Method for redundant data transmission in a communication network, communication system, and configuration device

Abstract

To achieve redundant data transmission in a communication network, at least a first (201) and a second virtual local area network (202), VLAN, are each configured on communication devices (101-104, 112-113, 122-123, 132-133, 142-144) of the communication network that forward datagrams (300). Different spanning trees are determined for each of the first (201) and second VLAN (202) within the communication network by applying a spanning tree protocol, said spanning trees being used to specify, for each of the first and second VLAN, active paths for forwarding datagrams via the forwarding communication devices. At least selected datagrams (300) to be transmitted are duplicated by a transmitter-side communication device (111) and transmitted both via the first VLAN (201) and via the second VLAN (202). Received datagrams (300) associated with the VLANs (201, 202) are each checked for identity with an already received datagram by receiver-side communication devices (121, 131, 141) and discarded if an already received identical datagram is present.

Description

[0001] 202416279 Foreign version

[0002] 1

[0003] Description

[0004] Methods for redundant data transmission in a communication network, communication system and configuration device

[0005] The present invention relates to a method for redundant data transmission in a communication network, in particular for the transmission of control data in an industrial automation system, a communication system suitable for carrying out the method, and a configuration device, for example an engineering system or a central network controller.

[0006] An industrial automation system typically comprises a large number of automation devices interconnected via an industrial communication network and serves to control or regulate plants, machines, or equipment within the context of manufacturing or process automation. Due to time-critical conditions in industrial automation systems, real-time communication protocols such as PROFINET, PROFIBUS, Real-Time Ethernet, or Time-Sensitive Networking (TSN) are predominantly used for communication between automation devices.

[0007] Due to their use for often highly diverse applications, problems can arise in Ethernet-based communication networks, for example, when network resources for transmitting data streams or data frames with real-time requirements are competing for the transmission of data frames with large payloads without specific quality of service requirements. This can lead to data streams or data frames with real-time requirements not being transmitted according to the requested or required quality of service.

[0008] EP 2 127 329 B1 relates to a method for filtering redundant frames, which have at least one MAC source address, one frame ID, and one CRC value, in a network node with at least two ports, each with a transmitting and a receiving device. The transmitting device has a send list in which frames to be transmitted are stored. The receiving device has a receive memory for storing a received frame. To filter redundant frames in a network node, after receiving an initial frame at one of the at least two ports of the

[0009] The receiving device searched the transmission list of one of the at least two ports for a second frame with the same MAC source address and frame ID. If the second frame is found, the output is: 202416279 Foreign Version

[0010] 2

[0011] The first frame is not forwarded to a local application or to other ports.

[0012] EP 2 343 857 B1 describes a network node for a communication network comprising a first subnetwork and a second subnetwork connected to it. While a spanning tree protocol is used in the first subnetwork, a different protocol is used in the second subnetwork. The network node is configured as an element for the second subnetwork and is designed for communication within the second subnetwork. Furthermore, the network node is configured and designed as a spanning tree main node for monitoring and controlling the second subnetwork by means of spanning tree functionality. This allows the second subnetwork to be treated as a virtual network node by the spanning tree protocol used in the first subnetwork, as the network node, acting as the spanning tree main node, performs a spanning tree protocol application for other network nodes of the second subnetwork.

[0013] From EP 2 693 700 B1, a method for message transmission in a redundantly operated communication network is known, comprising a first subnetwork with a tree topology and a second subnetwork. In the first subnetwork, messages are transmitted according to a spanning tree protocol. Network nodes of the first subnetwork exchange messages containing topology information with each other to form a tree topology. In the second subnetwork, messages are transmitted according to a parallel or ring redundancy protocol. A virtual network node is configured as the root network node of the first subnetwork, which is connected to all network nodes of the second subnetwork via a virtual connection that cannot be interrupted by a fault.

