What is the purpose of hold-down timers in distance-vector protocols?

• A. To prevent premature changes by waiting a period before accepting new route information after a potential failure.
• B. To immediately replace a route with any new information.
• C. To count to infinity by increasing metrics gradually.
• D. To disable all updates during network instability.

Answer: (A)

Hold-down timers exist to stabilize routing information by delaying acceptance of new route updates for a destination after it might have failed. When a route is deemed unreachable, the router marks it with an infinite metric and starts the hold-down timer. During this period, it won’t immediately act on conflicting or fluctuating updates for that destination, even if those updates suggest a different path. This prevents premature changes, reduces route flapping, and avoids potential routing loops as the network settles. Once the timer expires, normal updates resume if the route has stabilized.

This isn’t about replacing a route immediately, nor about counting to infinity or disabling updates entirely; it’s about giving the network a chance to confirm that a route failure is real before changing the path.