Abstract: A data communication system having a plurality of input/output ports, a first group and a second group of switching units. Each switching unit has a plurality of input/output interfaces, whereby at least one of the plurality of input/output interfaces of each switching unit of the first group forms an input/output port of the communications system, and at least one other input/output interface of each of the switching units of the first group is connected via a communication link with an input/output interface of one of the switching units of the second group, so that a network is formed having at least one loop. The network is logically divided in at least two virtual subnetworks, each forming a spanning tree. Each switching unit of the first group is configured to assign an incoming data packet arriving at a particular input/output port to a predetermined one of the at least two virtual sub-networks.