OSPF stub areas, totally stubby areas, and not-so-stubby (NSSA) areas are specialized area types that control the routing information distributed within the area. Stub areas allow only Type 1 and 2 LSAs, and the ABR will advertise a default route into the stub area. NSSAs function similarly but also allow an ASBR to advertise external routes using Type 7 LSAs, which the ABR converts to Type 5 LSAs. A totally stubby area receives no Type 3 LSAs, while a totally NSSA area also filters Type 3 LSAs and relies on a default route from the ABR.

1. OSPF Stub Areas

2. Area 0
Type 1/2
Area 1
Type 1/2
Type 3
Type 5
Type 4
ASBR
Type 4 LSA’s are injected into the backbone by the ABR of an area which
contains an ASBR. This ensures all other OSPF routers can reach the ASBR
ABR
Normal OSPF Area

3. Area 0
Type 1/2
Stub Area 1
Type 1/2
Type 3
R1(config-router)# area 1 stub
R2(config-router) # area 1 stub (Enabled on all routers in the area)
The ABR will advertise a default route into the stub area.
External Routes (LSA5) and ASBR advertisements (LSA4) are removed
R1-ABR
Default Route
Stub Area
R2
Type 5

4. Area 0
Type 1/2
Stub Area 1
Type 1/2
Type 3
R1(config-router)# area 1 stub no-summary
R2(config-router) # area 1 stub
The ABR will advertise a default route into the stub area.
A totally stubby area will not receive any type 3 LSAs
The stub no-summary config only goes on the ABR
R1-ABR
Default Route
Totally Stubby Area
R2

5. Area 0
Type 1/2
Stub Area 1
Type 1/2
The only problem with a stub and totally stubby area is neither type can
contain an ASBR as type 4 and type 5 LSAs are not permitted inside the
area. To solve this problem Cisco introduced the not-so-stubby area NSSA
R1(config-router)# area 1 nssa
R2(config-router) # area 1 nssa
All routers in the area must be configured as an nssa
Not-so-stubby Area
R1-ABR R2

6. Area 0
Type 1/2
Stub Area 1
Type 1/2
An NSSA makes use of type 7 LSAs. This allows an ASBR to advertise
external links to an ABR, which converts the type 7 LSAs into type 5 before
flooding them to the rest of the OSPF domain.
ABR does not originate default automatically
ABR
Not-so-stubby Area
ASBR
TYPE 7TYPE 5
TYPE 4

7. Area 0
Type 1/2
Stub Area 1
Type 1/2
An NSSA can function as a stub or totally stubby area. To define as a stub.
R1(config-router)# area 1 nssa
Type 3 LSAs will pass into and out of the area, Unlike a normal stub, the ABR will not inject
a default route into an NSSA unless explicitly configured.
R1(config-router)# area 1 nssa default-information-originate
R1-ABR
Not-so-stubby Area
R2-ASBR
TYPE 7TYPE 5
TYPE 4
Type 3

8. Area 0
Type 1/2
Stub Area 1
Type 1/2
router(config-router)# area 1 nssa default-information-originate
With this command configured the ABR will advertise a default route
ABR
Not-so-stubby Area
ASBR
TYPE 7TYPE 5
TYPE 4
Type 3
Default

9. Area 0
Type 1/2
Stub Area 1
Type 1/2
Finally there is one more area called the Totally not-so-stubby area, to configure this:
R1(config-router)# area 1 nssa no-summary
R2(config-router) # area 1 nssa
Type 3 LSAs are removed and the default route is advertised
R1-ABR
Totally not-so-stubby Area
R2-ASBR
TYPE 7TYPE 5
TYPE 4
Type 3Default