You lost me with your topology explanation. However, the two common topologies we recommend are documented here https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair#Recommended_Topologies MXs don't participate in STP, but the BPDUs from switches will flow through the MX LAN ports as long as you aren't dropping untagged traffic. As long as STP is configured properly on your switches spanning tree behavior should behave just as if the switches were directly connected as the MX LAN is just kind of a passive bump along the way in this design. I also have an animated deck showing the behavior of MX HA and dual switches in this deck https://docs.google.com/presentation/d/1yFB7tffTaBexz2mq7UWBJaUiiGhV45bzIZRmuo3b7R8/edit?usp=sharing
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