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Home > Tech Library > Configuring N-TRON's 500 Series Switches For use with Rockwell Automation's EtherNet/IP

Download PDFConfiguring N-TRON's 500 Series Switches For use with Rockwell Automation's EtherNet/IP

Introduction

This application note will explain how switches utilizing IGMP snooping can significantly reduce traffic from bandwidth-intensive IP multicast traffic generated by Rockwell Automation products, Additionally, it will describe the Internet Group Multicast Protocol (IGMP) and provide step-bystep instructions for configuring N-TRON 500 series switches with the -A option to use IGMP Snooping. It will also discuss Port QoS and how it can improve determinism and reduce latency when used with Rockwell Automation's PLC's.

EtherNet/IP

Rockwell Automation EtherNet I/P devices support two distinct message types - 'explicit' messages and 'implicit' messages. Explicit messages are information messages used for device configuration and diagnostics, which are highly variable in both size and frequency. With explicit messaging, nodes must interpret each message, execute the requested task, and generate responses. On the other hand, Real-time control or 'implicit' messaging demands different performance. Here, the protocol needs to be able to support multi-casting (send to a number of nodes), while ensuring processing time in the node is kept to a minimum. This is due to the timecritical nature of the transfer.

EtherNet/IP realizes explicit messaging by using TCP/IP encapsulation. With this approach, the data field carries both protocol information and instructions for service performance. By employing a different protocol to realize implicit messaging - the User Datagram Protocol/Internet Protocol (UDP/IP). The UDP/IP data field contains no protocol information, only real-time I/O data. The meaning of the data is predefined at the time the connection is established and processing time in the node is therefore minimized during runtime. Such messages are low overhead, short, and provide the time-critical performance required for control functionality.

diagram 1

By using both TCP/IP and UDP/IP protocols to construct network messages, both implicit realtime I/O and explicit messaging can occur. When using this methodology, EtherNet/IP allows prioritization of I/O data over regular non time-critical messaging data. As a result, EtherNet/IP supplies every service that is essential in control and device-level networks - from polled, cyclic and change-of-state trigger mechanisms to multicast and point-to-point data transfer. This provides Ethernet users with real-time I/O, device-configuration and diagnostic capabilities, along with the interoperability and interchangeability demanded by modern industrial applications.

Figure 1

Layer-2 Switches and Multicast Packets

By default, an unmanaged layer 2 Ethernet switch floods multicast traffic within the broadcast/multicast domain. This consumes a lot of bandwidth if several multicast servers are sending streams to a LAN segment. Multicast traffic is flooded because a switch usually learns MAC addresses by looking into the source address field of all the frames it receives. However, a multicast Group Destination Address (GDA) MAC address (01:00:5E:XX:XX:XX) never appear as a source address for a packet. Therefore a layer-2 ethernet switch cannot "learn" the port associated with source of the multicast packet, hence it must forward the multicast packet to all ports.

IGMP Overview

IGMP protocol starts with a query packet. According to the RFC's the router with the lowest IP address on the network is responsible for generating the IGMP query packet. All nodes that support IGMP require a querier to be present on the network (EtherNet I/P is no exception). The use of routers for this purpose is often not desired in a stand alone control systems environment due to the delays they create in communication and added expense. N-TRON switches do not require the use of a router by providing the configuration option to enable the switch to generate query packets.

In response to the IGMP query packet, participating nodes will respond with the appropriate Join or Leave messages. Unsolicited (early) joins or leaves are also supported by N-TRON's implementation of IGMP. If a node joins a group, the address is added to the MAC filtering database. If a node leaves a group, the address is removed. EtherNet I/P implicit messages generate multicast IGMP packets that could impact the determinism of a large network by loading it down with multicast traffic. Managed switches that support IGMP Snooping have the capability to "learn" IGMP group members, and "route" multicast traffic intelligently, instead of flooding.

Figure 2

Joining a Multicast Group

When a host decides to join a multicast group, it sends an IGMP Report message to the GDA it wants to join. The IGMP snooping switch will recognize the IGMP Report message and add a GDA MAC address of the associated port in the MAC Filtering Database. When multicast traffic is transmitted to the switch, it will forward the traffic to the ports associated with this GDA MAC address regarding the Filtering Database.

Router Data Filter

According to the IGMP RFC's, the router port should receive all IGMP packets. This strict implementation will generate unnecessary data traffic on the uplink/router port. The N-TRON implementation (Designed in conjunction with Rockwell Automation), provides an option to filter the data packets from the router port, and thus reduces unnecessary network loading. IGMP Join and Leave control messages will still be sent to the router regardless of this setting. By default, the Router Multicast Data Filter is enabled once IGMP is enabled, so the selected router port will not receive the I/O data packets.

