The invention relates to a method of routing a data packet via a router (12) in a packet communications network (1) supported by a transport network, said method including a step of determining from said packet a required back-up level and a required route. It further includes: a preliminary step of establishing a table storing for a given route links (Ifp1-Ifp6) of the transport network able to provide said route and classified by back-up level; and a step of selecting a link of said transport network in the table on the basis of the required back-up level and the required route.