A router classifies packets assigned to X*Y classes of service into X classes of service supported by the router, each of the X classes of service having Y loss-priority levels. The router maintains a free queue that links available entries of a buffer in which packets are stored. A weighted average depth of the free queue is used to determine whether to retain a given packet. If the weighted average is above a maximum threshold, the packet is retained. If the weighted average is below a minimum threshold, the packet is discarded. If the weighted average is between the two thresholds, a probability of discard that is based on the X*Y classes of service is calculated and compared to a random value to determine whether the packet should be retained.