What is OSPF area design and types?
Why Interviewers Ask This
This is a differentiating question used for senior and lead roles. Interviewers want to see if you can explain not just what happens, but why — and what the trade-offs are in different approaches.
Answer
OSPF uses hierarchical area design to scale large networks. The backbone is Area 0 — all other areas must connect directly or virtually to it. Regular areas contain internal routers, receive full routing information. Stub area — does not accept external LSAs (routes redistributed into OSPF from other protocols); uses a default route instead — reduces LSDB size. Totally Stubby area (Cisco) — more restrictive; does not accept inter-area or external LSAs, uses only default route. NSSA (Not-So-Stubby Area) — like a stub area but allows limited redistribution of external routes via Type 7 LSAs (converted to Type 5 at ABR). Router types: Internal Router (all interfaces in one area), ABR (Area Border Router) (connects multiple areas), ASBR (Autonomous System Boundary Router) (redistributes routes from/to other routing domains). Good area design minimizes LSA flooding, reduces LSDB size, and limits SPF recalculations to specific areas.
Common Mistake
Many candidates answer correctly but can't explain the 'why'. Always be prepared to justify your answer with a concrete example or use case from your Networking experience.