Validating a path in an optical layer of a communications network, for client traffic having an associated service level, involves setting an optical quality margin according to the service level associated with that client traffic. The optical quality margin indicates how close an estimated optical quality of the path can approach a level which produces a threshold error rate. This margin is used to check whether the estimated optical quality is within the optical quality margin set according to the client traffic service level. Making the optical quality margin dependent on client traffic service level, can enable increased optical reach. This can give more flexibility in path selection and enable better matching to service levels of client traffic.