[0014] EP 2 838220 B1 relates to a method for redundant message transmission in a communication network with an arbitrarily meshed network topology. Several communication devices each comprise a first and a second transmitting and receiving unit. Each transmitting and receiving unit is assigned a signal processing unit, through which messages to be sent redundantly by the communication device are duplicated and sent by the transmitting and receiving units, and messages redundantly received by the transmitting and receiving units at the communication device are detected. For a communication link between two network nodes that is at least partially redundant, independent paths are determined within the communication network. Redundant to 202416279 Foreign version

[0015] 3. Transmitting messages are exchanged in the communication network according to forwarding rules corresponding to the determined paths.

[0016] US Patent 2008 / 253385 A1 discloses a method for redundant data transmission in a communication network, in which at least one first and one second virtual local area network (VLAN) are configured for communication devices forwarding datagrams in the communication network. At least selected datagrams to be sent are duplicated by a sending communication device and transmitted via both the first and the second VLAN. Received datagrams assigned to the VLANs are checked by receiving communication devices for identity with a previously received datagram and discarded if an identical datagram has already been received.

[0017] Numerous redundancy protocols have been developed, particularly for industrial communication systems, to compensate for line or component failures or malfunctions. It is important to distinguish between interruptible redundancy methods, such as MRP (Media Redundancy Protocol) or RSTP / MSTP (Rapid / Multiple Spanning Tree Protocol), which switch from primary to backup paths during reconfiguration, and non-interruptible redundancy methods. Non-interruptible redundancy methods, such as HSR (High-availability Seamless Redundancy), PRP (Parallel Redundancy Protocol), or MRPD (Media Redundancy for Planned Duplication), generally do not require reconfiguration of the communication system, thus avoiding even brief communication interruptions.The use of the above redundancy protocols often requires special hardware, meaning that commercially available Ethernet components are not sufficient.

[0018] The present invention is based on the objective of providing a method for redundant data transmission in a communication network which, when using any standard-compliant Ethernet hardware, enables the realization of a seamless redundancy method for a multitude of network topologies, as well as providing a suitable implementation for carrying out the method.

[0019] This problem is solved according to the invention by a method with the features specified in claim 1, by a configuration device with the features specified in claim 13, and by a communication system with the features specified in claim 14. 202416279 Foreign version

[0020] 4

[0021] Features solved. Advantageous embodiments of the present invention are specified in the dependent claims.

[0022] According to the inventive method for redundant data transmission in a communication network, at least one first and one second virtual local area network (VLAN) are configured on communication devices forwarding datagrams, such as switches or bridges, of the communication network, in particular port-based or tag-based. For the first and the second VLAN, different spanning trees are determined within the communication network using a spanning tree protocol such as STP, RSTP, or MSTP. These spanning trees define active paths for forwarding datagrams, especially frames, via the forwarding communication devices for the first and second VLANs, respectively. Preferably, the active paths for forwarding datagrams are defined by blocking ports of the forwarding communication devices that would lead to loop formation within the communication network.

[0023] According to the invention, at least selected datagrams to be transmitted are duplicated by a sender-side communication device, such as a programmable logic controller (PLC) or a host with an installed virtual PLC, and transmitted via both the first and second VLANs. Conversely, received datagrams assigned to the VLANs are checked for identity with a previously received datagram by receiver-side communication devices, such as distributed I / O modules of a PLC or input / output devices or I / O modules, and are discarded if an identical datagram has already been received. Advantageously, the sender-side and receiver-side communication devices are configured to send and receive at least the selected datagrams via the first and second VLANs. Furthermore, the sender-side and receiver-side communication devices can be configured to...Receiver-side communication devices can detect the presence of a previously received identical datagram based on a sequence number assigned or inserted by the sender, or based on an identical hash value calculated over the received datagram. In particular, the sequence number can be inserted by the sender into a payload area of ​​the respective datagram, which is possible with any Ethernet hardware.

