In distance-vector routing, what best defines a triggered update and when is it used?

• A. A periodic update sent at fixed intervals to refresh routes.
• B. A message sent only to the directly connected neighbor when a route changes.
• C. An immediate update sent when a topology change occurs, speeding convergence beyond regular timer-based updates.
• D. An update that is used to hold routing information during a topology change.

Answer: (C)

Triggered updates happen when a topology change is detected, and they are sent immediately to speed up convergence. In distance-vector routing, routers normally share their distance vectors at regular, fixed intervals. If a link goes down or a route becomes invalid, waiting for the next timer could leave other routers with outdated information for longer, risking loops or dropped traffic. An immediate update delivers the new distance information right away, so neighbors can recompute their routes and propagate the new facts quickly across the network.

This update is typically sent to all directly connected neighbors, not just one, so the new topology information spreads promptly. The idea isn’t about holding or delaying routing data during a change; it’s about broadcasting the change as soon as it’s known. The option describing periodic timer updates captures the normal behavior, while the one about holding information describes a hold-down-like mechanism, which isn’t the same as a triggered update.