What is IGMP?
IGMP (Internet Group Management Protocol) is a network protocol that allows devices to join or leave multicast groups. In practice, it tells a managed switch or router which ports actually need a multicast stream, so that the traffic is forwarded only to the relevant receivers instead of being broadcast everywhere. Without IGMP, multicast traffic behaves like broadcast, flooding the entire network and consuming unnecessary bandwidth.
IGMP and the ST12MHDLAN Series
Our ST12MHDLANxx Series products (HDMI-over-IP) transmit video as multicast traffic. With a single transmitter and a single receiver, IGMP isn’t strictly required: the traffic simply flows between the two devices. However, as soon as multiple receivers are added, or especially when multiple transmitters are active, IGMP becomes essential to prevent the network from being saturated with multiple video streams going to devices that don’t need them. A clear way to visualize this is by using a laptop at the edge of the network running Wireshark:
- Without IGMP, the laptop will see the full video data streams even though it isn’t a receiver.
- With IGMP enabled, the laptop won’t see those video packets because the switch only sends them to the RX ports that requested them.
Credit: Wikipedia
Applying IGMP in Practice
On a managed switch such as the IES101002SFP, enabling IGMP Snooping alone is not enough, you also need to configure a VLAN entry with IGMP Querier enabled. The querier periodically sends “Are there any members?” queries, and the receivers (RX units) respond to stay subscribed. By contrast, on many routers (e.g., TP-Link AC5400), enabling IGMP Snooping/Proxy is typically a single toggle that handles both snooping and querier functions automatically. It’s important to note that IGMP support is required on every network segment between a transmitter and receiver, and that only one device should be the querier in a broadcast domain, similar to how only one DHCP server should exist in a network. This ensures that multicast streams are efficiently managed without duplication or conflict.
TLDR: Without IGMP, the transmitter is shouting for all to hear. With IGMP, the signal is properly channeled only to the devices that need it.