Entry type: FAQ, Entry ID: 9822272, Entry date: 10/16/2015
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How do you use and configure IP multicast for UDP connections in Industrial Ethernet CPs / CMs?

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IP multicast is a special connection option that is supported and can be configured only for UDP connections (UDP - User Datagram Protocol) in Industrial Ethernet CPs / CMs. IP multicast was established to send messages from one station to multiple partner stations.

Multicast messages are sent with a special multicast address. The IP address range 224.0.1.0 to 239.255.255.255 is used exclusively for IP multicast.
A broadcast message is sent to all participants in the network.
Broadcast messages are needed, for example, when searching for the MAC address of an IP address (ARP request). This is why a communication module has to generally process and evaluate broadcast messages. If there are too many broadcast messages on the network, the network performance drops. This is because the single modules have to process all broadcast messages to determine which ones are meant for them.

In the case of S7-300 / S7-400 Industrial Ethernet CPs you should pay attention to the following points with regard to broadcast messages:

  • After receipt, the broadcast messages are filtered out as high priority in all S7-300 / S7-400 Industrial Ethernet CPs. All messages which are not usable (ARP request, for example), are discarded immediately. This prevents the negative influence of broadcast messages on the other connections.
  • S7-300 / S7-400 Industrial Ethernet CPs can send data, but can`t receive data, over UDP connections that are projected.

Since STEP 7/ NCM V5.1 + SP2 and higher projecting of IP multicast for UDP connections in S7-300 / S7-400 Industral Ethernet CPs is available to send a message to a specific group of communication partners.

Behavior of the communication processor
The communication processor is generally also blocked for multicast messages. The time is an exception here. If you now enable a multicast group in the configuration, it is also enabled in the controller. In this way that one group is active and the communication processor continues to be immune to broadcast messages from the network. Each configured multicast group has to be registered in the controller.
This is why multicast is always a better solution when messages are to be sent to groups of partner stations.

  • Under entry ID 16767769 you can find information about the max. number of multcast groups that is supported by the S7-300 Industrial Ethernet CPs.
  • Under entry ID 15368142 you can find information about the max. number of multicast groups that is supported by the S7-400 Industrial Ethernet CPs.
  • In the manual of S7-1200 / S7-1500 CP / CM you can find information about the max. number of multicast groups that is supported.
  • The data length is limited to 2048 bytes just as with UDP.
  • The communication processor continues to be immune to broadcast loads.
  • The requirement is of course that all the partner stations also support multicast.
  • The messages are sent without any security mechanisms (acknowledgment).
  • IP multicast messages can be sent to external IP subnets over routers.

The messages are sent without an acknowledgment request because the UDP protocol does not provide for acknowledgments. If, for example, a message is sent to 100 partners, then 100 acknowledgments would arrive simultaneously (one acknowledgment per partner). Such floods of data could not be evaluated by the sender module.

Configuration of multicast connections

  1. Insert a new connection of the "UDP connection" type in NetPro. As connection partner you select "All multicast stations".
       

    Fig. 1
       
  2. In the Properties dialog of the UDP connection you open the "Addresses" tab. Here you define the multicast group. The IP addresses 224.0.1.0 to 239.255.255.255 are available specially for IP multicast. Messages from this IP address range are recognized as multicast messages by each module. The IP address range is used exclusively for IP multicast via UDP connections.
    The local and remote ports can be enabled from 1 to 65535). The first multicast group is assigned the IP address 224.0.1.0. The IP address of multicast group can be changed by the user. The IP address range 224.0.1.0 to 239.255.255.255 is possible.
        

    Fig. 2
     
  3. In NetPro the UDP connection is displayed in the list of configured connections. In the case of a multicast connection the entry "All multicast nodes" is displayed in the Partner column.
       

    Fig. 3
     

Configuration recommendations
It is recommended to follow the rules below when configuring multicast connections:

  1. Select identical port numbers for the local and remote ports  of a multicast connection.
  2. Consider the selection of Multicast addresses on the LAN

Select identical port numbers for the local and remote ports  of a multicast connection
If multicast messages have passed the controller, only the port number is relevant.

Exactly one multicast connection is configured between two stations. In station 1 und 2 you have to configure the same port number for the local and remote port of the multicast connection. Thus station 1 can receive data which station 2 sends to the multicast group and station 2 can receive data which station 1 sends to the multicast group.
 

Station 1 Station 2
IP address 140.90.36.1 140.90.37.1
Multicast group 224.0.1.0 224.0.1.0
Local port 2000 2000
Remote port 2000 2000

Table 1

Consider the selection of Multicast addresses on the LAN
With multicast the 3 low bytes of the IP address are copied into the last bytes of the MAC address 01.00.5E.00.00.00. This is then the MAC address which is entered in the controllers for the separate multicast groups. This ensures that the messages can pass the respective controllers. Furthermore, the top bit of the first copied byte of the address is ignored and is always 0.
The created MAC address is also visible as target MAC address in the messages on the LAN.

