What is OSPF?
Why Interviewers Ask This
Mid-level Networking roles require deep understanding of this topic. Interviewers ask this to separate candidates who truly understand the mechanics from those who only know surface-level concepts.
Answer
OSPF (Open Shortest Path First) is a link-state routing protocol used within a single autonomous system (interior gateway protocol). Each OSPF router maintains a complete map of the network topology (Link State Database/LSDB) and uses Dijkstra's SPF (Shortest Path First) algorithm to calculate the best path to every destination. Routers exchange Link State Advertisements (LSAs) to share topology information. OSPF is organized into areas to limit LSA flooding and reduce the size of routing tables — Area 0 is the backbone; all other areas must connect to Area 0. OSPF uses the cost metric (based on bandwidth) to determine the best path. OSPF quickly detects and responds to topology changes, converges fast, supports VLSM/CIDR, and has no hop count limit. Widely used in enterprise and ISP networks. OSPFv2 for IPv4, OSPFv3 for IPv6.
Pro Tip
Before answering, structure your response: one-line definition → real-world analogy → concrete example from a project. This makes even complex Networking answers easy to follow.