swIPv6NeighborCacheReachState reports the IPv6 neighbor discovery reachability state of a cache entry, one of incomplete, reachable, stale, delay, probe, or static. Admins use this to diagnose neighbor-resolution problems, since a neighbor stuck in incomplete or repeatedly cycling through probe suggests it is not responding to neighbor solicitations. For example, an entry reporting stale(3) means the device has not recently confirmed reachability to that neighbor and neighbor discovery will re-verify it before further use.