How does an EIGRP router indicate that a path computation is required for a specific route?

EIGRP update packets with a topology change notification flag are used to inform neighbors about changes, but a query with an infinite metric is the specific mechanism for initiating a path computation for a lost route.

Sending an EIGRP update packet with a metric value of zero is not the standard way to indicate that a path computation is required; an infinite metric signals a route is unreachable or needs recomputation.

When an EIGRP router loses the successor and any feasible successor for a route, it enters an 'active' state for that route and sends EIGRP query packets to its neighbors. These queries signify that the router is actively looking for a new path, indicated by the route's metric (specifically, the delay) being set to infinity in the query.

Sending a route withdrawal notifies neighbors to remove a route from their topology tables because it's no longer available, whereas a query seeks an *alternative* path for a route that is now 'active' on the originating router.