OSPF Stub Areas

OSPF Stub Area Concepts – Comprehensive Technical Notes

1. Introduction

In large-scale OSPF (Open Shortest Path First) networks, stub areas are used to optimize scalability and reduce unnecessary routing overhead. They achieve this by filtering specific LSA (Link-State Advertisement) types, minimizing the Link-State Database (LSDB) size, and replacing external routes with a default route.

Key Benefits:

• 🚀 Scalability & Performance – Smaller LSDB → faster SPF calculations, reduced CPU/memory usage.
• 🔒 LSA Filtering – Blocks certain LSA types from entering the area.
• 🌐 Default Route Injection – ABR injects a 0.0.0.0/0 summary route to guide traffic outside the area.

2. OSPF Area Types and LSA Behavior

OSPF Area Type	LSAs Allowed	LSAs Blocked	Default Route?
Standard Stub Area (OSA)	Type 1, 2, 3	Type 4, 5	Yes (ABR generates via Type 3 LSA)
Totally Stubby Area (TSA)	Type 1, 2	Type 3, 4, 5	Yes (ABR generates via Type 3 LSA)
Not-So-Stubby Area (NSSA)	Type 1, 2, 3, 7	Type 4, 5	Optional (manual via default-information-originate)
Totally NSSA (TNSSA)	Type 1, 2, 7	Type 3, 4, 5	Yes (ABR generates via Type 3 LSA)

Key LSA Handling

• Type 4 (ASBR Summary): Blocked in stub areas; suppresses ASBR reachability info.
• Type 5 (External): Blocks redistributed external routes (e.g., from BGP/EIGRP).
• Type 7 (NSSA External): Used only inside NSSA/TNSSA for redistributed routes; ABR converts Type 7 → Type 5 for backbone.
• Default Route: Always injected as Type 3 Summary LSA.

3. Configuration Commands

A. Standard Stub Area (OSA)

• All routers in area : area <area-id> stub

B. Totally Stubby Area (TSA)

• ABR only : area <area-id> stub no-summary

• Other routers : area <area-id> stub

C. Not-So-Stubby Area (NSSA)

• All routers in area : area <area-id> nssa

• Optional default route on ABR : area <area-id> nssa default-information-originate

D. Totally NSSA (TNSSA)

• ABR only : area <area-id> nssa no-summary

• Other routers : area <area-id> nssa

4. Technical Requirements & Restrictions

1. Uniform Configuration: All routers in an area must agree on stub type (checked via OSPF Hello options).

2. Backbone Restriction: Area 0 cannot be a stub area.

3. ASBR Restriction:

   • ❌ Not allowed inside Stub/Totally Stubby areas (no Type 5 LSAs).
   • ✅ Allowed inside NSSA/TNSSA (Type 7 LSAs supported).

4. Virtual Links: Cannot be created across stub areas.

5. Verification & Troubleshooting

🔍 Verification Commands

• Check OSPF Process:
  • OSPFv2 → ` show ip ospf `
  • OSPFv3 → ` show ipv6 ospf or show ospfv3 `
• Check Routes:
  • OSPFv2 → show ip route ospf
  • OSPFv3 → show ipv6 route ospf

✅ Example (TSA Routing Table):

O*IA 0.0.0.0/0 [110/2] via 10.34.1.3, GigabitEthernet0/0

⚡ Troubleshooting Adjacencies

• Common Issue: Mismatched stub type → adjacency failure.
• Debugging Commands:
  • debug ip ospf hello (OSPFv2)
  • debug ospfv3 hello (OSPFv3)
• Why? – Stub/Transit area bit mismatch in OSPF Hello packets.

6. Interview & Revision Key Points

• Stub areas = scalability + reduced LSDB.
• Totally stubby = most restrictive (only intra-area + default route).
• NSSA = allows redistribution (Type 7 LSAs).
• TNSSA = NSSA + no Type 3/4/5 LSAs.
• Area 0 cannot be stub.
• Virtual links not supported in stub areas.
• Always check default route injection (Type 3).

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