Constrained-path LSP in which the path is obtained by querying an external source, such as a PCE server. In the case that an LSP is defined to be externally queried, it may also have associated explicit definitions (which are provided to the external source to aid computation); and the path that is returned by the external source is not required to provide a wholly resolved path back to the originating system - that is to say, some local computation may also be required