Which technique uses sending an unreachable metric to a failed route to prevent propagation of stale information?

• A. Split Horizon
• B. Route Poisoning (Poison Reverse)
• C. Triggered Updates
• D. Hold-Down Timers

Answer: (B)

When a route goes down, routers need to clearly tell neighbors that this path is no longer usable. Route poisoning does exactly that by advertising an unreachable metric for the failed route, effectively marking it as invalid so other routers stop using it and remove it from their tables.

In many RIP-like protocols, the unreachable metric is called infinity (for RIP, typically 16). By propagating this infinity, the network prevents stale information from circulating and helps avoid loops, since neighbors learn that the route is no longer valid.

A related technique, poison reverse, is a way of applying this poison on the interface toward a specific neighbor to ensure that neighbor doesn’t keep using a route that was learned from you. However, the core concept here is the act of advertising an unreachable metric to stop propagation of stale information.

Split horizon prevents sending updates back out the same interface, which helps avoid certain loops but doesn’t broadcast an unreachable metric. Triggered updates are about sending updates immediately when changes occur, not about labeling a route as unreachable. Hold-down timers delay accepting changes to a route to prevent flapping, but they don’t inform neighbors that the route is gone.