The maximum number of frames that can be queued to this
interface for this combination of traffic class and drop
precedence. Arriving frames are discarded when the queue
has reached its maximum length. A value of 0 denotes that
all frames shall be discarded. A value of -1 denotes that
the device automatically assigns the threshold value. The
value must either be set to automatic threshold assignment
(-1), or manually assigned (i.e. greater than 0) for all
drop precedences on a queue.