I also doubt this is an MX issue ... But let's try and break the problem down further. Perhaps next Monday, can you plug the MX circuit directly into the primary MX (this will obviously mean your warm spare is not working). This will bypass your switch, and a couple fo connections. If the issue still happens, you know it is directly related to the ISP and primary MX (since everything else was not plugged in). Does the uplink graph in the dashboard show any big traffic flows at this time (perhaps someone is doing a big upload/download). This is the list of things monitored on the uplink: https://documentation.meraki.com/MX/Firewall_and_Traffic_Shaping/Connection_Monitoring_for_WAN_Failover#Failover_Connectivity_Tests Ask Meraki support to confirm which event is causing the failover. Perhaps it is the link going down. Perhaps the link is staying up, but the tests are failing.
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