PA System Fixed By Turning OFF IGMP Snooping... Why?

[ccooney]

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?

[FredrikB-NN2]

If you have no IGMP Querier in the VLAN, any multicast will work for 260 seconds (Query Interval 125 s x 2 + Max Response Time 10 s) with standard settings after the listener joins the stream. Run the command "show igmp vlan office" (or whatever you VLAN is called) and you should see this:

Learned multicast addresses(Last Querier=x.x.x.x):

If there is no querier, add an IP address to the VLAN on one of the Extreme switches and you get one. For production use, you should make sure you have a proper IGMP setup, not just a random IP on the first available switch...

[ccooney]

I got an output. The querier is up, I know that much, and it has returned an IP that is unfamiliar to me. I don't suppose you could help me decode what it's telling me? I'm in utterly unfamiliar territory here.

Query Interval : 125 sec
Max Response Time : 10 sec
Last Member Query : 1 sec
Robustness : 2

Interface on VLAN VLAN_0108 is enabled and up.
inet 0.0.0.0/0
Locally registered multicast addresses:

Learned multicast addresses(Last Querier=0.0.127.2):
224.0.2.60

s = static igmp member

Querier UpTime: 9 days 23 hours 13 minutes 6 seconds

Flags:
IP Fwding NO IPmc Fwding NO IGMP YES
IGMP Ver V2 Snooping NO Proxy Query YES
XmitRtrAlrt YES RcvRtrAlrtReq NO

[Gabriel_G · Accepted solution]

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!

[Stefan_K_]

---

IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic to control delivery of IP multicasts. Network switches with IGMP snooping listen in on the IGMP conversation between hosts and routers and maintain a map of which links need which IP multicast transmission.

Disabling IGMP snooping on the switch will cause Multicast-Traffic to just flood in the whole VLAN, that's probably why it fixed your issue.

• Are you only using Extreme Switches or are there maybe some other vendors? (e.g. small desktop switches)
• Did you do some IGMP troubleshooting? Some helpful commands can be found here: https://extreme-networks.my.site.com/ExtrArticleDetail?n=000002660 (show mcast cache, show igmp snooping "vlan" etc.)

Best regards
Stefan