Method for transmitting time-critical data, in which - datagrams are transmitted from a first communication device (103, 105) in a first subnet and to a second communication device in a second subnet via a third subnet, - the first and the third subnet are connected via at least a first and a second router and the second and the third subnet are connected via at least a third router, - datagrams are forwarded within the first and the second subnet in each case by way of switching, - datagrams are forwarded within the third subnet by way of routing, - the first subnet has a ring topology and comprises at least a first coupling communication device that is connected to the first router, and a second coupling communication device that is connected to the second router, wherein datagrams are transmitted within the first subnet in accordance with the Media Redundancy Protocol, MRP, - the second subnet comprises a third coupling communication device that is connected to the third router and to which a network layer address is assigned, - the first and second coupling communication device select a master coupling communication device among one another by way of the Virtual Router Redundancy Protocol, VRRP, wherein a common virtual network layer address is assigned to both coupling communication devices and the first or second coupling communication device not selected as master communication device is operated as reserve coupling communication device, - in the event of a changed selection or in the event of failure of the master coupling communication device, an MRP interconnection topology change is signalled, - as a result of signalling of the MRP interconnection topology change, in communication devices of the first subnet, relearning of their respective data link layer address tables is triggered, - a data link layer tunnel is set up through the third subnet via the first or second router and via the third router at least between the master coupling communication device and the third coupling communication device, wherein the datagrams transmitted from the first communication device to the second communication device are encapsulated in tunnel data frames within the data link layer tunnel, and the tunnel data frames in which the datagrams transmitted from the first communication device to the second communication device are each encapsulated are VXLAN frames or Layer 2 Tunnelling frames of the datagrams, - a first endpoint of the data link layer tunnel is configured with the virtual network layer address of the first and second coupling communication device, - a second endpoint of the data link layer tunnel is configured with the network layer address of the third coupling communication device. - datagrams are transmitted from a first communication device (103, 105) in a first subnet and to a second communication device in a second subnet via a third subnet, - the first and the third subnet are connected via at least a first and a second router and the second and the third subnet are connected via at least a third router, - datagrams are forwarded within the first and the second subnet in each case by way of switching, - datagrams are forwarded within the third subnet by way of routing, - the first subnet has a ring topology and comprises at least a first coupling communication device that is connected to the first router, and a second coupling communication device that is connected to the second router, wherein datagrams are transmitted within the first subnet in accordance with the Media Redundancy Protocol, MRP, - the second subnet comprises a third coupling communication device that is connected to the third router and to which a network layer address is assigned, - the first and second coupling communication device select a master coupling communication device among one another by way of the Virtual Router Redundancy Protocol, VRRP, wherein a common virtual network layer address is assigned to both coupling communication devices and the first or second coupling communication device not selected as master communication device is operated as reserve coupling communication device, - in the event of a changed selection or in the event of failure of the master coupling communication device, an MRP interconnection topology change is signalled, - as a result of signalling of the MRP interconnection topology change, in communication devices of the first subnet, relearning of their respective data link layer address tables is triggered, - a data link layer tunnel is set up through the third subnet via the first or second router and via the third router at least between the master coupling communication device and the third coupling communication device, wherein the datagrams transmitted from the first communication device to the second communication device are encapsulated in tunnel data frames within the data link layer tunnel, and the tunnel data frames in which the datagrams transmitted from the first communication device to the second communication device are each encapsulated are VXLAN frames or Layer 2 Tunnelling frames of the datagrams, - a first endpoint of the data link layer tunnel is configured with the virtual network layer address of the first and second coupling communication device, - a second endpoint of the data link layer tunnel is configured with the network layer address of the third coupling communication device.