Abstract: The invention relates to a network device for dispatching data packets from a transmitter on a data transmission network. The data transmission network also includes an abnormality detection module for detecting abnormality relative to the transmitter. The network device receives the data packets through an input port and transmits the data packets out through a plurality of output ports. The network device includes a port-isolating module which blocks the transmission of the data packets between the input port and some of the plurality of output ports in response to the abnormality detected by the abnormality detection module in accordance with a predetermined policy.