A method for controlling data flow including: measuring an source specific incoming data rate of one source; comparing the source specific incoming data rate with a corresponding predetermined data rate; if the source specific incoming data rate exceeds the predetermined data rate, eliminating incoming data packets for reducing the source specific incoming data rate to be smaller than the predetermined data rate, if the source specific incoming data rate does not exceed the predetermined data rate, transmitting the incoming data packets to switching means, connected to said input port, characterised in that a backpressure warning signal (bpws) is generated if the source specific incoming data rate exceeds at least one of the at least one predetermined data rates and said generated backpressure warning signal is sent to a source of the incoming data packets. Hereby further transmission of data packets is prevented.