I think you must be referring to the MX Warm Spare functionality, where a secondary MX unit will take over from a primary, should the latter fail. If this is indeed what you want to use, I'm afraid you won't be able to do it using an MX100 paired with an MX250 - the MX models must match exactly. For confirmation please see the Note near the beginning of the relevant documantation: https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair Unfortunately the MX100 is now End of Sale, so your chances of adding a second unit to it are probably slim. You may find that, through talking to your meraki account team, they can maybe help you with the cost of a new matched pair of devices, given that you have existing investment.
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