In this paper, we propose a set of ILP formulation with traffic grooming consideration for static multicast traffic in light-trail networks under the objective to minimize number of established light-trails. Our major idea is to reduce the multicast routing problem to several instances of unicast routing problems owing to the hop length limitation of light-trails. In the numerical analysis, we use two network topologies and different scenarios to derive the required light-trails and the light-trail assignment of given multicast requests in light-trail networks. Compared with similar previous work, our formulation offers an option with lower computational complexity and can be used as a performance bound to corresponding heuristic algorithms.