CCNA OSPF interview questions test whether you understand link-state routing and can diagnose adjacency or route problems. These concise answers cover the concepts most often discussed in networking interviews.
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What is OSPF?
Open Shortest Path First is a link-state interior gateway protocol. Routers exchange link-state information, build a shared topology database, and run the shortest path first algorithm.
What metric does OSPF use?
OSPF uses cost. Interface cost is derived from bandwidth and the configured reference bandwidth. A route with the lowest total path cost is preferred.
What is an OSPF area?
An area limits the scope of link-state flooding and SPF calculations. Area 0 is the backbone, and other areas normally connect to it through an area border router.
How do OSPF routers become neighbors?
They exchange Hello packets and must agree on key parameters such as area ID, timers, authentication, network type, and compatible addressing.
What are DR and BDR?
On multi-access networks, the designated router and backup designated router reduce the number of adjacencies and organize LSA exchange. Election considers interface priority and router ID.
What is an OSPF router ID?
It is a 32-bit identifier used by the OSPF process. It can be configured manually; otherwise, device selection rules determine it when the process starts.
What is an LSA?
A Link-State Advertisement describes routing information used to build the link-state database. Different LSA types represent routers, networks, summaries, and external destinations.
Which commands troubleshoot OSPF?
Use show ip ospf neighbor, show ip ospf interface, show ip ospf database, show ip protocols, and show ip route ospf.
Why might an adjacency fail?
Common causes include area mismatch, timer mismatch, authentication problems, duplicate router IDs, passive interfaces, subnet mismatch, MTU issues, and ACL filtering.
Interview Practice
Build a three-router lab, establish neighbors, change one parameter at a time, and predict the result. Explain the verification command before making a correction.