Draytek vigor2960 series User Manual page 427

Item Description
Rather than probe the neighbor immediately, however, delay
sending probes for a short while in order to give upper layer
protocols a chance to provide reachability confirmation.
PROBE - The neighbor is no longer to be reachable, and
unicast Neighbor Solicitation probes are being sent to verify
reachability.
Vigor2960 Series User's Guide
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