The emergence of multi-channel multi-radio wireless mesh networks has given us many new opportunities and challenges. Particularly, the issue on how to appropriately assign channels to interfaces has gathered great importance in the recent publications. To efficiently utilize the channels diversity, the communicating channels should be negotiated between interfaces, which would cause considerable overhead. To conquer this, a hybrid scheme was proposed to rule the way for assigning channels in literatures. In this paper, we formally model the hybrid scheme into an integer linear programming formulation. We provide the necessary as well as sufficient conditions of any feasible assignment. To optimize the link-layer performance, we suggest an objective function which minimizes the total potential interference. We also show that the approximability of the considered problem.