[0024] The present invention thus enables the efficient implementation of a high-performance redundancy method for virtually any network topology, based on standard-compliant Ethernet hardware and without reconfiguration times after faults or failures. (See also: 202416279 Foreign Version)

[0025] 5. Due to the fact that the redundancy method according to the invention has no impact on the PROFINET standard, no integration problems arise in PROFINET-based automation systems. Compared to the redundancy method according to IEEE 802.1CB – “Frame Replication and Elimination for Reliability” (FRER) – the present invention does not require any additional Ethernet headers or hardware functions. Compared to HSR and PRP, the present invention does not require any additional headers or trailers.

[0026] The present invention can be particularly advantageously applied when combining ring topologies with line or tree topologies, or with arbitrarily meshed topologies. For example, first forwarding communication devices can be interconnected in a ring topology, while second forwarding communication devices are interconnected in a line or tree topology. In this case, at least selected first forwarding communication devices are provided for coupling, each of which is connected to two first communication devices and to one second communication device.

[0027] According to a preferred embodiment of the present invention, spanning trees are generated according to the Multiple Spanning Tree Protocol (MSTP) for loop-free transmission of datagrams within the first and second VLANs. The datagrams are transmitted via these spanning trees. A first Multiple Spanning Tree instance is generated for the first VLAN, while a second Multiple Spanning Tree instance is generated for the second VLAN. Accordingly, the different spanning trees for the first and second VLANs can be easily and reliably defined by the generated Multiple Spanning Tree instances. Advantageously, an identifier of the first VLAN is assigned an identifier of the first Multiple Spanning Tree instance, while an identifier of the second VLAN is assigned an identifier of the second Multiple Spanning Tree instance.Furthermore, for example, a separate sender and receiver address table, in particular a forwarding database or MAC table, is provided for each VLAN in each forwarding communication device. According to a particularly preferred embodiment of the present invention, the spanning trees for the first and second VLANs are configured as Traffic Engineering Multiple Spanning Tree instances, so that different spanning trees for the VLANs can be efficiently provided and incorrect path learning for the VLANs is avoided, which would occur particularly with shared VLAN learning. 202416279 Foreign version.

[0028] 6

[0029] According to a further embodiment of the present invention, the first and second VLANs can be used for transmitting real-time data traffic, while a third VLAN is configured for non-real-time data traffic on the communication devices forwarding the datagrams. Accordingly, a separate spanning tree is also determined for the third VLAN within the communication network using a spanning tree protocol, which defines an active path for forwarding datagrams to the third VLAN. This allows available system resources to be used efficiently by providing a seamless redundancy method only for real-time data traffic and applying a less resource-intensive, interrupted redundancy method for non-real-time data traffic.

[0030] Preferably, the spanning tree for the third VLAN is configured as an Internal Spanning Tree instance, minimizing overhead. Loop formation within the third VLAN can be prevented, for example, using either the Multiple Spanning Tree Protocol (MSTP) or the Media Redundancy Protocol (MRP). Advantageously, the assignment of datagrams to be sent as real-time or non-real-time traffic is handled by a control application installed on the respective sender-side communication device.

[0031] The communication system according to the invention is designed to carry out a method as described above and comprises several communication devices for forwarding datagrams, each configured with a first and a second virtual local area network (VLAN), a sender-side communication device, and a receiver-side communication device. The sender-side communication device is configured to duplicate at least selected datagrams to be sent and transmit them via both the first and the second VLAN. In contrast, the receiver-side communication device is configured to check each received datagram assigned to the VLANs for identity with a previously received datagram and to discard any identical datagram that has already been received.Furthermore, the communication devices forwarding datagrams are each configured to determine different spanning trees for the first and second VLANs using a spanning tree protocol. These spanning trees define active paths for forwarding datagrams via the forwarding communication devices in the first and second VLANs, respectively. 202416279 Foreign version.

