What are common examiner traps in Distance Vector and Link-State Routing?

Assuming OSPF uses TCP or UDP rather than directly over IP (protocol number 89). Confusing intra-domain IGPs (OSPF, RIP) with inter-domain EGPs (BGP). Evaluating multi-round convergence in a question that explicitly asks for the state after 'the next round of updates' only. Forgetting that in Distance Vector, a node only uses the distance vectors received in the preceding round, not dynamically updating within the same exchange step unless specified. Believing split horizon completely prevents all routing loops (it only prevents two-node loops, not loops involving three or more nodes). Applying Dijkstra's SPF global topology logic to Distance Vector updates instead of neighbor-only vector additions.

GATE CSE Distance Vector and Link-State Routing PYQs

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GATE tests Distance Vector (DVR) and Link State Routing (LSR) through two primary lenses: step-by-step mathematical trace of Bellman-Ford vector updates and con… Read the guide

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Questions: 9
Asked: since 2011
Total marks: 13
Marks / paper: 0.5

9 GATE CSE Distance Vector and Link-State Routing PYQs with solutions, from Computer Networks. Browse all GATE CSE PYQs.

Questions

Every official paper that asked this topic, newest first.

What GATE tests in Distance Vector and Link-State Routing

Concepts, patterns, formulas and traps distilled from past papers.

GATE tests Distance Vector (DVR) and Link State Routing (LSR) through two primary lenses: step-by-step mathematical trace of Bellman-Ford vector updates and conceptual comparison of protocol implementations (RIP vs. OSPF). 2-mark questions focus on iterative distance vector computations after link cost changes or link failures, including routing loop and count-to-infinity dynamics. 1-mark questions evaluate protocol architecture, transport layer encapsulation, algorithmic complexity, and convergence properties.

Core concepts

Distance Vector Routing (Bellman-Ford algorithm, RIP, periodic neighbor exchange)
Link State Routing (Dijkstra's SPF algorithm, OSPF, LSA flooding to entire AS)
Next-hop determination from updated distance vectors
Count-to-infinity problem and routing loop formation
Split horizon and poison reverse mechanics and limitations
Transport layer encapsulation (RIP over UDP port 520, OSPF directly over IP proto 89, BGP over TCP)
Hierarchical OSPF design (backbone area 0, intra-domain vs inter-domain routing)
Computational overhead (O(|V|2) or O(|E| + |V| log |V|) for Dijkstra vs vector additions)

Recurring question patterns

Iterative Distance Vector Update Calculation
Commonmixed2 marks2021, 2011
Given a network topology or distance vectors of neighbors along with incident link costs, calculate a specific router's updated routing vector or next hop for designated destinations following a link cost change or link failure after exactly one round.
Protocol Property Comparison (RIP vs OSPF)
Commonmixed1 marks2025, 2023, 2017, 2014
Statement-based MCQs or MSQs assessing characteristics such as underlying algorithm (Bellman-Ford vs. Dijkstra), protocol layer encapsulation (RIP uses UDP port 520, OSPF encapsulates directly in IP protocol 89), routing scope (intra-domain IGP vs. inter-domain EGP), and hierarchical area support.
Convergence, Loop Formation, and Count-to-Infinity
Occasionalmixed2 marks2022, 2014
Questions evaluating behavior during link state changes, such as the comparative convergence speed, memory/computational overhead ( $O(V^2)$ vs hop additions), split horizon limitations, or computing the probability of transient routing loop formation.

Formulas GATE uses

Bellman-Ford Distance Vector Update

D_u(y) = \min_{v \in \text{Neighbors}(u)} \{ c(u, v) + D_v(y) \}

Used by a router $u$ to calculate the minimum cost to destination $y$ based on cost $c(u, v)$ to immediate neighbor $v$ and neighbor $v$ 's advertised distance $D_v(y)$ .

Independent Asynchronous Update Probability

P(\text{loop}) = P(\text{Node } A \text{ updates before Node } B) = \frac{\lambda_A}{\lambda_A + \lambda_B} = 0.5 \quad (\text{when } \lambda_A = \lambda_B)

Used to calculate the probability that an outdated neighbor advertises an old path before receiving the failure notice, triggering a count-to-infinity loop.

Examiner traps in Distance Vector and Link-State Routing

Assuming OSPF uses TCP or UDP rather than directly over IP (protocol number 89).
Confusing intra-domain IGPs (OSPF, RIP) with inter-domain EGPs (BGP).
Evaluating multi-round convergence in a question that explicitly asks for the state after 'the next round of updates' only.
Forgetting that in Distance Vector, a node only uses the distance vectors received in the preceding round, not dynamically updating within the same exchange step unless specified.
Believing split horizon completely prevents all routing loops (it only prevents two-node loops, not loops involving three or more nodes).
Applying Dijkstra's SPF global topology logic to Distance Vector updates instead of neighbor-only vector additions.

How questions are asked

Distance vector numerical update table / next hop selection
Multi-statement correctness evaluation (MCQ/MSQ)
Protocol and algorithm 2-column matching
Probability calculation of routing loop formation (NAT)

Paper trends

Transitioned from traditional linked MCQs calculating sequential rounds of distance vector updates to multi-select questions (MSQ) testing next-hop routing tables and protocol edge-case misconceptions.
2023, 2021, 2011
Introduction of probabilistic NAT problems analyzing asynchronous update order and count-to-infinity loop triggers.
2022
Consistent presence of fundamental 1-mark matching and multi-statement questions contrasting Bellman-Ford/RIP with Dijkstra/OSPF.
2025, 2017, 2014

Difficulty notes

Easy questions require standard matching or direct identification of routing protocols (RIP vs OSPF) and their underlying algorithms (Bellman-Ford vs Dijkstra). Medium questions require calculating single-round distance vector updates given neighbor arrays and link costs, or identifying multiple protocol properties in MSQ format. Hard questions involve multi-step failure propagations, asynchronous timing analysis for count-to-infinity loops, or assessing complex failure states across multi-hop topologies.