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.

Distance Vector and Link-State Routing

9 Qs · since 2011 · 13 marks · 0.5 marks/paperStandard yield

Take topic test

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 o… Guide

Type

GATE CSE 2025 Set 2Q17MCQ1 markEasyDiagramConsider the routing protocols given in List I and the names given in List II: For matching of items in List I with those in List II, which ONE of the following options is CORRECT?
GATE CSE 2023Q25MSQ1 markEasyWhich of the following statements is/are INCORRECT about the OSPF (Open Shortest Path First) routing protocol used in the Internet?
GATE CSE 2022Q57NAT2 marksMediumDiagramConsider a network with three routers P, Q, R shown in the figure below. All the links have cost of unity. The routers exchange distance vector routing information and have converged on the routing tables, after which the link Q–R fails. Assume that P and Q send out routing updates at random times, each at the same…
GATE CSE 2021 Set 2Q45MSQ2 marksMediumDiagramConsider a computer network using the distance vector routing algorithm in its network layer. The partial topology of the network is as shown below. The objective is to find the shortest-cost path from the router $R$ to routers $P$ and $Q$ . Assume that $R$ does not initially know the shortest routes to $P$ …
GATE CSE 2017 Set 2Q9MCQ1 markMediumConsider the following statements about the routing protocols, Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) in an IPv4 network. I: RIP uses distance vector routing II: RIP packets are sent using UDP III: OSPF packets are sent using TCP IV: OSPF operation is based on link-state routing Which…
GATE CSE 2014 Set 1Q23MCQ1 markMediumConsider the following three statements about link state and distance vector routing protocols, for a large network with 500 network nodes and 4000 links. [S1] The computational overhead in link state protocols is higher than in distance vector protocols. [S2] A distance vector protocol (with split horizon) avoids…
GATE CSE 2014 Set 2Q23MCQ1 markEasyWhich one of the following is TRUE about the interior gateway routing protocols – Routing Information Protocol (RIP) and Open Shortest Path First (OSPF)?
GATE CSE 2011Q52MCQ2 marksMediumDiagramStatement for Linked Answer Questions 52 and 53: Consider a network with five nodes, N1 to N5, as shown below. The network uses a Distance Vector Routing protocol. Once the routes have stabilized, the distance vectors at different nodes are as following. N1: (0, 1, 7, 8, 4) N2: (1, 0, 6, 7, 3) N3: (7, 6, 0, 2, 6) N4:…
GATE CSE 2011Q53MCQ2 marksHardAfter the update in the previous question, the link N1-N2 goes down. N2 will reflect this change immediately in its distance vector as cost, $\infty$ . After the NEXT ROUND of update, what will be the cost to N1 in the distance vector of N3?

Topic guide

What does GATE test in Distance Vector and Link-State Routing?

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| |V|) for Dijkstra vs vector additions)

Recurring question patterns

Iterative Distance Vector Update Calculation
common · mixed · 2 marks · 2021, 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)
common · mixed · 1 marks · 2025, 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
occasional · mixed · 2 marks · 2022, 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.