1. A system for validating a routing function for a network device, the system comprising: the network device, wherein the network devices comprises: a network interface for receiving data packets from a network, a run-time policy engine, and a memory to store a routing configuration file, wherein the run-time policy engine implements the routing configuration file to configure the network device to selectively forward data packets over the network via the network interface, the routing configuration file comprising one or more subroutine statements that define a routing function for network traffic; and a file editor arranged to load and edit the routing configuration file, the file editor comprising an editor policy engine that duplicates a functionality of the run-time policy engine, wherein the routing configuration file further comprises at least one validation portion, the validation portion comprising one or more route statements defining possible routes for data packets within the network, the one or more route statements having definitions indicative of expected routing function behavior for the one or more routes, wherein, following an edit of the routing configuration file: the file editor is arranged to parse the routing configuration file by implementing the routing configuration file using the editor policy engine, and the editor policy engine is arranged to apply the routing function defined by the one or more subroutine statements of the routing configuration file to the one or more route statements and to indicate where a result of the routing function for the one or more routes does not conform to the definitions in the validation portion of the routing configuration file that are indicative of expected routing function behavior.