The planetary system 55Cnc consists of four planets. Two of the planets, the companions b and c, seem to be in a 3:1 mean motion resonance and the corresponding dynamics is of special interest. First we study the stability of the system in the framework of the three body problem. We show that the system is located in the neighborhood of an asymmetric periodic orbit in phase space and the corresponding resonant angle variables librate. The asymmetry, which is obeyed by the motion, is essential for the long-term stability. If the system was in a symmetric configuration then the evolution would be chaotic. Additional numerical simulations are performed in order to show the effect of the rest planets in the stability of the overall planetary system.