We have an MX HA pair at each site and a stack of physical switches that they connect to. We cable one MX to one switch and the other MX to one of the other switches. Therefore if either MX fails you still have a connection to the switch stack. Equally if the switch connected to the active MX fails, the other MX takes over and as it is linked to a remaining working switch, you still have a connection. Worst case where you get a partial failure on a switch, as the MXs are connected to the dashboard via the WAN ports, you can force a failover.
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