I can confirm the stack is correctly configured on dashboard as well as connected with cables. We are missing the 5th stack cable, but this isnt the problem, that just gives us less resilience. Funnily enough that cable is arriving today 🙂   As highlighted - these are the trunk links back to the core. LACP is configured correctly on all ports on the MS250's and the 3750's (mode active is enabled on all 4 ports), however STP is blocking the ports on Switch05. I'm pretty confident the issue is pure STP. If I bring up VLAN1, STP should start recognising the core and LACP will hopefully come up properly. ... View more