Multicast & IGMP
One stream, many receivers — without flooding the network.
In a networked AV system, one encoder often feeds many decoders: a lectern PC shown on six displays, or a camera watched in three rooms. Sending a separate copy of a several-hundred-megabit stream to each receiver would quickly exhaust the encoder's link. Multicast solves this — but only if the network is configured for it. Misconfigured multicast is one of the most common reasons AV-over-IP systems misbehave.
Three ways to send
| Method | Who receives it | AV example |
|---|---|---|
| Unicast | One specific device | A control command to one display |
| Broadcast | Every device on the subnet or VLAN | Device discovery, DHCP requests |
| Multicast | Only devices that asked to join that group | One AV-over-IP stream to many decoders |
- Unicast
- Who receives itOne specific device
- AV exampleA control command to one display
- Broadcast
- Who receives itEvery device on the subnet or VLAN
- AV exampleDevice discovery, DHCP requests
- Multicast
- Who receives itOnly devices that asked to join that group
- AV exampleOne AV-over-IP stream to many decoders
A multicast stream is sent to a group address rather than to a device. IPv4 multicast addresses run from 224.0.0.0 to 239.255.255.255. The 239.x.x.x block is reserved for use inside an organisation (RFC 2365), which is why AV systems usually pick their stream addresses from it. The encoder sends one copy; the network copies it to wherever it's wanted.
IGMP: asking to join
Receivers say which groups they want using IGMP (Internet Group Management Protocol). When a decoder is routed to an encoder, it sends an IGMP membership report — “I'd like 239.1.1.1, please.” When it switches away it sends a leave. IGMP version 3 also lets a receiver ask for a group from one specific source.
IGMP snooping: delivering only where asked
A basic switch has no idea who wants a multicast stream, so it sends it out of every port, like a broadcast. With a few audio streams that may go unnoticed. With video, it is fatal: every port, including office laptops and the uplink, is flooded with traffic it never asked for.
IGMP snooping fixes this. The switch listens to the IGMP messages passing through it, notes which ports joined which groups, and forwards each stream only to those ports.
The querier
Snooping has a dependency that catches many installers out. Group memberships expire unless they are refreshed, and receivers refresh them by answering the regular queries of an IGMP querier. Normally the router for the VLAN acts as querier; on an isolated AV VLAN with no router, one switch must be configured to do it. With no querier, everything works at first, then memberships time out — typically after a few minutes — and streams either flood everywhere or stop, depending on how the switch handles unregistered multicast.
Simplified: one switch, one VLAN, illustrative bitrates. Real behaviour without a querier, and how uplinks to other switches are handled, depend on the switch's configuration.
Designing for multicast
- Enable snooping on every switch the streams pass through, and make sure each AV VLAN has a querier.
- Use fast leave where each port has a single receiver, so a decoder stops receiving the old stream as soon as it switches.
- Size the uplinks. A trunk between switches carries every stream that anything on the far side has joined — count the worst case.
- Keep streams inside one VLAN where possible. Routing multicast between VLANs needs a multicast routing protocol such as PIM on the layer-3 switch, which adds design and support effort.
- Follow the manufacturer's switch guide. AV-over-IP vendors document the exact snooping, querier and flooding settings they need, often for specific switch models.
Key takeaways
- 01Multicast sends one stream to a group address; the network copies it to the receivers that join.
- 02Receivers join and leave groups with IGMP; 239.x.x.x is the range set aside for use inside an organisation.
- 03Without IGMP snooping, a switch floods multicast to every port.
- 04Snooping needs an IGMP querier on the VLAN, or memberships expire and streams flood or stop.
- 05Enable snooping on every switch in the path, size uplinks for the worst case, and follow the vendor's switch guide.
Check your understanding
Q1Video on a new AV-over-IP system works after reboot, then freezes after a few minutes. Most likely cause?
Q2With IGMP snooping off, what happens to a multicast video stream on a switch?
Q3Why is multicast preferred over unicast for one encoder feeding eight decoders?