A method of assessing the feasibility of a composite optical path in an optical communications network in which the composite path is composed of two optical paths k1 and k2 is described. The method comprising the steps of defining, for each individual optical path k and with type i interface, at least one parameter indicating its feasibility, calculating a quality Q factor Q'_i,k1+k2 of the signal for the composite path, estimated considering the deterioration which affects transmission over the paths k1 and k2 and comparing this quality Q'_i,k1+k2 with a value Qbare_i which is defined as the lowest value which can be taken on by a mapping function Q_i(.) for interface i and which gives the Q factor as a function of the OSNR received evaluated under the conditions that are considered to be the worst case that can be accepted while ensuring the desired signal quality.