Hold-down timers make routers ignore potentially incorrect updates for a period after a route fails.
When a route becomes unreachable, the router starts a hold-down timer (typically seconds in RIP). During this period:
- Updates with same or worse metric are ignored
- Updates with better metric are accepted
- The route remains marked as "possibly down"
Why this helps: Old routing information might still circulate. Hold-down prevents accepting stale updates claiming the failed route is valid.