A multicast message is sent from Station 1 to Station 2.
   

Station 1 Station 2 Station 3
IP address 140.90.36.1 140.90.37.1 140.90.38.1
Multicast group 224.0.1.0 224.0.1.0 225.0.1.0
Local port 8000 8000 8000
Remote port 8000 8000 8000

Table 2

Since station 1 and station 2 are in the same multicast group and all the selected port numbers are identical unrestricted bidirectional data exchange is possible between Station 1 and Station 2.
However, multicast messages that are sent by station 1 and station 2 are also received by station 3. This is due to the multicast groups 224.0.1.0 and 225.0.1.0 have the same MAC address 01:00:5E:00:01:00.

Note

  • Different multicast IP addresses really represent the same multicast group. Therefore, you should pay attention when assigning the multicast group in order to avoid unwanted messages on non-addressed stations. This is due to a regulation based on the valid RFC 1112 (internet standard).
  • The Industrial Ethernet CPs / CMs support the IGMP protocol (Internet Group Management Protocol). Stations in external IP subnets are accessible,  due to the IGMP protocol affords IP multicasting on the internet. IP multicasting distributs simultaneous messages to several stations under one IP address. The management of multicast groups occurs in the routers, that are connected directly to the receivers of a multicast group. IGMP protocol affords functions that the Industrial Ethernet CPs / CMs can advise the router of the requirement of mulicast messages of a special multicast group. Multicast routing protocol adopt the coordination between the routers.

Modules that support multicast connections
Table 3 lists the S7-1200 / S7-1500 modules that support multicast and broadcast connections.
 

StationIntegrated interface of the CPUCommunication module (CM)
Communication processor (CP)
MulticastBroadcastMulticastBroadcastMulticastBroadcast
S7-1200NoSend1) / ReceiveCM not availableCM not availableNoNo
S7-1500NoSend1) / ReceiveSend / ReceiveSend / ReceiveSend / ReceiveSend / Receive

Table 3

1) Send via UDP connection is not supported.

Table 4 lists the S7-300 / S7-400 modules that support multicast connections.
   

 Module

Article number

Firmware versionMulticastBroadcast
CP 343-1 PN

6GK7343-1HX00-0XE0

V1.0 and higherSend / ReceiveSend
CP 343-1 Standard6GK7343-1EX10-0XE0V2.1 and higherSend / ReceiveSend
CP 343-1 ERPC6GK7343-1FX10-0XE0V1.0 and higherSend / ReceiveSend
CP 343-1 BACNET6FL4343-1CX10-0XE0V1.0 and higherSend / ReceiveSend
CP 343-1 Lean6GK7343-1CX10-0XE0V2.0 and higherSend / ReceiveSend

CP 343-1 Standard

6GK7343-1EX11-0XE0

V2.0 and higher

Send / ReceiveSend

CP 343-1 Standard

6GK7343-1EX20-0XE0

V1.0 and higher

Send / ReceiveSend
CP 343-1 Standard6GK7343-1EX21-0XE0V1.0 and higherSend / ReceiveSend
CP 343-1 Standard6GK7343-1EX30-0XE0V2.0 and higherSend / ReceiveSend
CP 343-1 IT

6GK7343-1GX11-0XE0

V2.0 and higherSend / ReceiveSend
CP 343-1 IT

6GK7343-1GX20-0XE0

V1.0 and higherSend / ReceiveSend
CP 343-1 Advanced

6GK7343-1GX21-0XE0

V1.1 and higherSend / ReceiveSend
CP 343-1 Advanced

6GK7343-1GX30-0XE0

V1.0 and higherSend / ReceiveSend
CP 343-1 Advanced

6GK7343-1GX31-0XE0

V3.0 and higherSend / ReceiveSend

CP 443-1 Standard

6GK7443-1EX10-0XE0

V2.0.31 and higher

Send / ReceiveSend

CP 443-1 Standard

6GK7443-1EX11-0XE0

V2.0.31 and higher

Send / ReceiveSend
CP 443-1 Standard

6GK7443-1EX20-0XE0

V1.0 and higherSend / ReceiveSend
CP 443-1 Standard

6GK7443-1EX30-0XE0

V3.0 and higherSend / ReceiveSend
CP 443-1 IT6GK7443-1GX11-0XE0V2.0 and higherSend / ReceiveSend

CP 443-1 Advanced

6GK7443-1EX40-0XE0

V1.0 and higher

Send / ReceiveSend

CP 443-1 Advanced

6GK7443-1EX41-0XE0

V1.0 and higher

Send / ReceiveSend

CP 443-1 Advanced

6GK7443-1GX20-0XE0

V2.0 and higher

Send / ReceiveSend

CP 443-1 Advanced

6GK7443-1GX30-0XE0

V3.0 and higher

Send / ReceiveSend

CP 443-1 RNA

6GK7443-1RX00-0XE0

V1.0 and higher

Send / ReceiveSend

Table 4