GATE 2022 CS – Question 57
Consider a network with three routers P, Q, R connected in a line P—Q—R. 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 average rate. The probability of a routing loop formation (rounded off to one decimal place) between P and Q, leading to count-to-infinity problem, is ___________.
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Correct answer: 0.5
Explanation
A loop forms only if P advertises its stale route to R to Q before Q advertises the failure to P. With both sending updates at random times at the same rate, either order is equally likely, so the probability is 0.5.