1. 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.