A switching unit of a cross connecting device is expanded with no momentary interruption, while data loss and data overlapping is eliminated. To achieve such an expansion, a cross connecting device comprises a base set having a switching unit for determining an output channel based on virtual path information for cell data that is input and for outputting the cell data. An expanded set has a switching unit for receiving the cell data from the base set and cell data from another base set, and for determining an output channel based on virtual path information for both inputs of cell data and for outputting the cell data. A filter prevents the transmission, to the expanded set, of cell data for which the output channel is determined by the switching unit of the base set.