Discontinuity in the resonant voltage (traveling wave resonance) as a function of applied magnetic field has been observed in large Josephson tunnel junctions. It is believed that our results can be explained in terms of peculiar magnetic properties of large junctions of finite lengthL≫&lgr;J, i.e., the quantization of internal fields and magnetic hysteresis. The discontinuity corresponds to the change of number of vortices penetrating the junction barrier. The experimental voltage jumps are in fairly good agreement with these calculated from the theory.