[0032] 7

[0033] The configuration device according to the invention is suitable for carrying out a method as described above and is configured to configure at least one first and one second virtual local area network (VLAN) on communication devices that forward datagrams. Furthermore, the configuration device is configured to configure the forwarding communication devices to determine different spanning trees for the first and second VLANs, respectively, using a spanning tree protocol. These spanning trees define active paths for forwarding datagrams via the forwarding communication devices for the first and second VLANs.Furthermore, the configuration device is set up to configure at least one communication device, to duplicate at least selected datagrams to be sent and to transmit them over both the first VLAN and the second VLAN, and to configure at least one communication device to check each received datagram assigned to the VLANs for identity with a datagram already received and to discard it if an identical datagram has already been received.

[0034] The present invention is explained in more detail below using an exemplary embodiment with reference to the figure. It shows the

[0035] Figure a communication system for redundant data transmission, in which communication devices are connected to each other in a ring topology or in a line topology,

[0036] The communication system depicted in the figure comprises, by way of example, first communication devices 101-104, which are interconnected in a ring topology, and second communication devices 111-113, 121-123, 131-133, 141-144, which are interconnected in a line topology. Each first communication device 101-104 is connected to two first communication devices and to one second communication device. In principle, the redundancy method described below is applicable to arbitrarily complex mesh topologies. In the present embodiment, each communication device 101-104, 111-113, 121-123, 131-133, 141-144 has both a forwarding function, such as a switch or a bridge, and a function as a receiving station, for example, as an I / O controller or programmable logic controller, like communication device 111, or as an I / O device, input / output device, or...Decentralized peripheral device, such as communication devices 101-104, 112-113, 121-123, 131-133, 141-144. Additionally, a network controller 100 is included as a configuration device 202416279 (foreign version).

[0037] 8 provided that the configuration data described below 200 is distributed to the communication devices 101-104, 111-113, 121-123, 131-133, 141-144.

[0038] For seamless redundant transmission of real-time data traffic, at least one first VLAN 201 and one second VLAN 202 are configured on each of the communication devices 101-104, 111-113, 121-123, 131-133, and 141-144 in this embodiment. Furthermore, a third VLAN 203 is configured on each of the communication devices 101-104, 111-113, 121-123, 131-133, and 141-144 for the redundant transmission of non-real-time data traffic with interrupts. In the figure, the paths logically assigned to VLANs 201-203 are graphically marked differently (VLAN 201 dashed, VLAN 202 dotted, VLAN 203 dash-dotted). In the present embodiment, all communication devices 101-104, 111-113, 121-123, 131-133, 141-144 are set up to send and receive datagrams 300 with control data via the first 201, the second 202 and the third VLAN 203.

[0039] For the first VLAN 201 and the second VLAN 202, different spanning trees are determined using a spanning tree protocol, specifically MSTP. These spanning trees define active paths for forwarding datagrams (300), particularly frames, containing control data via communication devices 101-104, 111-113, 121-123, 131-133, and 141-144 for both VLANs (201) and VLANs (202). Advantageously, switches or bridges are configured to generate (at most) different spanning trees for each VLAN (201) or VLAN (202) using MSTP. The active paths for forwarding the datagrams are defined, in particular, by blocking ports on the forwarding communication devices that would create a loop within the communication system.In the present embodiment, these are port P1 on communication device 101 for the first VLAN 201 and port P2, also on communication device 101, for the second VLAN 202. A separate spanning tree is also determined for the third VLAN 203 using a spanning tree protocol, which defines an active path for forwarding datagrams in the third VLAN 203. For example, to prevent loops in the third VLAN 203, port P3 on communication device 103 is blocked from forwarding datagram 300.

[0040] Within the third VLAN 203, loop formation can, for example, be prevented using Media Redundancy Protocol (MRP) as an alternative to Rapid or Multiple Spanning Tree Protocol (RSTP / MSTP). In addition to the direct application of MSTP in a real communication network, the above spanning tree determination can also be carried out using the foreign version 202416279.

