at the NSSA ABR and flooded throughout the OSPF domain.

How to Configure OSPF Not-So-Stubby Area (NSSA)

CONFIGURE:
R1:
conf t
router ospf 1
network 10.10.12.0 0.0.0.255 area 0
redistribute connected subnets
end
wr

R2:
conf t
router ospf 1
network 10.10.12.0 0.0.0.255 area 0
network 10.10.23.0 0.0.0.255 area 1
end
wr

R3:
conf t
router ospf 1
network 10.10.23.0 0.0.0.255 area 1
redistribute connected subnets
end
wr

Convert Area 1 (Stub Area) to be a Not-So-Stubby Area (NSSA):
R2:
conf t
router ospf 1
area 1 nssa default-information-originate
end
wr


R3:
conf t
router ospf 1
area 1 nssa
end
wr


VERIFY:
show ip ospf neighbor
show ip route ospf
show ip ospf database
clear ip ospf process

REFERENCES:
Types of OSPF Areas
https://www.cisco.com/c/en/us/support...

OSPF Not-So-Stubby Area (NSSA)
https://www.cisco.com/c/en/us/support...

How OSPF Injects a Default Route into a Not So Stubby Area
https://www.cisco.com/c/en/us/support...

What Are OSPF Areas and Virtual Links?
https://www.cisco.com/c/en/us/support...

OSPF Database Explanation Guide
https://www.cisco.com/c/en/us/support...

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