At the present time, the FCFS router is the only router type included with the x-sim distribution. It is beyond the scope of this document to explain how to implement other types of routers. However, the best approach is to modify the FCFS code to produce the desired behavior. The FCFS code is found in /usr/xkernel/simulator/sim/fcfs.c, where /usr/xkernel signifies the root of your x-kernel tree.