Examples:

• ip neigh add 10.0.0.3 lladdr 0:0:0:0:0:1 dev eth0 nud perm

  -- add a permanent ARP entry for the neighbour 10.0.0.3 on the device eth0.

• ip neigh chg 10.0.0.3 dev eth0 nud reachable

  -- change its state to reachable.