AV-over-IP Fundamentals: Multicast, IGMP Snooping & QoS

Est. 60 minCompetency: Explain multicast, IGMP snooping, and QoS for AV-over-IP.

Learning objectives

Explain unicast vs multicast for video.
Describe IGMP snooping and why it matters.
Apply QoS to prioritize AV traffic.
Identify bandwidth considerations for video.

Vocabulary

Unicast: One-to-one traffic; one stream per receiver.
Multicast: One-to-many; one stream reaches many receivers efficiently.
IGMP snooping: Switch feature limiting multicast to ports that asked for it.
QoS (DSCP): Marking traffic so switches prioritize AV over best-effort data.
Jitter: Variation in packet arrival time; hurts AV quality.

Lesson content

AV-over-IP sends video and audio as network streams. The choices that keep it clean are multicast, IGMP snooping, and QoS.

Unicast vs multicast

Unicast — one stream per receiver. Simple but bandwidth multiplies with receivers.
Multicast — one stream delivered to many receivers who 'join' the group. Efficient for one-to-many video.
Multicast is standard for distributing a source to many displays.

IGMP snooping

1Without it, a switch floods multicast to every port — every link carries every stream.
2IGMP snooping lets the switch forward multicast only to ports whose devices joined the group.
3This keeps multicast video from saturating links that do not need it.
4Enable IGMP snooping on the AV VLANs and set a querier if there is no multicast router.

Field caution — multicast flood

Forgotten IGMP snooping is a classic AV-over-IP killer: one source to four displays floods the whole switch and everything glitches. Enable it before adding streams.

QoS and bandwidth

Mark AV traffic (often with DSCP values) so switches queue it ahead of best-effort data.
Video bandwidth scales with resolution, frame rate, color depth, and compression — plan the uplink capacity.
Match QoS settings end-to-end across every switch in the path.

Worked example

One encoder feeds six displays over multicast. With IGMP snooping, only the six links to joined displays carry the stream; the rest of the switch is quiet. QoS marks the video so it is never starved by file traffic on the same VLAN.

Implementation checklist

✓Multicast used for one-to-many video.
✓IGMP snooping enabled with a querier.
✓QoS/DSCP marking applied end-to-end.
✓Uplink bandwidth sized for the stream count.

Downloadable study guide

study-guide.txtcopy-paste ready

01. Unicast = per receiver; multicast = one-to-many via group joins.
02. IGMP snooping limits multicast to joined ports; prevents floods.
03. QoS/DSCP prioritizes AV over best-effort traffic end-to-end.
04. Size uplinks for resolution × frame rate × count.