This paper studies how to assign radio interfaces to work on multiple channels in multi-channel and multi-interface wireless networks. Different from previous studies that focused on the MAC layer, we study this problem from a global perspective. We use a layered graph to model multi-channel wireless networks, and develop effective interface assignment and routing path computation algorithms based on the layered graph. We have evaluated the performance of our algorithm and compared it to a sequential interface assignment algorithm. The numerical results show that our algorithm significantly outperforms the sequential interface assignment.