System area networks (SANs), which usually accept irregular topologies, have been used to connect nodes in PC/WS clusters or high-performance storage systems. Since wormhole or virtual cut-through transfer is used for low latency communication, deadlock-free routings are essential in SANs. We propose a novel deadlock-free deterministic routing called descending layers (DL) routing for SANs. In order to reduce both nonminimal paths and traffic congestion, the network is divided into layers of subnetworks with the same topology using virtual channels, and a large number of paths across multiple subnetworks are established. The DL routing is implemented on a real PC cluster called RHiNET-2, and execution results show that its throughput is improved up to 33% compared with that of up∗/down∗ routing. Its execution time of a barrier synchronization is also improved 29% compared with that of up∗/down∗ routing. Simulation results of various sizes and topologies also show that the DL routing achieves up to 266% improvement on throughput compared with up∗/down∗ routing.