@Zac67, yes - the default VLAN would be the native VLAN that all ports on a switch will deafult to without any configuration overriding it. The default VLAN is also referenced in smartport configuration. Other switches on the network are setup with a default vlan of 11, and without being able to change that on the SG350, the switch will complain of a mismatched default VLAN... or maybe the message is actually referring to native VLAN, I'd have to check.

I made our firewall the IGMP router. I'm no longer seeing duplicate IGMP general membership queries. I think the issue is with a bad firmware for this switch. I'll have to ask the guy at Cisco that's dealing with my support case to check this in their lab and see if they have the same issue on the firmware I'm running (latest).

When monitoring with Wireshark, I actually see two IGMP general queries from the IGMP querier address. The two packets have different identifiers - so they're not the same packet bouncing around and hitting the same host twice... either something is, as @RickyBeam says echoing the query, or the switch really is sending out two queries, which can only be a bug in the firmware.

Hi Ricky. RSTP is enabled on all switches on the network. I've used Wireshark to see if there's anything odd going on, but apart from these spurious messages that the switch is generating, I can't see anything that suggests there's a loop in the network. I'm wondering now if the issue is purely because the switch is in router mode and there's something different I need to do with the configuration... or perhaps I should move IGMP querier onto a switch that's in layer 2 mode?

Great answer, @Eddie. The guys who setup our Cisco switches used the default VLAN 1 for our main data LAN and VLAN 2 for our voice. We're setting up a new site and the two are going to be linked via Ethernet. The new site will use VLANs 11 and 12. Is there some way of preventing VLAN 1 on one side of the link from going across to the other?