[0041] 9

[0042] It is possible to apply an analog method in the network controller 100 for a digital twin of the communication network.

[0043] To determine the spanning trees for VLANs 201-203, in this embodiment, messages 400 containing topology information are forwarded from a communication device operating as a root bridge. This information includes, in particular, path costs for transmitting user data to the root bridge. Specifically, the topology information includes details of existing links between communication devices 101-104, 111-113, 121-123, 131-133, and 141-144. These messages 400 containing the topology information are, in particular, Bridge Protocol Data Units (BPDUs). Based on these messages 400 containing the topology information, communication devices 101-104, 111-113, 121-123, 131-133, and 141-144 select the communication device operating as the root bridge. This is done, for example, using node identifiers and priorities that are assigned to the communication devices 101-104, 111-113, 121-123, 131-133, 141-144.

[0044] Based on the messages containing topology information or BPDUs 400, communication devices 101-104, 111-113, 121-123, 131-133, and 141-144 configure their respective ports for loop-free user data transmission. Loop-free connections from the respective root bridge to the other communication devices 101-104, 111-113, 121-123, 131-133, and 141-144 are established by determining the minimum path costs to the root bridge. The path costs can vary for each VLAN 201-203. In particular, the communication devices 101-104, 111-113, 121-123, 131-133, 141-144 configure their respective ports using the BPDUs 400 for blocking the forwarding of user data or topology information, for forwarding user data or topology information, for learning MAC addresses, and for creating forwarding rules.By configuring the ports of communication devices 101-104, 111-113, 121-123, 131-133, 141-144, links between communication devices or network nodes are activated or deactivated.

[0045] Datagrams 300 containing the control data are preferably transmitted within VLANs 201-203 using Multiple Spanning Tree Protocol (MSTP) between communication devices 101-104, 111-113, 121-123, 131-133, and 141-144. Accordingly, a first Multiple Spanning Tree instance is created for the first VLAN, 201. Similarly, a second Multiple Spanning Tree instance is created for the second VLAN, so that the different spanning trees for the first VLAN, 201, and the second VLAN, 202, are represented by [202416279 Foreign Version].

[0046] 10. The generated Multiple Spanning Tree instances are specified. The spanning trees for the first VLAN (201) and the second VLAN (202) are advantageously configured as Traffic Engineering Multiple Spanning Tree instances, while the spanning tree for the third VLAN (203) is configured as an Internal Spanning Tree instance. Further details on the functionality of the Multiple Spanning Tree Protocol can be found, for example, on Wikipedia (see https: / / en.wikipedia.org / wiki / Multiple_Spanning_Tree_Protocol). The Multiple Spanning Tree Protocol is also specified in IEEE 802.1Q-2022.

[0047] Traffic Engineering Multiple Spanning Tree instances are generally operated without a protocol to prevent loop formation. Therefore, in this case, a separate Forwarding Database (FDB) for each VLAN is configured on switches or bridges via an engineering interface alongside the respective Multiple Spanning Tree instance. Since no FDB learning takes place when using Traffic Engineering Multiple Spanning Tree instances, any number of Multiple Spanning Tree instances can be created. Switches or bridges can be configured for each Multiple Spanning Tree instance so that a) no FDB learning occurs, b) FDB learning occurs collectively for the same Multiple Spanning Tree instance (Shared VLAN Learning), or c) FDB learning occurs individually.

[0048] In the present embodiment, an identifier of the first VLAN 201 is assigned an identifier of the first Multiple Spanning Tree instance. Similarly, an identifier of the second VLAN 202 is assigned an identifier of the second Multiple Spanning Tree instance. This applies analogously to the third VLAN 203. Based on this, each forwarding communication device 101-104, 111-113, 121-123, 131-133, 141-144 provides a separate sender and receiver address table, in particular a forwarding database or MAC table, for each VLAN 201-203.

