Just to confirm are you using an iPhone to test with? For the sake of testing, I would try with 802.11r disabled and see if it makes any difference, or fully on. Your on the right path, using a 'test SSID' will allow you to figure out where the trouble areas are on your own. I'm going to assume the port configurations for each AP are correct, and all on the same L2 network. So assuming that, what I would do is have this new SSID only broadcast on AP's with tags, and then pick two AP's near each other and just walk back and forth and confirm if roaming is having an issue or not. I would tune your power levels, or at least see where Meraki is having them land on. You want it to be around 11 - 17dBm range, so if you have AP's at 5dBm or 30dBm then that's not good. This allows for proper TX power matching between clients and APs (again this also comes down to the RF design). Another thing to consider (not sure what country your in) is if your using DFS channels (UNII-2 and UNII-2e), which has impact on roaming. Clients are not allowed to actively scan and probe on DFS, they can only listen for beacons, so its takes longer for them to discover if there is an AP on a DFS channel for it to roam to, which means a delay in the roaming etc. 802.11k is supposed to help with this but its not perfect, so its something to consider. If all configurations are good, and your still having issues, its going to come down to the RF design which is a physical thing and these forums won't really be able to assist you with other than recommend you have an outside source perform a survey and come up with better AP locations. Have a look at this guide here (although I recommend ignoring the SSID bandwidth limiting they suggest): https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/Best_Practice_Design_-_MR_Wireless/Wireless_VoIP_QoS_Best_Practices This link here is about Cisco WLCs, but they go over some RF design stuff that is worth a review: https://mrncciew.files.wordpress.com/2012/12/brkewn-2000-voice-over-wifi-best-practices.pdf
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