Rockwell Automation 20-COMM-ER 20-COMM-ER Dual-Port EtherNet/IP Communication Adapter User Manual - Manual (Page 18)

18 Rockwell Automation Publication 20COMM-UM015B-EN-P - July 2013
Chapter 2 Installing the Adapter
Preparing for an
Installation
Before installing the adapter, do the following:
• Make sure the Ethernet switch is the correct type. A ‘managed’ switch that
supports IGMP snooping is usually recommended. An ‘unmanaged’
switch can be used instead if RSLogix 5000 software, version 18.00 or
later, is used and all devices on the network are configured for ‘unicast’
I/O. For more details, see the following documents:
– EtherNet/IP Media Planning and Installation Manual,
ODVA publication 148
– EtherNet/IP Network Infrastructure Guidelines,
ODVA publication 35
– Ethernet Design Considerations Reference Manual,
publication ENET-RM002
• Understand IGMP Snooping/Ethernet Switches
The 20-COMM-ER adapter is a multicast device. In most situations, an
IGMP snooping (managed) switch is required. If more than one or two
20-COMM-ER adapters are connected to the switch, a managed switch is
required—otherwise the drive may fault on a DPI Port x network loss. The
20-COMM-ER, RSLogix 5000 software, version 18.00 or later, and a
ControlLogix or CompactLogix controller will support unicast. Unicast
setup is required when adding the drive to the I/O. When all adapters are
set up as unicast devices, then an IGMP snooping (managed) switch is not
needed.
Much of EtherNet/IP implicit (I/O) messaging uses IP multicast to
distribute I/O control data, which is consistent with the CIP producer/
consumer model. Historically, most switches have treated multicast
packets the same as broadcast packets. That is, all multicast packets are re-
transmitted to all ports.
IGMP snooping constrains the flooding of multicast traffic by dynamically
configuring switch ports so that multicast traffic is forwarded only to ports
associated with a particular IP multicast group.
Switches that support IGMP snooping (managed switches) ‘learn’ which
ports have devices that are part of a particular multicast group and only
forward the multicast packets to the ports that are part of the multicast
group.
Be careful as to what level of support a switch has of IGMP snooping.
Some layer 2 switches that support IGMP snooping require a router
(which could be a layer 3 switch) to send out IGMP polls to learn what
devices are part of the multicast group. Some layer 2 switches can use
IGMP snooping without a router sending polls. If your control system is a
standalone network or is required to continue performing if the router is
out of service, make sure the switch you are using supports IGMP
snooping without a router being present.