A method of operating a second network access node comprises configuring the second network access node to act as a secondary network access node for a dual connectivity mode for a terminal device in which a first network access node acts as a master network access node. The method further comprises establishing, while acting as a secondary network access node for the dual connectivity mode, that the second network access node should switch to acting as a master network access node, deriving a new master network access node security key for use by the second network access node when switched to acting as a master network access node for the dual connectivity mode, and configuring the second network access node to act a master network access node for the dual connectivity mode using the new master network access node security key.