An Internet Protocol router device (10) for use in a data network comprising a plurality of nodes (10, 20, 30), wherein packets of data are sent from one node in the network to another on a hop by hop basis. The router includes: a data store (14) for storing a routing table indicating the next hop destination for a received packet on the basis of the ultimate destination specified by the received packet; a digital processor (13) operable to maintain the routing table on the basis of communications received from neighboring router devices (20, 30) within the network; and a timer (15) for timing the length of time elapsed since last receiving a communication, or a communication of a type which is one of a subset of the possible types of communication, from each of one or more of the router device's neighbors; wherein the data store (14) is further operable to store in respect of each of said one or more of the router device's neighbors a router dead interval; and the digital processor (13) is further operable to compare said length of time elapsed with the corresponding router dead interval in respect of each of said one or more of the router device's neighbors and to ascertain that the corresponding router device is no longer operating correctly if the comparison indicates that a length of time equal to or greater than the router dead interval has elapsed without receiving a said communication, or a said communication of a type which is one of a subset of the possible types of communication, from said neighboring router device; the router device being characterised in that the digital processor (13) is further operable to dynamically vary the or each router dead interval in accordance with one or more properties of the network.