IGMP — Internet Group Management Protocol

Internet Group Management Protocol

For full multicast and IGMP snooping configuration, see networking/multicast-and-igmp-snooping.

IGMP is the protocol by which IP hosts signal to their local router or switch that they want to join or leave a multicast group. Multicast allows a single stream (e.g., a Dante audio channel or an AV-over-IP video stream) to be received by multiple subscribers simultaneously without duplicating traffic — the network only delivers the stream to devices that have subscribed to it. Without IGMP snooping, managed switches flood multicast traffic to every port on the VLAN, consuming link bandwidth and potentially overwhelming devices that receive unwanted multicast.

How IGMP Snooping Works

IGMP snooping is a switch-level feature that allows the switch to inspect IGMP membership reports from hosts and querier messages from the router. Instead of flooding all multicast to all ports, the switch builds a table of which ports have subscribed to each multicast group, and only forwards the multicast traffic to those ports.

Process:

1. A receiver (Dante device, NVX decoder) sends an IGMP Membership Report: "I want to receive multicast group 239.x.x.x"
2. The switch with IGMP snooping enabled records this: "port 12 wants group 239.x.x.x"
3. When the sender (Dante device, NVX encoder) transmits to that multicast address, the switch forwards it only to port 12 (and any other ports that subscribed), not to every port on the VLAN

IGMP Versions

Dante uses IGMPv2 multicast. AES67 and some AV-over-IP platforms use IGMPv3 for source-specific multicast (a receiver subscribes to a specific source address, not just a group address). Configure switches to support at least IGMPv2; IGMPv3 where required.

IGMP Querier

IGMP requires a querier — a device that periodically sends IGMP General Queries to the VLAN to prompt hosts to re-assert their group memberships. Without a querier, switches eventually age out their membership tables and start flooding multicast again. The querier is typically a router (the default gateway for the VLAN) or a Layer 3 switch.

If the AV VLAN has no router, configure IGMP Querier on the switch itself (available on Cisco, Aruba, Netgear, and most managed switches as "ip igmp snooping querier" with a source IP in the VLAN's subnet).

IGMP Fast-Leave

IGMP fast-leave (IGMPv2 Immediate Leave) allows a switch port to immediately remove a host from a multicast group when it receives a Leave Group message, without waiting for the querier's Last Member Query timeout (default 2 seconds). For AV-over-IP systems with many receivers joining and leaving streams, enabling fast-leave reduces the time stale multicast traffic floods to a port after the receiver stops listening. Fast-leave is enabled per VLAN or per port in most switch configurations.

Common Pitfalls

• IGMP snooping disabled, multicast floods all ports. Without snooping, Dante multicast streams and AV-over-IP video saturate every port on the VLAN regardless of subscription. A 100 Mbps Dante multicast stream delivered to every port in a 48-port switch consumes 4.8 Gbps of switch backplane capacity. Fix: enable IGMP snooping on the VLAN on every switch in the Dante/AV-over-IP network.

• No IGMP querier on isolated AV VLAN. The AV VLAN has no router and IGMP querier is not enabled on the switch. After a few minutes, the switch ages out its membership table and starts flooding multicast to all ports. Fix: enable the IGMP querier function on the switch with a static IP in the AV VLAN's subnet.

• IGMP snooping enabled but querier not enabled — multicast still floods. Snooping requires a querier to function correctly; enabling snooping alone is not sufficient if there is no querier on the segment. Fix: verify querier status with show ip igmp snooping querier (Cisco) or equivalent; enable if absent.

• IGMPv3 required by AES67 but switch configured for IGMPv2 only. AES67 source-specific multicast uses IGMPv3 INCLUDE reports; an IGMPv2-only switch cannot process these and falls back to flooding. Fix: verify switch supports IGMPv3 snooping and enable it on the AES67 VLAN.