Method for operating an industrial automation system communication network comprising a plurality of communication devices, in which - a plurality of mutually redundant control units (101-103; 101-102, 103-104) control functions of a plurality of assigned communication devices, wherein the mutually redundant control units comprise identically designed and configured state machines, which, in an error-free state, generate identical output variables (11-13; 11-12) given identical input variables, wherein the communication devices are assigned to a Software Defined Network which comprises one communication control plane designated as a Control Plane and one data transmission plane designated as a Data Plane, and wherein the control units (101-103; 101-102, 103-104) are assigned to the Control Plane and the communication devices are assigned to the Data Plane, - the control units (101-103; 101-102, 103-104) for user-side communication service requests each determine a path between a first communication device at a first end node and a second communication device at a second end node via third communication devices, wherein the third communication devices comprise routers and/or switches and the control units (101-103; 101-102, 103-104) specify flow tables, from which routing tables and/or forwarding tables for third communication devices respectively assigned to the control units are derived, - the control units (101-103; 101-102, 103-104) verify whether necessary system resources are available along the determined paths during a scheduled usage period, - the control units (101-103; 101-102, 103-104) reserve system resources necessary for the communication service requests in each case in the event of a positive verification result and control routing or switching functions of the third communication devices in accordance with the determined paths, - the communication devices transfer communication-specific input variables to all associated redundant control units (101-103; 101-102, 103-104), - the output variables (11-13; 11-12) generated by the mutually redundant control units (101-103; 101-102, 103-104) are checked for deviations relative to each other, - in the event of deviations a switchover to reserve control units is performed or control units having output variables that deviate with respect to a majority of mutually redundant control units (101, 103) are deactivated. - a plurality of mutually redundant control units (101-103; 101-102, 103-104) control functions of a plurality of assigned communication devices, wherein the mutually redundant control units comprise identically designed and configured state machines, which, in an error-free state, generate identical output variables (11-13; 11-12) given identical input variables, wherein the communication devices are assigned to a Software Defined Network which comprises one communication control plane designated as a Control Plane and one data transmission plane designated as a Data Plane, and wherein the control units (101-103; 101-102, 103-104) are assigned to the Control Plane and the communication devices are assigned to the Data Plane, - the control units (101-103; 101-102, 103-104) for user-side communication service requests each determine a path between a first communication device at a first end node and a second communication device at a second end node via third communication devices, wherein the third communication devices comprise routers and/or switches and the control units (101-103; 101-102, 103-104) specify flow tables, from which routing tables and/or forwarding tables for third communication devices respectively assigned to the control units are derived, - the control units (101-103; 101-102, 103-104) verify whether necessary system resources are available along the determined paths during a scheduled usage period, - the control units (101-103; 101-102, 103-104) reserve system resources necessary for the communication service requests in each case in the event of a positive verification result and control routing or switching functions of the third communication devices in accordance with the determined paths, - the communication devices transfer communication-specific input variables to all associated redundant control units (101-103; 101-102, 103-104), - the output variables (11-13; 11-12) generated by the mutually redundant control units (101-103; 101-102, 103-104) are checked for deviations relative to each other, - in the event of deviations a switchover to reserve control units is performed or control units having output variables that deviate with respect to a majority of mutually redundant control units (101, 103) are deactivated.