A network interface device for a data processing system comprising: one or more network connection ports for connection to a network; a routing table supported at a memory of the network interface device; a controller operable to modify the routing table; and a control plane interface of the controller arranged to receive routing table update instructions carried in one or more control packets received at the said ports; wherein the control plane interface is configured to, on receiving a routing table update instruction, cause the controller to update the routing table in accordance with the routing table update instruction.