The Storm-Constrain function is used to avoid the
packet-storm on a port.  Three types of flux can be
set observed respectively: broadcast, multicast and
unicast.  If an observed flux exceeds its upper limit,
this kind of flux will be recognized as over-upper-flux.
This kind of flux will be considered as normal-flux
until it falls below its lower limit.  If any type of
observed flux is over-upper-flux, the port will change
its status to controlled-status, that is to stop packet
forwarding to avoid storm.  Only when all the observed
fluxes are not over-upper-flux, will the port return to
normal(forwarding) status.