This blog originated from a question asked by one of our resellers, who encountered something during an AV-over-IP project that many installers will recognize:

“WHY AM I SEEING DROPPED FRAMES IN THE LOGGING? AND WHY DOES THE IGMP QUERIER BEHAVIOR NOT MATCH WHAT I EXPECT?”

During our technical discussion, it quickly became clear that these questions had one common factor: IGMP.

IGMP determines how multicast behaves, how switches respond to receivers, how a querier is elected, and why this can sometimes seem “strange” — especially in AV networks, where timing, bandwidth, and stability are critical.

In this first blog, I will share the full background you need to understand in order to predict multicast behavior. In next week’s blog, I will explain how this works specifically in NETGEAR Engage, including the “Planned Querier” setting and what it does — and does not — do.

What Is IGMP?

IGMP stands for Internet Group Management Protocol. It is the protocol that devices use to indicate which multicast streams they want to receive.

When a device needs an AV stream (for example, a Dante group or an IPMX video stream), it registers via IGMP with the switch. The switch uses that information to forward traffic only to the correct ports. Without IGMP, multicast traffic would be sent everywhere — leading to congestion, jitter, packet loss, and collapsing AV systems.

IGMP handles three basic functions:

• Joining a stream (Membership Report)
• Leaving a stream when it is no longer needed (Leave Group)
• Periodically checking whether receivers are still listening (General Query)

IGMP is simple in theory, but in practice it is highly sensitive to order, timing, and topology.

The IGMP Querier — The “Traffic Controller” of Multicast

In every VLAN where multicast is present, there must be exactly one IGMP Querier.

This querier sends a General Query every 30–60 seconds to determine which receivers still want to receive multicast traffic.

How Does the Network Choose the Querier?

IGMP follows a strict standard rule:

THE SWITCH WITH THE LOWEST IP ADDRESS IN A VLAN AUTOMATICALLY BECOMES THE QUERIER.

This happens completely automatically — whether you intend it or not.

This can lead to surprising situations:

• An edge switch becomes querier simply because it has a lower IP than the core.
• The “preferred querier” is ignored because it has a higher IP address.
• A switch that has just booted up wins the election because the IP plan was not designed properly.

As soon as two switches are querier-capable, the IP address alone determines the outcome.

This explained part of our reseller’s observations: the network was behaving according to the standard — not according to his expectations.

Why Does IGMP Sometimes Behave “Strangely”?

1. Incorrect or Unpredictable Querier Due to IP Numbering

If IP addresses are not deliberately planned, the “wrong” switch may win.

2. Multiple Queriers Active Simultaneously

If a switch has Election Participate disabled, it does not take part in the election — but it may still believe it should act as querier if it does not detect another one.

3. No Active Querier

In that case, multicast stops after a timeout and streams disappear unexpectedly.

4. Incorrect Snooping Configuration

If IGMP snooping is enabled but no querier is present, the switch has either insufficient or incorrect information.

5. Unexpected Video Loss

A misconfigured variable somewhere in the chain (jitter, buffering, microbursts, packet loss) can lead to visible consequences — especially with AV codecs that have very little tolerance for loss.

Why NETGEAR Developed IGMP Plus™

Many AV engineers indicate that multicast configuration is:

• too complex,
• too error-prone,
• and often fails under time pressure.

That is why NETGEAR added IGMP Plus™ to all Pro AV models (M4250, M4300, M4350, M4500).

What Does IGMP Plus Actually Do?

IGMP Plus:

• Automatically enables all required IGMP snooping settings
• Eliminates advanced IGMP configuration (one setting instead of ten)
• Prevents flooding
• Prevents incorrect querier elections in multi-switch environments
• Ensures multicast works immediately
• Is optimized for AV protocols such as Dante, AES67, Q-SYS, sACN, NDI, and IPMX

IGMP Plus is therefore not “just another feature,” but a safety net that prevents AV engineers from getting lost in the complexity of IGMP standards.

Conclusion

IGMP sometimes appears “strange” because:

• Querier election is uncompromising (lowest IP wins)
• Small IP planning mistakes can have major consequences
• Multicast depends on a single central control point
• Multicast propagates through mechanisms that provide little visual feedback

With IGMP Plus™, this is structurally resolved:
multicast works reliably, predictably, and without complex manual configuration.

👉 Next week, I will explain how all of this appears in NETGEAR Engage, how Engage visualizes the querier, and why the “Planned Querier” setting does not influence the election process.

Eric Lindeman, NETGEAR ProAV Staff Systems Engineer Benelux

For more information about NETGEAR AV Switching, please contact the NETGEAR Pro AV Design Team via email: ProAVdesign@netgear.com

If you’d like to delve deeper into AV over IP switching, I invite you to check out our Online Academy via the link: https://academy.netgear.com/

On our training portal, you can find both AV and IT-related training courses. These courses are free to attend after registration, and at the end of each course, you can take an exam to earn a certificate.