Daniel, depending on your deployment architecture/scenario, there are a couple of things to consider. Check out some of the deployment guides https://documentation.meraki.com/MX-Z/Deployment_Guides such as Warm Spare, DC-DC Redundancy, and SD-WAN solutions if that applies to your specific architecture. If you're simply referring to mitigating an MX failure in a single site, that's simply the Warm Spare (NAT HA) scenario for an active/passive design. Along with other obvious things like having both MX units on separate and redundant UPS power, dual power supplies in the higher-end models, physical separation between the units as appropriate (like in different racks or rooms or buildings), and then you'd consider redundant ISP connections, cellular WAN backup, and SD-WAN scenarios, etc. And if for whatever reason a 2nd MX for Warm Spare functionality isn't in the budget, consider a Z1 or Z3 along with a 4G cellular dongle, you could drop the Z appliance in place of a failed MX for emergency connectivity, and/or use the 4G USB dongle for ISP outages. Neither might not be close to the same bandwidth/performance, but then again it's for emergency backup use. I've used my own Z1 and 4G USB dongle together multiple times for on-the-fly connectivity, works great.
