OSPF Interview Questions and Answers
OSPF is the most-asked routing protocol in interviews. These questions separate people who memorised commands from those who understand why OSPF behaves as it does — which is exactly what interviewers probe. Ground the concepts in our OSPF guide and areas guide.
How to prepare
OSPF interviews reward understanding the state machine and troubleshooting logic. Know why neighbours get stuck (MTU, timers, area, subnet), what each LSA type does, and how the DR reduces adjacencies. If you can explain a "routes are missing" troubleshoot end to end, you're ahead of most candidates.
Frequently asked questions
What are the requirements for two routers to become OSPF neighbours?
Matching area ID, hello/dead timers, subnet/mask, authentication, and compatible MTU (for full adjacency). A mismatch in any prevents neighbourship.
What does the FULL neighbour state mean?
The routers have fully synchronised their link-state databases — a healthy adjacency. On multi-access networks, 2WAY between two non-DR routers is also normal.
Why would OSPF neighbours be stuck in EXSTART/EXCHANGE?
Almost always an MTU mismatch between the interfaces — align the MTUs or use ip ospf mtu-ignore.
What is the purpose of the DR and BDR?
On multi-access segments, the Designated Router (and Backup) reduce adjacencies — routers form full adjacency only with the DR/BDR instead of every peer, cutting LSA flooding.
How is the OSPF DR elected?
By highest interface priority; ties break on highest router ID. Priority 0 means never DR. There's no preemption — a new higher-priority router won't take over an existing DR.
What is OSPF area 0?
The backbone area — every other area must connect to it, and inter-area traffic transits it.
What is the difference between a Type 1 and Type 5 LSA?
Type 1 (Router) describes a router's own links within its area; Type 5 (External) advertises routes redistributed into OSPF from another source, flooded domain-wide.
How does OSPF calculate its metric?
Cost = reference bandwidth (default 100 Mbps) ÷ interface bandwidth, minimum 1. Lower total cost wins.
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