We develop the use of piecewise linear test functions for the analysis
of stability of multiclass queueing networks and their associated flu
id limit models. It is found that if an associated LP admits a positiv
e solution, then a Lyapunov function exists. This implies that the flu
id limit model is stable and hence that the network model is positive
Harris recurrent with a finite polynomial moment. Also, it is found th
at if a particular LP admits a solution, then the network model is tra
nsient.