Stub Area Types
| Type | Blocks | Default Route | Use Case |
|---|---|---|---|
| Stub | Type 5 | ABR injects Type 3 | Standard stub |
| Totally Stubby | Type 3, 4, 5 | ABR injects Type 3 | Maximum reduction (Cisco only) |
| NSSA | Type 5 | Optional | Stub + redistribution |
| Totally NSSA | Type 3, 4, 5 | ABR injects Type 3 | NSSA + max reduction |
Stub Area Configuration
Rules
- All routers in area must be configured as stub
- No ASBRs allowed in stub area
- Area 0 cannot be stub
Configuration
ABR (R1):
router ospf 1
area 10 stub
Internal Router (R4):
router ospf 1
area 10 stub
That's it! ABR automatically injects default route
Verification
R4 (internal router):
R4# show ip route ospf
O*IA 0.0.0.0/0 [110/2] via 172.16.14.1, Gi0/0
O IA 10.0.12.0/30 [110/2] via 172.16.14.1, Gi0/0
Key: O*IA = Default route from ABR as inter-area (Type 3)
Totally Stubby Area (Cisco Proprietary)
Blocks: Type 3, 4, 5 LSAs
Allows: Only Type 1, 2, and default route
Configuration
ABR only:
router ospf 1
area 10 stub no-summary
Internal routers:
router ospf 1
area 10 stub
Note: Only ABR needs no-summary
Verification
R4:
R4# show ip route ospf
O*IA 0.0.0.0/0 [110/2] via 172.16.14.1, Gi0/0
Only default route—no other inter-area routes!
NSSA (Not-So-Stubby Area)
Use when: Stub area needs limited redistribution
Example: Branch router redistributing static route
Configuration
All routers in area:
router ospf 1
area 10 nssa
ASBR in NSSA (R4 redistributing static):
ip route 172.16.50.0 255.255.255.0 Null0
router ospf 1
area 10 nssa
redistribute static subnets
How NSSA Works
- ASBR generates Type 7 LSA (NSSA external)
- Type 7 floods within NSSA
- ABR converts Type 7 → Type 5 LSA
- Type 5 floods to rest of OSPF domain
Verification
R4 (NSSA internal):
R4# show ip ospf database nssa-external
LS Type: Type-7 AS External Link
Link State ID: 172.16.50.0
Advertising Router: 4.4.4.4
R1 (ABR):
R1# show ip ospf database external
LS Type: AS External Link
Link State ID: 172.16.50.0
Advertising Router: 4.4.4.4 ← Type 7 converted to Type 5
Totally NSSA
Combines: NSSA + Totally Stubby
ABR only:
router ospf 1
area 10 nssa no-summary
Internal routers:
router ospf 1
area 10 nssa
Default Route in NSSA
By default: NSSA doesn't get default route from ABR
To inject default:
router ospf 1
area 10 nssa default-information-originate
Configuration Example
Topology
Area 0: R1 (ABR) --- R2
Area 10 (NSSA): R1 --- R4 (ASBR redistributing static)
R1 (ABR)
router ospf 1
router-id 1.1.1.1
area 10 nssa default-information-originate
network 10.0.12.0 0.0.0.3 area 0
network 172.16.14.0 0.0.0.3 area 10
R4 (NSSA ASBR)
ip route 172.16.50.0 255.255.255.0 Null0
router ospf 1
router-id 4.4.4.4
area 10 nssa
network 172.16.0.0 0.0.255.255 area 10
redistribute static subnets
Summary
Now you know:
✅ Stub area — Blocks Type 5, ABR injects default
✅ Totally stubby — Blocks Type 3, 4, 5 (Cisco only)
✅ NSSA — Stub + allows redistribution via Type 7
✅ Totally NSSA — NSSA + blocks Type 3, 4, 5
✅ Configuration — All routers must agree on area type
Next Step:
Stub areas block LSAs. Sometimes you need OSPF Virtual Links to connect broken areas.
Internal Links:
- ← OSPF Authentication (Article 13)
- → OSPF Virtual Links (Article 15)
- → LSA Types Explained (Article 26)
