GATE CS 2023 Q.25: Routing Algorithms (Distance Vector, Link State) and BGP/OSPF

Which of the following statements is/are INCORRECT about the OSPF (Open Shortest Path First) routing protocol used in the Internet?

OSPF implements Bellman-Ford algorithm to find shortest paths.

OSPF uses Dijkstra's shortest path algorithm to implement least-cost path routing.

OSPF is used as an inter-domain routing protocol.

OSPF implements hierarchical routing.

Official answer and solution

Solution

Recall the operational characteristics and algorithm used by OSPF.

OSPF is a Link-State routing protocol that uses Dijkstra's algorithm to compute the shortest-path tree from the source router to all destinations. Bellman-Ford is used by Distance-Vector protocols like RIP. Thus, statement (A) is INCORRECT.
OSPF uses Dijkstra's algorithm. Thus, statement (B) is CORRECT.
OSPF is an Intra-domain (Interior Gateway Protocol, IGP) routing protocol designed for use within a single Autonomous System (AS). BGP is the protocol used for inter-domain routing. Thus, statement (C) is INCORRECT.
OSPF supports hierarchical routing by dividing an Autonomous System into Areas (with Area 0 as the backbone). Thus, statement (D) is CORRECT.

The question asks for INCORRECT statements, so (A) and (C) are the correct answers.

Final answer

Options (A, C)

Common trap

Selecting the correct statements instead of the INCORRECT ones requested in the stem.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

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