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. In this paper, we propose a novel deadlock-free deterministic routing called descending layers (DL) routing for SANs. In order to reduce both non-minimal paths and traffic congestion, the network is divided into layers of sub-networks 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.