Packet transmission apparatus, line interface unit, and control method for packet transmission apparatus

A packet transmission apparatus includes: a plurality of communication ports; a switch for switching packets; a storage unit which stores a plurality of queues for respectively holding packets to be output from the plurality of communication ports; and a stop signal generating unit. The stop signal generating unit generates a stop signal for stopping packets directed to one of the communication ports from being input into the switch, if the amount of packets held in the queue for the one communication port is equal to or larger than a certain first threshold value, and if the sum of the amounts of packets held in the plurality of queues is equal to or larger than a certain second threshold value.