Port QoS

Port Quality of Service (QoS) provides an effective way to assign high priority to the traffic entering from the ports you designate. By default, all ports will have the same priority status assigned. Therefore, without the use of Port QoS, the switch will process all packets in the order in which it received them. Port based QoS is disabled by default and can be enabled by selecting any number of ports. Typically, the ports that have a device connected, such as an AB ControlLogix PLC controller, would be selected for this feature. Once enabled, the traffic received on the ports you designated is given higher priority and will be processed before the lower prioritized traffic. Port QoS is especially useful when using legacy devices that are not IGMP compliant. This feature will allow you to further control latency and determinism in a controls network environment that may be loaded down with multicast messages. NOTE: port QoS will override 802.1p tagged QoS.

N-TRON Ethernet Switch Configuration

Enabling IGMP Snooping and the Query Function

By default, IGMP is disabled in N-TRON 500 series switches. In most cases, simply enabling our IGMP Snooping feature will prevent the unnecessary multicast traffic on the LAN from overloading the network. However, you may have some devices that are not IGMP aware. If this is the case, you can manually enter static filters to achieve the same results. This will over-ride the dynamic IGMP Snooping when conflicts exist. To configure an N-TRON 500 series switch for dynamic IGMP snooping, you will need to perform the following steps:

  1. Connect a standard serial cable to the COM port and load a terminal program on the PC.
  2. Supply power to the switch.
  3. Press [ESC].
  4. When prompted to do so, log in with User Name: admin [ENTER].
  5. Enter the Password: admin [ENTER].
  6. Type SWITCH and press [ENTER].
  7. Type IGMP and press[ENTER]
  8. Type ENABLE and press [ENTER]
  9. Type QUERYON and press [ENTER]. Then, to accept the calculated address answer NO [ENTER].
  10. Cycle power to the switch to initiate new settings.

NOTE: Do not use QUERYON if you have a router or another querier device on your LAN.

Enabling Multicast Static Filters

Setting up a static filter address and assigning the filter to a given port will allow control over which ports addresses are allowed to egress from. Although this filter was originally intended as a multicast address, unicast addresses can also be filtered using this mechanism. To manually configure static filters, you will need to perform the following steps:

  1. Connect a standard serial cable to the COM port and load a terminal program on the PC.
  2. Supply power to the switch.
  3. Press [ESC].
  4. When prompted to do so, log in with User Name: admin [ENTER].
  5. Enter the Password: admin [ENTER].
  6. Type SWITCH and press [ENTER].
  7. Type FILTERS and press [ENTER].
  8. Type SFILTER [ENTER].
  9. Type in the SFILTER Group Number when prompted and press [ENTER].
  10. Type the 12 digit address (Ex. 01005e0a0a0a) and press [ENTER].
  11. Type in the ports that are not to be filtered as prompted and press [ENTER].
  12. Cycle power to the switch to initiate new settings.

Note: To disable a group, select the group and enter all zeroes for the address.

Changing the Router Port

To manually change the router port (default is port 1), you will need to perform the following steps:

  1. Connect a standard serial cable to the COM port and load a terminal program on the PC.
  2. Supply power to the switch.
  3. Press [ESC].
  4. When prompted to do so, log in with User Name: admin [ENTER].
  5. Enter the Password: admin [ENTER].
  6. Type SWITCH and press [ENTER].
  7. Type IGMP and press [ENTER].
  8. Type ROUTER and press [ENTER].
  9. Type in the port number the router is connected to when prompted and press [ENTER].
  10. Cycle power to the switch to initiate new settings.

Enabling the Broadcast Filter

By enabling the Broadcast filtering feature, the N-Tron switch can control which ports to receive broadcast traffic. To enable the broadcast filter, you will need to perform the following steps:

  1. Connect a standard serial cable to the COM port and load a terminal program on the PC.
  2. Supply power to the switch.
  3. Press [ESC].
  4. When prompted to do so, log in with User Name: admin [ENTER].
  5. Enter the Password: admin [ENTER].
  6. Type SWITCH and press [ENTER].
  7. Type FILTERS and press [ENTER].
  8. Type BFILTER and press [ENTER].
  9. Enter the ports that are not to be filtered as prompted and press [ENTER].
  10. Cycle power to the switch to initiate new settings.

Note: To disable the broadcast filter feature, enter all port numbers in step 9.

Enabling Port Quality of Service QoS

By enabling the Port QoS, the N-Tron switch will assign high priority to the traffic it receives from the ports you select. To enable the Port QoS, you will need to perform the following steps:

  1. Connect a standard serial cable to the COM port and load a terminal program on the PC.
  2. Supply power to the switch.
  3. Press [ESC].
  4. When prompted to do so, log in with User Name: admin [ENTER].
  5. Enter the Password: admin [ENTER].
  6. Type SWITCH and press [ENTER].
  7. Type QOS and press [ENTER].
  8. Type SET_PORT and press [ENTER].
  9. Enter the ports that you want to designate with high priority as prompted and press [ENTER].
  10. Type EN_PORT and press [ENTER].
  11. Type INFO [ENTER] to verify your configuration.
  12. Cycle power to the switch to initiate new settings.

The information in this white paper was first published by N-TRON. In an effort to bring out customers the best possible information, B&B Electronics has received permission to republish this document. B&B Electronics thanks N-TRON for allowing us to distribute this information to our customers. You can visit their site at http://www.n-tron.com.

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