Emerging parallel real-time and multimedia applications broaden the range of performance requirements imposed on the interconnection network. This communication typically consists of a mixture of different traffic classes, where guaranteed packets require bounds on latency or throughput while good average performance suffices for the best-effort traffic. This paper investigates how multicomputer routers can capitalize on low-latency routing and switching techniques for best-effort traffic while still supporting guaranteed communication. Through simulation experiments, we show that certain architectural features are best-suited to particular performance requirements. Based on these results, the paper proposes and evaluates a router architecture that tailors low-level routing, switching, and flow-control policies to the unique needs of best-effort and guaranteed traffic. Careful selection of these policies, coupled with fine-grain arbitration between the classes, allows the guaranteed and best-effort packets to share network bandwidth without sacrificing the performance of either class.