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PA System Fixed By Turning OFF IGMP Snooping... Why?
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03-06-2024 06:00 AM
Hello. This one had us stumped and in many cases, still does.
For context, I am running 5420M-48W-4YE edge switches, and over the winter break (we are a k-12 school) I pushed a firmware update to my switches, bringing them to 3.6.1.5.
Over the past few weeks, classroom teachers have been complaining that the IP speakers in their rooms have been quiet, or altogether not working. As I worked with the hardware vendor, the problem seemed to get worse. I pushed a firmware update to the devices, which seemed to fix the issue, only for it to slowly come back. We worked with my multicast paging device, similarly with no luck.
In desperation I turned to Google, and noticed that one person years ago was having a similar issue, with completely dissimilar equipment. What stood out was the solution they used that worked for them was to activate IGMP Snooping on their switches. I noted with dismay that IGMP Snooping is on by default on my models.
The thought occurred: why not try toggling it off?
Sure enough, turning OFF the IGMP Snooping fixed the issue; my IP speakers sprang to life immediately.
I'm not sure anyone might be able to explain what's going on with that? Is IGMP Snooping broken in the firmware version I'm running on?
Solved! Go to Solution.
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03-08-2024 10:44 AM
The IGMP querier for a VLAN is responsible for:
-Receiving all multicast streams that exist in that VLAN
-asking all devices in that VLAN what multicast streams they want
-Forwarding relevant multicast streams towards the devices that have requested them
Importantly, the IGMP querier is the switch/router with IGMP enabled (not to be confused with IGMP snooping) that has the LOWEST IP in that VLAN.
Note that the IP Address of the querier can be literally anything, it does not have to belong to the subnet associated with the VLAN.
Based on your output, you have some device in VLAN 18 that has IGMP enable and that has IP address 0.0.127.2.
That seems very strange and that seems to be some 'rouge' device in your network that is hijacking IGMP querier operation.
You'll need to find whatever device is 0.0.127.2 and remove it from your network or reconfigure it.
The easiest way to identify this device will probably be to perform a packet capture for this VLAN (wherever is easiest for you, maybe on a laptop or server in this VLAN), and look for IGMP queries coming from IP Address 0.0.127.2. Then look for the MAC address that sent this IGMP query. Then you can run the command 'show fdb <MAC>' on any of your EXOS switches to see where in the network that MAC address is learned to find this device.
Hope that helps!
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03-06-2024 09:50 AM
Having reviewed the page I'm not immediately seeing anything that might help illuminate what the causes of my issue might be; then again, this is all new to me so I'm not really sure how to read the outputs. With snooping turned off I'm not sure it's going to give me any usable metrics, and snooping needs to remain off for the PA system to work. I will otherwise have to wait for a school vacation day; I was hoping maybe someone might be aware of any possible problems with leaving snooping on.
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03-07-2024 01:05 AM
IGMP snooping needs to be enabled in order for the commands to give an output. 🙂
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03-07-2024 05:20 AM
I figured as much, but that will bork my PA system. Seems I'll have to wait for a school vacation or something.
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03-06-2024 07:30 AM
We are using extreme switches with the exception of my core switch, which is Aruba. Everything was working fine prior to the firmware update, and seemed to slowly degrade over time until the speakers were barely turning on at all.
I'll look through those troubleshooting commands and see what they have to say.
