@whistleblower that's interesting, on the one hand I can see you don't see the 15.42.3 option required, but on the other, you still have MS12.28.1 which is no longer an option for me.   It looks like Meraki maintain legacy stable versions in an organisation dashboard where they are upgrades, but not downgrades.  I have an MX HA pair as a VPN concentrator running 15.42, and you must have some 12.<28.1 switches...   In this case I'd ask support to apply 15.42.3 to your MX pair. ... View more

In case anyone else finds this useful, Meraki Support pointed me to the Docs where it does state that it is guaranteed to be unique in the Org and not just in networks bound to the template: "The subnet will be randomly selected based on the address space and subnet mask, but will not use any subnets that have previously been used in the organization." https://documentation.meraki.com/General_Administration/Templates_and_Config_Sync/Managing_Multiple_Networks_with_Configuration_Templates#MX_-_Template_VLAN_IP_Address_Range_Allocations Don't know how I missed that! ... View more