@merda_sottile wrote: if the switch is down, this is no problem, but if both switches are up, i have usually the problem having a loop between mx <- switch1 - stackconnection - switch2 -> mx, and then the connection goes havoc. This typically happens when BPDUs from switch1 to the MX are not received back on switch2. One reason could be that you don't have a VLAN1 on the MX that can pass the BPDUs.
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