What is IGMP Snooping and why enable it?

IGMP Snooping is a Layer 2 feature that allows switches to intelligently forward multicast traffic only to ports that requested it, rather than flooding all ports.

The Problem It Solves

Without IGMP snooping, multicast traffic (video streams, IEC 61850 GOOSE, industrial protocols) floods to every port in a VLAN — wasting bandwidth and CPU on non-interested devices.

How It Works

1. Switch monitors IGMP membership reports between hosts and multicast routers
2. Builds a table of ports subscribed to each multicast group
3. Forwards multicast frames only to subscribed ports
4. Prunes ports on IGMP leave messages

Key Terms

• IGMP Querier: Router that periodically queries for members
• IGMP Snooping Querier: Switch-based function when no router present
• Fast Leave: Immediately removes port on leave message
• Static Multicast: Manually configured group memberships

Industrial Use Cases

• IEC 61850 Sampled Values (SV) and GOOSE messaging
• IP video surveillance with multicast streams
• PROFINET IRT communication
• Multicast SCADA polling

Best Practices

• Enable IGMP snooping on all VLANs carrying multicast
• Configure snooping querier if no L3 device present
• Enable fast-leave on access ports (single host)
• Set appropriate query intervals (default 125s often too long)