Measures for transferring routing protocol information between a software defined network and one or more external networks are provided. A control node in a software defined network (SDN) is configured to maintain a protocol information database comprising routing protocol information for one or more switching nodes in the SDN. In response to detecting occurrence of a predetermined trigger event, the control node modifies the routing protocol information in the protocol information database, and determines whether the one or more switching nodes include an edge switching node. In response to the determination, a routing protocol message is transmitted from the control node to the edge switching node for forwarding via an external network interface. The routing protocol message includes data generated on the basis of the modified routing protocol information.