Ticket#14 – OSPF Routes Missing: Area Type Mismatch (Stub vs NSSA) Fixed [CCNP Enterprise]

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Problem Summary

An enterprise network running OSPF faced missing routes in one part of the topology. The issue was traced to an OSPF area type mismatch — one router was configured as a Stub area, while its neighbor was configured as NSSA (Not-So-Stubby Area). This mismatch caused OSPF adjacency to form, but route types were filtered, leading to incomplete routing tables.

Symptoms Observed

• OSPF adjacency is up, but some routes (especially external or inter-area) are missing.
• Routes learned in one area are not propagating into another area.
• The OSPF database shows incomplete LSAs in the affected area.
• show ip route ospf shows fewer entries than expected.
• Application traffic depending on redistributed routes fails intermittently.

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Root Cause Analysis

• OSPF area 1 on Router A was configured as a Stub.
• OSPF area 1 on Router B was configured as an NSSA.
• Mismatch in area type doesn’t break adjacency but causes LSA filtering:
  • Stub blocks Type 5 LSAs (external routes).
  • NSSA allows external routes but uses Type 7 LSAs.
• This mismatch led to:
  • Route suppression of external and inter-area LSAs.
  • Routing table inconsistency and partial reachability.

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The Fix

1. Identify area types on both routers: show ip ospf show ip ospf interface brief
2. Compare configurations: show run | section router ospf
3. Fix the mismatch:
   • Decide on a unified area type — either both Stub or both NSSA.
   • Reconfigure as needed: router ospf 1 area 1 nssa ! or area 1 stub
4. Clear OSPF process (if needed) to refresh LSAs: clear ip ospf process

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EVE-NG Lab Topology

• R2 connects Area 0 and Area 1.
• R3 was configured as NSSA, while R2 had Area 1 as Stub.
• This caused missing external routes on R3.

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Verification

Commands to verify post-fix:

show ip ospf neighbor
show ip ospf database
show ip route ospf
show run | section router ospf

Check:

• All neighbors in FULL state.
• Presence of Type 7 LSAs if using NSSA.
• External routes correctly redistributed.
• No routing loop or route blackhole.

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Key Takeaways

• OSPF adjacency can still form even if area types are mismatched.
• Area type defines which LSAs are allowed — mismatches silently drop routes.
• Always maintain consistent area types between adjacent routers.
• Stub vs NSSA:
  • Stub: No external LSAs (Type 5), uses default route.
  • NSSA: Accepts redistributed routes using Type 7 LSAs.

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Best Practice / Design Tips

• Document all area types in network design.
• Use NSSA over Stub if redistribution is required in that area.
• Match area configuration on both sides of the interface.
• Avoid unnecessary stub area usage in core or redistribution zones.
• Use router ID and LSA checks to debug OSPF database mismatch.

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FAQs

1. Why did OSPF adjacency not fail despite mismatched area types?

OSPF neighbors still form adjacency as long as hello/dead intervals and area IDs match. Area type mismatches don’t stop adjacency but can block LSA types.

2. What is the main difference between Stub and NSSA areas?

Stub areas block external routes (Type 5), while NSSA allows external routes using Type 7 LSAs.

3. Can I use Stub areas in redistribution scenarios?

No. Stub areas do not support route redistribution. Use NSSA instead.

4. What are Type 5 and Type 7 LSAs?

Type 5 LSAs are used for external routes. Type 7 LSAs are used in NSSA areas and converted to Type 5 by the ABR.

5. How do I convert an area from Stub to NSSA safely?

Ensure both routers in that area are updated simultaneously and restart OSPF process.

6. Will changing the area type impact routing traffic?

Yes, routes may temporarily disappear during reconvergence. Schedule during a maintenance window.

7. What command shows current area types?

show ip ospf interface brief

8. What’s the role of no-summary in Stub/NSSA?

It makes the area a Totally Stubby or Totally NSSA — it blocks even inter-area routes, leaving only a default route.

9. Why are external routes missing in my NSSA area?

Check if ABR is configured to translate Type 7 to Type 5 and redistribution is active.

10. Can a Stub and NSSA area connect directly?

No. Mismatch in area types causes route suppression issues. Both sides must agree on area type.

11. Do area type changes require OSPF reset?

Yes, usually a clear ip ospf process is required for the changes to take full effect.

12. What’s the safest area type for branch sites?

Stub or NSSA with default route injection, depending on need for redistribution.

13. Can mismatched area types lead to routing loops?

Not loops, but blackholes where traffic is dropped due to missing routes.

14. How to confirm if Type 7 LSAs are present?

show ip ospf database nssa-external

15. Does NSSA support default route injection?

Yes, using area <id> nssa default-information-originate.

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YouTube Video

Watch the Complete CCNP Enterprise: OSPF Routes Missing: Area Type Mismatch (Stub vs NSSA) Fixed Lab Demo & Explanation on our channel:

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Final Note

Understanding how to differentiate and implement OSPF Routes Missing: Area Type Mismatch (Stub vs NSSA) Fixed is critical for anyone pursuing CCNP Enterprise (ENCOR) certification or working in enterprise network roles. Use this guide in your practice labs, real-world projects, and interviews to show a solid grasp of architectural planning and CLI-level configuration skills.

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