I'm going to guess that for a specific layer 3 vlan, you have two or more Meraki switches plugging directly into the 9500 rather than each other? What I'm guessing is happening is a client is generating multicast or broadcast traffic, and multiple Meraki switches are seeing it. The switches involved that are seeing it are not connected through a Meraki switch, so they are unable to determine the true source of the client traffic. If you had a Meraki core switch this issue is unlikely to happen. Are you able to configure thre VLAN trunking on the 9500 so that each IDF Meraki switch sees a unique set of VLANs, or operates in seperate layer 3 domains? If you trunk all (or many) VLANs to all IDF switches, are you able to make a pair of Meraki switches a sub-core (distribution layer), so that all the other Meraki switches can plug into, and plug the sub-core into your 9500?
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