  We have investigated numerically the phase--locking behavior of
two-dimensional Josephson junction arrays,taking into account a finite
inductance ($l\stackrel{>}{\sim}1$) of the unit cell and external magnetic
fields. Within this model we demonstrate the existence of multi--valued {\it
I--V} curves of the network. Different branches of the {\it I--V} curve
correspond to "in--phase" and "anti--phase" coherent states with high and low
levels of output power, respectively. The arrays show remarkable hysteretic
transitions between these states under influence of external magnetic fluxes in
the range $ - \Phi_0/4 < \Phi_{ext} < \Phi_0/4 $.
