GATE CS 2023 Q15 — Routing Protocols: Shortest Path, Flooding, Distance Vector, Link State

To identify the INCORRECT statements about the OSPF (Open Shortest Path First) routing protocol, let us analyze each option:

• Statement A: "OSPF implements Bellman-Ford algorithm to find shortest paths."
  Analysis: OSPF is a link-state routing protocol and uses Dijkstra's algorithm to compute the shortest paths. The Bellman-Ford algorithm is used by distance-vector routing protocols such as RIP (Routing Information Protocol). Thus, this statement is INCORRECT.


• Statement B: "OSPF uses Dijkstra's shortest path algorithm to implement least-cost path routing."
  Analysis: This statement is correct. Each router running OSPF constructs a complete topology map (Link State Database) of its area and runs Dijkstra's algorithm to find the least-cost path to all destinations.


• Statement C: "OSPF is used as an inter-domain routing protocol."
  Analysis: OSPF is an intra-domain routing protocol (Interior Gateway Protocol, or IGP) used to route packets within a single Autonomous System (AS). Protocols like BGP (Border Gateway Protocol) are used for inter-domain routing. Thus, this statement is INCORRECT.


• Statement D: "OSPF implements hierarchical routing."
  Analysis: This statement is correct. OSPF allows an Autonomous System to be divided hierarchically into multiple areas (with a backbone area, Area 0, at the core) to improve scalability and reduce routing overhead.

Since the question asks for the INCORRECT statements, options A and C are the correct choices.