The process of computing routes that network traffic must follow throughout Internet has become more complex in the last years. Nowadays, this process is subject to the application of several constraints related to traffic engineering, resources management, quality of the offered services, security or robustness. The application of all these constraints has caused an increase of complexity of those nodes in charge of path computation. Sometimes, the amount of time and resources spent to compute routes is superior to resources used in the main task of these nodes: traffic classification and forwarding. PCE (Path Computation Element) architecture is being developed to diminish that problem in the context of constraint-based path computation for MPLS (Multiprotocol Label Switching). Although research work in relation to this topic is making progress quickly, there are still some unsolved aspects. As a contribution to PCE development, in this work we present a mechanism called PILEP (Procedure for Interdomain Location of External PCEs) that allows the dynamic discovery of routes computation elements in interdomain environments, making use of the existing routing protocols.