What is IGMP Snooping and why is it important for Ethernet/IP applications?

IGMP Snooping and IGMP Query are a process by which network switches listen to IGMP, or multicast, conversations between network devices and then filter out unnecessary traffic. IGMP Snooping is very important in order to ensure proper performance on networks with heavy multicast traffic, like those using Ethernet/IP.

Phoenix Contact managed switches feature IGMP snooping and IGMP query for use on Ethernet/IP networks. In fact, the Lean Managed Switch "-E" version are specifically designed for quick and easy use with Ethernet/IP right out of the box: Simply give it an IP Address and it works.Ethernet/IP devices can generate a lot of multicast traffic and therefore it is necessary to limit which end devices receive the traffic -- to avoid overloading them and causing them to fail.

When a switch without IGMP Snooping receives multicast messages, it floods all ports, potentially overloading end devices and other network switches. However, a switch with IGMP Snooping can forward multicast messages to only the devices that request the traffic and are designed to handle it.

When an Ethernet/IP device wants to consume multicast data, it will transmit an IGMP join message. These join messages are received by all IGMP Snooping switches and the switch "snoops" on the join messages as they pass in order to determine which ports will receive the multicast data. This restricts the multicast data to only the ports-and connected end devices-that expect and can handle the traffic.

For IGMP Snooping to work properly, one or more switches or routers in the network must provide IGMP Query support. The IGMP Querier will periodically ask each end device in the network which multicasts they wish to receive, refreshing the IGMP multicast/port associations.

NOTE: If no switch or router is present that supports IGMP Query, multicast traffic problems will likely occur. Such problems include conditions where the IGMP Snooping switches act erroneously and either (1) forget the learned multicast/port associations and then flood all ports and devices, or (2) don't forward multicast traffic to any devices at all, including those that should receive it.

Ethernet/IP requires IGMP Query support on at least one switch or router in the network. IGMP Query supports multiple IGMP Queriers in the network. If there are multiple, then the IGMP Querier with the lowest IP address will act as the network Querier. If it was to fail or be removed, then the next lowest IP address will take over. Clearly, IGMP Query is critical to IGMP snooping.