Sets how long a neighbor is considered reachable after the device gets positive confirmation of connectivity, per the IPv6 Neighbor Discovery protocol and this object's description. Admins tune this to control how often the device re-verifies neighbor reachability; too short wastes bandwidth on unnecessary re-probing, while too long delays detection of a neighbor that has gone offline. Setting zyNdpReachableTime to 30000 milliseconds, or 30 seconds, means the device treats a neighbor as reachable for half a minute before needing to re-confirm via another solicitation.