What is route poisoning and when is it used?

• A. Advertises an unattainable distance to indicate a failed route, accelerating convergence and preventing stale routes.
• B. Floods all routes with the maximum metric to confuse neighbors.
• C. Uses split horizon to prevent routing loops.
• D. Forbids updating routes to discontiguous networks.

Answer: (A)

Route poisoning is signaling that a previously known route is no longer valid by advertising it with an infinite or unattainable metric. This makes neighboring routers immediately invalidate that path and look for alternatives, which speeds up convergence and helps prevent stale or looping routes from persisting in the network. It’s commonly used in distance‑vector protocols like RIP, where a route shown with the maximum metric is treated as unreachable. When a route goes down, the router propagates that “dead” route to its neighbors, encouraging rapid re‑routing. Some environments also use poison reverse to ensure the poisoned route isn’t sent back toward the router that advertised it, further helping to avoid loops.