A data network, of the type in which a number of inter-connected router devices forward received packets of data towards a destination node in accordance with a routing table associated with each router is operated. The network is operated by: receiving routing information at one of the routers, determining if the information is such that, if it were correct, it would cause the router to update its routing table in respect of one or more entries, and, if so, sending out two test packets, one of which is sent out according to the existing information contained in the routing table and the other of which is sent out according to the information which would be included in the routing table if it were updated in accordance with the received information; comparing the results of the two test sendings; and updating the routing table to reflect the received information if the comparison indicates that the received information is correct, but otherwise ignoring the information and maintaining the routing table unchanged.