[0049] Datagrams 300 containing control data, classified as real-time traffic, are duplicated by a sender-side communication device 101-104, 111-113, 121-123, 131-133, 141-144 and transmitted via both the first VLAN 201 and the second VLAN 202. A classification of the datagrams 300 into real-time and non-real-time traffic can be performed, for example, by a control program running on the respective communication device 101-104, 111-113, 121-123, 131-133, 141-144. In particular, the datagrams to be sent can also be duplicated by this control program. 202416279 Foreign version

[0050] 11

[0051] Received datagrams 300 assigned to the first VLAN 201 or the second VLAN 202 are checked at the receiver in the respective communication device 101-104, 111-113, 121-123, 131-133, 141-144 for identity with a previously received datagram 300. If an identical datagram 300 has already been received, a subsequently received duplicate is discarded, for example by the control program, which otherwise classifies and, if necessary, duplicates datagrams 300 to be sent.

[0052] The sender- and receiver-side communication devices 101-104, 111-113, 121-123, 131-133, 141-144 can detect the presence of an already received identical datagram 300, in particular based on a sequence number assigned or inserted by the sender, or based on an identical hash value calculated over the respective received datagram 300. If a sequence number is used, it can be inserted by the sender into a payload area of ​​the respective datagram 300, for example by the control program mentioned above.

[0053] The network controller 100 supports the implementation of the redundancy procedure described above and is designed to configure at least VLANs 201-203 on the communication devices 101-104, 111-113, 121-123, 131-133, 141-144. Furthermore, the network controller 100 is configured to configure the communication devices 101-104, 111-113, 121-123, 131-133, 141-144, to determine different spanning trees for the first 201 and the second VLAN 202 using a spanning tree protocol, through which active paths for forwarding the datagrams 300 via the communication devices 101-104, 111-113, 121-123, 131-133, 141-144 are specified for the first 201 and the second VLAN 202.Furthermore, the network controller is set up to configure the communication devices 101-104, 111-113, 121-123, 131-133, 141-144, to duplicate datagrams 300 to be sent with control data and to transmit them via both the first VLAN 201 and the second VLAN 202.

[0054] Finally, the network controller is designed to configure the communication devices 101-104, 111-113, 121-123, 131-133, 141-144, to check received datagrams 300 assigned to VLANs 201, 202 for identity with an already received datagram 300 and to discard if an identical datagram 300 has already been received.

Claims

202416279 Foreign version 12 Patent claims 1. Method for redundant data transmission in a communication network, in which - at least one first (201) and one second virtual local area network (VLAN) (VLAN) are configured for each communication device (101-104, 112-113, 122-123, 132-133, 142-144) of the communication network that forwards datagrams (300). - for the first (201) and the second VLAN (202), different spanning trees are determined within the communication network using a spanning tree protocol, through which active paths for forwarding datagrams via the forwarding communication devices are specified for the first and second VLANs respectively, - at least selected datagrams to be sent (300) are duplicated by a sender-side communication device (111) and transmitted via both the first VLAN (201) and the second VLAN (202), - Received datagrams (300) assigned to the VLANs (201 , 202) are checked by receiver-side communication devices (121 , 131 , 141) for identity with a previously received datagram and are discarded if an identical datagram has already been received.

2. Method according to claim 1, wherein, for loop-free transmission of the datagrams within the first (201) and the second VLAN (202), spanning trees are generated according to Multiple Spanning Tree Protocol, MSTP, via which the datagrams are transmitted, wherein a first Multiple Spanning Tree instance is generated for the first VLAN, wherein a second Multiple Spanning Tree instance is generated for the second VLAN, and wherein the different spanning trees for the first and the second VLAN are specified by the generated Multiple Spanning Tree instances.

