A method authenticates a device in a communication network of an automation installation, in which authentication information indicating the device is transmitted to an authentication server that admits or rejects the device in the communication network as a subscriber. To perform an authentication of the device in a communication network configured with redundancy, the communication network has a communication ring that, besides the device, has first and second neighbor devices. At the start of the authentication the device sends authentication requests containing the authentication information to the neighbor devices. The neighbor devices duplicate the authentication information and send it via the communication ring in both transmission directions to an authentication server which uses the authentication information to perform a respective check on the authenticity of the device and admits or rejects the device in the communication network as a subscriber as the result of the check.