Consider a network P-Q-R. Link Q-R fails. P and Q send routing updates at random times at the same a

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GATE-CS is a competitive examination conducted in India. This question is from the 2022 GATE-CS Computer Networks paper.

Q: How can I prepare for GATE-CS Computer Networks?

To prepare for GATE-CS Computer Networks, practice previous year questions (PYQs), understand concepts thoroughly, and solve mock tests regularly. This 2022 question covers Routing Protocols (Shortest Path, Flooding, Distance Vector, Link State) topic.

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This GATE-CS 2022 Computer Networks question has a medium difficulty level and is marked as an important question.

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Question

Consider a network P-Q-R. Link Q-R fails. P and Q send routing updates at random times at the same average rate. The probability of a routing loop forming between P and Q (leading to count-to-infinity) is ___ (rounded to one decimal place)

Accepted Answer

Answer: 0.5Concept -- Distance Vector Routing and Count-to-Infinity (from Kurose and Ross, Computer Networking):Before Q-R fails: Q knows distance to R = 1 (direct link). P knows distance to R = 2 (via Q).After Q-R fails: Q detects failure and updates its distance to R as infinity. Now Q needs to update P. P still has old distance 2 to R (via Q). If P sends its update to Q before Q propagates the failure to P: Q sees P has distance 2 to R, Q thinks it can reach R via P with cost 3. P then updates to cost 4 via Q, etc. This creates a routing loop (count-to-infinity).Whether a loop forms depends on who sends their update first. If Q sends first: Q tells P that R is unreachable, P correctly updates to infinity. No loop. If P sends first: P tells Q that R is reachable via P (cost 2), Q uses this incorrect route. Loop forms. Since both P and Q send at the same average rate and timing is random, P sends first with probability 1/2. Probability of routing loop = 0.5.

Consider a network P-Q-R. Link Q-R fails. P and Q send routing updates at random times at the same average rate. The probability of a routing loop forming between P and Q (leading to count-to-infinity) is ___ (rounded to one decimal place)

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Consider a network P-Q-R. Link Q-R fails. P and Q send routing updates at random times at the same average rate. The probability of a routing loop forming between P and Q (leading to count-to-infinity) is ___ (rounded to one decimal place)

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More Questions from Routing Protocols (Shortest Path, Flooding, Distance Vector, Link State)

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