What are the LAN port settings of the MXs? You don't want to have them set as drop untagged traffic. That will results in loops due to STP running on the untagged VLAN. Practical example of a spoke I have. Diagram and failover animations here. MX has VLANs: 10, 20, 30, 31, 32, 50. VLAN 30 is the mgmt VLAN for the switches and also what I use as the native VLAN. So, MX LAN ports are trunks, native 30, allow 10, 20, 30, 31, 32, 50. Switches ports are the same exact config - trunks, native 30, allow 10, 20, 30, 31, 32, 50. As for MS side STP options. This doc covers the use case for each option. My personally, I wouldn't use any guard mode for the switchports connected to the MX LAN ports.
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