Quality of service (QoS) based routing provides QoS guarantees to multimedia applications and an efficient utilization of the network resources. Nevertheless, the QoS-routing algorithm must be simple because a costly procedure does not scale with the size of the network. This paper proposes and analyzes the performance of the single mixed metric (SMM) routing protocol in two versions: one based on distance-vectors and the other on link-states. SMM takes into account three metrics: propagation delay, available bandwidth, and loss probability. A heuristic based on the residual loss probability and metric-combination is used to turn the algorithm scalable and solvable in polynomial time. The simulation results show that SMM outperforms other solutions that implement QoS routing based on three metrics.