Correct.. This is not recommended. You can ignore the above idea. Primary MX and secondary MX need to be configured using warmspare VIP. You may need to have /29 subnet - as we will 6 usable ip address. / 30 subnet wont work. Below switch need to be separate not in stack to forward the VRRP packets. If you have the Cisco FTD in active / passive scenario, you need to add two more connections. Cisco FTD Primary to Secondary MX Cisco FTD Secondary to Primary MX.
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