The emergence of Elastic Optical Networks allowed a more flexible spectrum allocation for routing traffic demands within telecommunication networks. From this context arises the Routing and Spectrum Assignment problem, which consists of routing a given set of origin-destination traffic demands and assigning them to contiguous spectrum frequencies such that no frequency slot is assigned to more than one demand within a network link. This work deals with the variant where each demand route must additionally satisfy a maximal-length constraint. In this paper we propose a compact extended formulation for the Constrained Routing and Spectrum Assignment Problem. We show that our extended formulation is theoretically stronger than formulations known in the literature. Experimental results demonstrate the efficiency of our approach.