What is the count-to-infinity problem in distance-vector protocols and how is it mitigated?

• A. A rapid change in all routes to infinity when a topology changes; mitigated by frequent updates.
• B. A mechanism where routes are quickly aged out using random timers.
• C. A slow route metric increase toward infinity due to incorrect updates; mitigated by split horizon, poison reverse, triggered updates, and hold-downs.
• D. A feature that prevents any routing updates after a failure for a fixed period.

Answer: (C)

Count-to-infinity in distance-vector routing happens when routers learn about a destination through each other in a way that creates a loop. If a route becomes invalid, a mistaken update can cause neighbors to keep advertising that same route, and each hop increases the reported distance by one. Because routers base their decisions on the distances received from neighbors, this can cause the metric to creep upward all the way toward infinity, even though the destination is unreachable. This slow convergence is the essence of the problem.

To prevent or reduce it, several techniques are used. Split horizon stops a router from advertising a route back onto the same interface from which it learned it, which helps break potential loops. Poison reverse takes that idea further by advertising the route back to the neighbor with an infinite metric, clearly signaling that the path via that neighbor is no longer usable. Triggered updates push information immediately when a route changes, rather than waiting for the regular refresh, accelerating convergence and reducing the chance that stale, looping information persists. Hold-downs place a recently invalid route into a temporary quiet state, preventing rapid oscillations and dampening the effects of unstable updates. Together these mechanisms address the loop creation and slow metric increases that lead to count-to-infinity.