OSPF Areas - Technical Notes
🛰️ OSPF Special Areas & LSAs – Complete Technical Notes
🧩 I. OSPF Link State Advertisement (LSA) Fundamentals
OSPF (Open Shortest Path First) operates using a Link State Database (LSDB), which contains Link State Advertisements (LSAs).
Each LSA type shares specific information about the OSPF topology.
Special areas (Stub, NSSA, etc.) function primarily by filtering or blocking certain LSA types to optimize performance.
📜 LSA Types Overview
| LSA Type | Name | Purpose / Content | Scope & Border Behavior | Prefix Info? |
|---|---|---|---|---|
| 1 | Router LSA | Lists router links, costs, and neighbors (in point-to-point). | Flooded only within the area. | ✅ Yes |
| 2 | Network LSA | Lists router IDs of all routers on a broadcast network (DR/BDR). | Flooded only within the area. | ❌ No (Router ID info only) |
| 3 | Summary LSA | Contains inter-area prefixes (O IA). Generated by ABRs. | ABR blocks LSA 1/2, translates into LSA 3 when moving between areas. | ✅ Yes |
| 4 | ASBR Summary LSA | Contains Router ID of the ASBR. | Generated only by the ABR. Used to calculate the path to ASBR. | ❌ No |
| 5 | External LSA | Contains external prefixes (e.g., redistributed RIP, EIGRP, BGP). | Flooded throughout the OSPF domain. | ✅ Yes |
| 7 | NSSA External LSA | External routes generated by an ASBR inside an NSSA. | Flooded only within NSSA. ABR translates to LSA 5. | ✅ Yes |
Insight:
The main goal of special OSPF areas is to reduce LSDB size and routing table entries, lowering CPU and memory usage by limiting LSA flooding.
🗺️ II. OSPF Special Area Types
Special OSPF areas modify which LSAs are flooded to reduce routing overhead.
They mainly block LSA 3 (inter-area), LSA 4 (ASBR Summary), and LSA 5 (External).
🔹 1. Stub Area
Purpose: Prevent external routing information (LSA 4, 5) from entering the area.
| Feature | Details |
|---|---|
| LSAs Blocked (Incoming) | LSA 4, LSA 5 |
| Default Route | ABR automatically generates a Type 3 Summary LSA (0.0.0.0) |
| Rules / Restrictions | - Cannot be Area 0 - No Virtual Links - No ASBR present - All routers must have stub flag set |
| Configuration | area <area-id> stub |
🔹 2. Totally Stub Area
Purpose: A stricter version of Stub Area that also blocks inter-area routes (LSA 3).
| Feature | Details |
|---|---|
| LSAs Blocked (Incoming) | LSA 3, LSA 4, LSA 5 |
| Database Content | Contains only LSA 1 (intra-area), LSA 2 (DR info), and one default LSA 3 (0.0.0.0) |
| Default Route | ABR automatically generates Type 3 default route |
| Rules / Restrictions | Same as Stub Area |
| Configuration | On ABR: area <area-id> stub no-summaryOn other routers: area <area-id> stub |
🔹 3. Not-So-Stubby Area (NSSA)
Purpose: Allows ASBR presence inside a stub area (for route redistribution).
| Feature | Details |
|---|---|
| LSAs Blocked (Incoming) | LSA 4, LSA 5 (from other areas) |
| Internal ASBR Behavior | Generates LSA 7 instead of LSA 5 |
| LSA 7 Translation | ABR converts LSA 7 → LSA 5 and floods it externally |
| Default Route | Not generated automatically |
| Manual Default Route | area <area-id> nssa default-information-originate |
| Configuration | On all routers: area <area-id> nssaOn ABR (for default route): area <area-id> nssa default-information-originate |
🔹 4. Totally Not-So-Stubby Area (Totally NSSA)
Purpose: Most restrictive area type that still allows ASBRs. Blocks LSA 3, 4, and 5.
| Feature | Details |
|---|---|
| LSAs Blocked (Incoming) | LSA 3, LSA 4, LSA 5 |
| Internal ASBR Behavior | Generates LSA 7, which ABR translates to LSA 5 |
| Default Route | ABR automatically generates Type 3 default route (0.0.0.0) |
| Configuration | On all routers: area <area-id> nssaOn ABR: area <area-id> nssa no-summary |
🌐 Summary – Default Route Generation
| Area Type | Blocked LSAs | Default Route Type (Generated by ABR) | Automatic? |
|---|---|---|---|
| Stub | LSA 4, LSA 5 | Type 3 (O IA) | ✅ Yes |
| Totally Stub | LSA 3, LSA 4, LSA 5 | Type 3 (O IA) | ✅ Yes |
| NSSA | LSA 4, LSA 5 | Type 7 (O N1/N2) | ❌ No |
| Totally NSSA | LSA 3, LSA 4, LSA 5 | Type 3 (O IA) | ✅ Yes |
⚙️ Key Takeaways
- LSA control = core of OSPF scalability.
- Stub & Totally Stub Areas limit external routes for smaller LSDBs.
- NSSA & Totally NSSA allow limited redistribution while restricting flooding.
- ABRs handle LSA generation, summarization, and translation.
- Ensure area-type consistency across routers to form stable adjacencies.
📘 Use Case Summary
| Environment | Recommended Area Type | Purpose |
|---|---|---|
| Backbone (Area 0) | Normal | Full LSDB visibility |
| Branch / Remote Sites | Stub / Totally Stub | Lightweight routing |
| Partner / Edge Zones | NSSA / Totally NSSA | Limited redistribution with control |
🧠 Interview Prep Questions
- What is the difference between Stub and Totally Stub areas?
- How does an NSSA handle redistributed routes?
- Which LSA types are area-scoped vs domain-wide?
- What happens if an ASBR exists inside a Stub Area?
- How does the ABR handle LSA 7 translation?
✅ End of OSPF Special Areas & LSAs Notes
🙌 Connect With Me
