@buschtrommelXXL Glad to hear you got it working.   Here are the guide lines for setting up the STP guard features:   BPDU Guard should be enabled on all end-user/server access ports to avoid rogue switch introduction in network Loop Guard should be enabled on trunk ports that are connecting switches  Root Guard should be enabled on ports connecting to switches outside of administrative control These come from this document, https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/Best_Practice_Design_-_MS_Switching/General_MS_Best_Practices.  However, between the MX and MS devices I wouldn’t enable any of the STP guards since the MX doesn’t participate in STP, it just forwards any BPDUs it receives. ... View more