Rockwell Automation Ethernet Design Considerations Reference Manual - Manual (Page 55)

Rockwell Automation Publication ENET-RM002C-EN-P - May 2013 55
Ethernet Infrastructure Features Chapter 3
Internet Group Management
Protocol (IGMP) The IGMP is a communication protocol used to manage the membership of IP
multicast groups. Much of EtherNet/IP implicit (I/O) messaging uses IP
multicast to distribute I/O control data, which is consistent with the CIP
produced/consumer model. Without IGMP, switches treat multicast packets the
same as broadcast packets. Multicast packets are re-transmitted to all ports.
The behavior of an unmanaged switch is to flood multicast packets to all ports
within the same VLAN. This behavior is not typically desirable. To resolve this
the following occurs:
• Querier functionality manages a table that lists the devices that are
participating in multicast groups.
• Snooping functionality inspects packets from devices and forwards
multicast data to devices that only request the data.
IGMP snooping constrains the flooding of multicast traffic by dynamically
configuring switch ports so that multicast traffic is forwarded to ports associated
with only a particular IP multicast group.
If you have a router (Layer 3 device) on the network, make it the querier. IGMP
protocol has versions 1, 2, and 3. Rockwell Automation products support
versions 1 or 2. IGMP protocol version 2 negotiates the active querier
automatically and that task is assigned to the IGMP capable device with the
lowest IP address on a given VLAN. Therefore, assign the first available IP
address on a given VLAN to the router (Layer 3 device).
If you do not have a router, the querier function must be placed on a centrally
located IGMP capable device on the network by configuring it to the first
available IP address on a given VLAN.