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Malvineous
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Joined: Wed Jul 14, 2021 6:21 pm

IGMP snooping broken? Won't deliver multicast traffic after a reboot

Sat Jul 31, 2021 11:02 am

I have noticed that in SwOS (2.13) on a CRS-309 if you enable IGMP Snooping it seems to break multicast streams.

With IGMP snooping active, starting a new multicast stream works fine. Traffic is delivered and the multicast group appears on the IGMP tab.

But if you reboot the switch while the stream is active, although after a reboot the multicast group reappears on the IGMP tab, the switch won't route traffic for that multicast group to any ports, even if you rejoin the multicast group.

To avoid going and stopping and restarting each active multicast stream, if I disable IGMP snooping then suddenly all the multicast data arrives. But if I go tick the box again, all existing multicast streams stop, even though they are listed on the IGMP tab against the correct interfaces.

Am I doing something wrong? I thought IGMP snooping was supposed to direct multicast traffic to only the ports that had requested it, not stop it from being delivered to any port.

I have IGMP snooping enabled on a Cisco switch and that's now it works there, so I'm trying to work out whether Mikrotik are doing something different or whether IGMP Snooping in SwOS is just broken.

What confuses me even more is that if I have a multicast IP streaming data on two different UDP ports, and I only restart one stream, then the Mikrotik will only deliver the multicast traffic on one of the UDP ports, even though the device is transmitting two streams on that IP address. I thought IGMP worked at the IP address level (ignoring the port) rather than the IP + port level so I find this quite surprising. It seems something in the switch is able to take all multicast traffic from a single IP and drop traffic for one port but permit traffic for another.

Anyone else using this and have any experience with it?

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