1. A network control device for managing data packet routes used on at least one network according to at least one of an interior gateway protocol and an exterior gateway protocol, comprising: a policy engine arranged to receive, over a network interface, a proposed network route for data packets within the at least one network, and to process said proposed network route based on a plurality of policy elements, to implement at least one of the interior gateway protocol and the exterior gateway protocol, wherein each policy element comprises: data defining a proposition, the proposition relating to at least one property of a received network route within the at least one network; data indicative of a probability of the proposition being true of a randomly-selected network route for the at least one network, and data indicative of a probability of the proposition being true of a randomly-selected network route that is suitable for use in routing network traffic within the at least one network, wherein the policy engine is configured to process the proposed network route by evaluating the plurality of policy elements in accordance with at least one property of the proposed network route using Bayesian inference to determine a confidence value relating to use of the proposed network route, wherein the policy engine is configured to select an action to be performed in relation to the proposed network route based on the confidence value, wherein said action determines how the proposed network route is to be used by at least one network routing device within the at least one network in implementation of at least one of the interior gateway protocol and the exterior gateway protocol.