Which IGP uses the Dijkstra algorithm?

• A. OSPF
• B. RIP
• C. EIGRP
• D. IS-IS

Answer: (A)

Dijkstra-based shortest-path calculation is used by SPF, the method behind most link-state interior gateways. In this approach, a router first builds a complete view of the network topology by exchanging and collecting link-state advertisements, forming a Link-State Database. When it’s time to compute routes, the router runs the Dijkstra algorithm on that topology to generate a shortest-path tree rooted at itself. The resulting tree tells the router the best next hop for every destination, based on the chosen cost metric.

Among the options, OSPF is the protocol that uses this SPF/Dijkstra process to determine routes. RIP, by contrast, relies on a distance-vector approach (Bellman-Ford) rather than a global graph search. EIGRP uses its own algorithm (DUAL) rather than the full Dijkstra-based SPF. IS-IS also uses a shortest-path calculation, but OSPF is the classic example most people associate with Dijkstra in the context of IGPs.