An ATM routing switch has a buffer circuit for holding cells located on queues at output ports, the buffer having a first reserve buffer capacity for cells of a first type requiring integrity of cell transmission and a first designation for use in determining a permitted path through the network, a second reserve buffer capacity for cells of the first type having a second designation for use in determining a different permitted path in the network and a third reserve buffer capacity for cells of a second type accepting some loss of cells in transmission, flow control circuitry operating to limit input of cells of either the first or second type if predetermined thresholds for the first, second or third buffer capacities are reached.