3. Method according to claim 2, wherein an identifier of the first VLAN (201) is assigned an identifier of the first Multiple Spanning Tree instance, wherein an identifier of the second VLAN (202) is assigned an identifier of the second Multiple Spanning Tree instance, and wherein in each forwarding communication device (101-104, 112-113, 122-123, 132-133, 142-144) a separate sender and / or receiver address table, in particular a forwarding database or MAC table, is provided for each VLAN. 202416279 Foreign version 13 4. Method according to one of claims 2 or 3, wherein the spanning trees for the first and second VLAN are configured as Traffic Engineering Multiple Spanning Tree instances.

5. Method according to one of claims 1 to 4, wherein the sender and / or receiver communication devices are configured to send and receive at least the selected datagrams via the first and the second VLAN.

6. Method according to any one of claims 1 to 5, wherein the sender-side and / or receiver-side communication devices detect the presence of an already received identical datagram on the basis of a respective sender-side assigned and / or inserted sequence number and / or on the basis of an identical hash value calculated over the respective received datagram.

7. Method according to claim 6, wherein the sequence number is inserted into a payload area of ​​the respective datagram on the sender side.

8. Method according to any one of claims 1 to 7, wherein the active paths for forwarding datagrams are specified by blocking ports of the forwarding communication devices that would lead to loop formation within the communication network.

9. Method according to any one of claims 1 to 8, wherein the first (201) and the second VLAN (202) are used for the transmission of real-time data traffic, wherein a third VLAN (203) is configured for non-real-time data traffic on the communication devices (101-104, 112-113, 122-123, 132-133, 142-144) that forward datagrams (300), and wherein a separate spanning tree is determined for the third VLAN within the communication network using a spanning tree protocol, by which an active path for forwarding datagrams is specified for the third VLAN.

10. The method of claim 9, wherein the spanning tree for the third VLAN is configured as an internal spanning tree instance. 202416279 Foreign version 14 11. Method according to one of claims 9 or 10, wherein loop formation within the third VLAN is prevented by means of Multiple Spanning Tree Protocol, MSTP, or by means of Media Redundancy Protocol, MRP.

12. Method according to any one of claims 9 to 11, wherein first forwarding communication devices are interconnected in a ring topology, wherein second forwarding communication devices are interconnected in a line or tree topology, and wherein at least selected first forwarding communication devices are each connected to two first communication devices and to a second communication device.

13. Configuration device (100) for carrying out a method according to one of claims 1 to 12, wherein the configuration device is configured for this purpose, - to configure at least one first (201) and one second virtual local area network (VLAN) (202) for each communication device (101-104, 112-113, 122-123, 132-133, 142-144) that forwards datagrams (300), - to configure the forwarding communication devices, to determine different spanning trees for the first and second VLANs using a spanning tree protocol, which specify active paths for forwarding datagrams via the forwarding communication devices for the first and second VLANs respectively, - to configure at least one communication device (111), to duplicate at least selected datagrams to be sent and to transmit them over both the first VLAN and the second VLAN, - to configure at least one communication device (121 , 131 , 141), to check each received datagram assigned to the VLANs for identity with an already received datagram and to discard it if an identical datagram has already been received.

14. Communication system for carrying out a method according to any one of claims 1 to 12, wherein the communication system at least - a configuration device according to claim 13, - several datagram forwarding (300) communication devices (101-104, 112-113, 122-123, 132-133, 142-144), each with a first (201) and a second virtual local area network (202), VLAN, configured, 202416279 Foreign version 15 - a sender-side communication device (111) configured to duplicate at least selected datagrams to be sent and transmit them over both the first VLAN and the second VLAN, and - includes a receiver-side communication device (121, 131, 141) that is configured to check received datagrams assigned to the VLANs for identity with a previously received datagram and to discard them if an identical datagram has already been received, - wherein the datagram forwarding communication devices are each further configured to determine different spanning trees for the first and second VLANs using a spanning tree protocol, by which active paths for forwarding datagrams via the forwarding communication devices are specified for the first and second VLANs.

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