A method for a router participating in computation of a constraint-based label switched path, LSP, that spans a plurality of areas (310, 320, 315, 325) to reach a destination, the router included within a first area of the plurality of areas (310, 320, 315, 325), the method comprising the steps of: computing a path segment that meets a set of one or more constraints, from the router to at least one border router (335, 340) of the first area, wherein the at least one border router (335, 340) lies in a path necessary to reach the destination; transmitting a path computation request, PCReq, message to a respective path computation element (334, 336) in each neighboring area of the plurality of areas (310, 320, 315, 325), wherein the PCReq message includes a set of one or more attributes for each computed path segment that is used by the respective path computation element (334, 336) to compute one or more path segments towards the destination of the constraint-based LSP; adding the respective path computation element (334, 336) to a PCReq pending list in response to the transmission of the PCReq message to the respective path computation element (334, 336) in each neighboring area of the plurality of areas (310, 320, 315, 325); initiating a timer for receiving a path computation reply, PCRep, message in response to the transmitted PCReq message; receiving the PCRep message from the respective path computation element (334, 336) of each neighboring area of the plurality of areas (310, 320, 315, 325), wherein the PCRep message specifies a set of one or more computed path segments that meet the set of one or more constraints and that were computed by one or more path computation elements (334, 336) downstream from the router; removing the respective path computation element (334, 336) from the PCReq pending list in response to the receipt of the PCRep message from the respective path computation element (334, 336) of each neighboring area of the plurality of areas (310, 320, 315, 325), wherein if removing the respective path computation element (334, 336) from the PCReq pending list makes the PCReq pending list empty, the timer is cancelled; and stitching at least one of the set of one or more computed path segments that are specified in the PCRep message, received from the respective path computation element (334, 336) of each neighboring area of the plurality of areas (310, 320, 315, 325), with the path segment computed by the router to form an optimal constraint-based LSP. computing a path segment that meets a set of one or more constraints, from the router to at least one border router (335, 340) of the first area, wherein the at least one border router (335, 340) lies in a path necessary to reach the destination; transmitting a path computation request, PCReq, message to a respective path computation element (334, 336) in each neighboring area of the plurality of areas (310, 320, 315, 325), wherein the PCReq message includes a set of one or more attributes for each computed path segment that is used by the respective path computation element (334, 336) to compute one or more path segments towards the destination of the constraint-based LSP; adding the respective path computation element (334, 336) to a PCReq pending list in response to the transmission of the PCReq message to the respective path computation element (334, 336) in each neighboring area of the plurality of areas (310, 320, 315, 325); initiating a timer for receiving a path computation reply, PCRep, message in response to the transmitted PCReq message; receiving the PCRep message from the respective path computation element (334, 336) of each neighboring area of the plurality of areas (310, 320, 315, 325), wherein the PCRep message specifies a set of one or more computed path segments that meet the set of one or more constraints and that were computed by one or more path computation elements (334, 336) downstream from the router; removing the respective path computation element (334, 336) from the PCReq pending list in response to the receipt of the PCRep message from the respective path computation element (334, 336) of each neighboring area of the plurality of areas (310, 320, 315, 325), wherein if removing the respective path computation element (334, 336) from the PCReq pending list makes the PCReq pending list empty, the timer is cancelled; and stitching at least one of the set of one or more computed path segments that are specified in the PCRep message, received from the respective path computation element (334, 336) of each neighboring area of the plurality of areas (310, 320, 315, 325), with the path segment computed by the router to form an optimal constraint-based LSP.