Method for operating a switch device of a motor vehicle communication network, having the steps: - receiving a device recognition of a device of the motor vehicle at a first port of the switch device, - carrying out an authenticity check of the device on the basis of the device recognition, - in the case that a check result of the authenticity check is positive, receiving communication data, addressed to at least one further device of the motor vehicle, of the device at the first port and transmission of the communication data in a first VLAN of the communication network to the at least one further device, - in the case that the check result is negative, rejection of the communication data at the first port such that in the case of a negative check result, if the device is not authenticated, its communication data is not transferred into the first VLAN and hereby remaining devices are protected from the communication data of the device characterised by the steps: - to the device are outputted also no communication data of the remaining devices out of the first VLAN via the first port, - receiving a diagnosis request for the device from a diagnosis device at a second port of the switch device, wherein the diagnosis device signals by means of the diagnosis request that it wishes to obtain diagnosis data from the device connected at the first port, - independently from the check results, forwarding the diagnosis request via the first port to the device in a second VLAN of the communication network such that the diagnosis capability for the device is also retained in the case that its communication data is rejected at the first port because the device is not been successfully authenticated. - receiving a device recognition of a device of the motor vehicle at a first port of the switch device, - carrying out an authenticity check of the device on the basis of the device recognition, - in the case that a check result of the authenticity check is positive, receiving communication data, addressed to at least one further device of the motor vehicle, of the device at the first port and transmission of the communication data in a first VLAN of the communication network to the at least one further device, - in the case that the check result is negative, rejection of the communication data at the first port such that in the case of a negative check result, if the device is not authenticated, its communication data is not transferred into the first VLAN and hereby remaining devices are protected from the communication data of the device - to the device are outputted also no communication data of the remaining devices out of the first VLAN via the first port, - receiving a diagnosis request for the device from a diagnosis device at a second port of the switch device, wherein the diagnosis device signals by means of the diagnosis request that it wishes to obtain diagnosis data from the device connected at the first port, - independently from the check results, forwarding the diagnosis request via the first port to the device in a second VLAN of the communication network such that the diagnosis capability for the device is also retained in the case that its communication data is rejected at the first port because the device is not been successfully authenticated.