Multicast, IGMP, and the Querier

One sender, several interested receivers

With unicast, a source sends a separate copy to each receiver. A 700 Mbps encoder feeding ten decoders could therefore require 7 Gbps from the encoder. With multicast, the encoder sends one stream to a multicast group address and the network duplicates it only where paths branch toward interested receivers.

A multicast address such as 239.10.10.50 identifies a group, not a physical device. The source does not need a list of every decoder. Receivers join the group when they want the stream and leave when they no longer need it.

IGMP manages receiver membership

IGMP—Internet Group Management Protocol—does not carry the audio or video. It carries membership information. A decoder sends a membership report that effectively says it wants traffic for a particular multicast group.

IGMPv2 supports ordinary group joins and leaves and remains common in commercial AV. IGMPv3 adds source filtering, which allows Source-Specific Multicast: a receiver can request group G specifically from source S. Seeing IGMPv3 in a switch configuration does not prove that every endpoint or AV platform is using SSM.

Snooping controls Layer 2 forwarding

A Layer 2 switch cannot learn multicast destinations through the ordinary unicast MAC-learning process. Without multicast awareness, it may flood a stream throughout the VLAN, including toward control processors, touchpanels, printers, or wireless access points that never requested it.

IGMP snooping lets the switch observe membership reports and build a table of multicast groups and interested ports. The switch then forwards each stream only toward its receivers and any required inter-switch or router ports.

• IGMP: receivers report which groups they want.
• IGMP snooping: switches learn which Layer 2 ports lead to those receivers.
• Multicast routing: a separate function that carries multicast between IP networks.

The querier keeps membership state alive

A receiver can disappear without sending a leave message. The IGMP querier periodically asks which memberships are still required. Active receivers respond, switches refresh their snooping tables, and stale entries eventually age out.

Snooping without an active querier may appear to work initially. Later, memberships can expire and behavior becomes platform-dependent: streams may stop, flood, or change unpredictably. That is why 'IGMP snooping enabled' is not a complete commissioning result.

The querier does not have to be the default gateway. A Layer 3-capable switch can often provide the querier function while an external firewall remains the VLAN gateway. Multiple querier-capable devices may participate in an election, but BMT should document the intended active device, its address, and any deliberate backup.

Membership belongs to a VLAN

IGMP snooping decisions are maintained per VLAN. A receiver in one VLAN cannot join a source in another through Layer 2 snooping alone. Crossing that boundary requires multicast routing, a proxy, or an application-specific gateway.

For many smaller AV systems, the cleanest design keeps high-bandwidth multicast sources and receivers together while routing management and control normally. Discovery must be documented separately because control, discovery, clocking, and media may use different traffic flows.