Method for checking the validity of sensor data of an Ethernet onboard network in a motor vehicle, in which the following steps are carried out: d) determining a propagation time of a first signal on a first connecting path between a first control unit of the Ethernet onboard network and a second control unit of the Ethernet onboard network; e) determining a maximum speed of the first connecting path on the basis of the propagation time; and f) determining a type of a transmission medium of the first connecting path on the basis of the maximum speed, characterized in that the following steps are carried out: - identification of at least a first control unit of the Ethernet onboard network; - synchronization of at least a first control unit of the Ethernet onboard network; - ascertainment of the synchronization interval of the first control unit; - ascertainment of a drift of a timer of the first control unit; - ascertainment of a timestamp of the first control unit; - reading a timestamp of the first control unit; - comparison of the timestamp of the first control unit with a reference clock of the Ethernet onboard network; - performance of a propagation time measurement on the first control unit; - ascertainment of the speed of the associated clock generator; - ascertainment of the time difference of the synchronization interval; - ascertainment of the last synchronization, when the last successful synchronization of a node took place and for how long the control unit of the node has not been synchronized again. d) determining a propagation time of a first signal on a first connecting path between a first control unit of the Ethernet onboard network and a second control unit of the Ethernet onboard network; e) determining a maximum speed of the first connecting path on the basis of the propagation time; and f) determining a type of a transmission medium of the first connecting path on the basis of the maximum speed, characterized in that the following steps are carried out: - identification of at least a first control unit of the Ethernet onboard network; - synchronization of at least a first control unit of the Ethernet onboard network; - ascertainment of the synchronization interval of the first control unit; - ascertainment of a drift of a timer of the first control unit; - ascertainment of a timestamp of the first control unit; - reading a timestamp of the first control unit; - comparison of the timestamp of the first control unit with a reference clock of the Ethernet onboard network; - performance of a propagation time measurement on the first control unit; - ascertainment of the speed of the associated clock generator; - ascertainment of the time difference of the synchronization interval; - ascertainment of the last synchronization, when the last successful synchronization of a node took place and for how long the control unit of the node has not been synchronized again. - identification of at least a first control unit of the Ethernet onboard network; - synchronization of at least a first control unit of the Ethernet onboard network; - ascertainment of the synchronization interval of the first control unit; - ascertainment of a drift of a timer of the first control unit; - ascertainment of a timestamp of the first control unit; - reading a timestamp of the first control unit; - comparison of the timestamp of the first control unit with a reference clock of the Ethernet onboard network; - performance of a propagation time measurement on the first control unit; - ascertainment of the speed of the associated clock generator; - ascertainment of the time difference of the synchronization interval; - ascertainment of the last synchronization, when the last successful synchronization of a node took place and for how long the control unit of the node has not been synchronized again.