Specifications

Optimizing Performance
84
EIO0000000051 6/2010
IGMP Snooping
Multicast Messaging
Internet Group Management Protocol (IGMP) is an essential feature of multicast
messaging. IGMP instructs routers and switches to forward Ethernet multicast
packets to only those device ports that have requested these packets.
In the absence of IGMP snooping, a switch forwards multicast packets out of all its
ports, resulting in greater network traffic, wasted network bandwidth, and degraded
network performance.
Be sure to configure one Ethernet network switch as the IGMP querier. This switch
periodically polls the field devices connected to the network, which causes all
connected devices to issue an IGMP Multicast Group Join message. The group
message is received by all network switches, which update their multicast
addressing information databases in response.
Similarly, when an Ethernet device transmits an IGMP Multicast Group Leave
message, all network switches update their multicast addressing information
databases by removing the device from their databases.
Multicast messaging reduces network traffic by:
z requiring that a message be sent only once
z sending the message only to devices for which the message is intended
Recommendation
Schneider Electric recommend the following:
z employ switches that support IGMP V2 or higher
z because IGMP snooping may be disabled by default, be sure to enable IGMP
snooping for each network switch
z be sure that one switch is configured as the IGMP querier