In order to provide reliable communication services, the cell loss probability of an ATM network is required to be less than some acceptable value, say 10^-5 or smaller. In light of the infrequent cell losses, and overloading of individual output ports is likely to occur at different time instances, an external buffer can be "time-multiplexed" to serve the overloaded output ports on demand. To achieve this, the switch will provide an additional feedback path for derouting cells destined for an overloaded output port to the shared buffer. When the traffic load at the corresponding output port drops and buffer space becomes available in the ouptut queue, the derouted cells will be removed from the shared buffer and routed via the switching fabric to their destinations. Simulation results reveal that under bursty traffic conditions the acceptable load can be more than doubled by incorporating a shared buffer of 1288 cells.