Failure detection for an MX Warm Spare pair uses VRRP heartbeat packets. These heartbeat packets are sent from the Primary MX to the Secondary MX on all configured VLANs in order to indicate that the Primary is online and functioning properly. As long as the Secondary is receiving these heartbeat packets, it functions in the spare state. If the Secondary stops receiving these heartbeat packets, it will assume that the Primary is offline and will transition into the master state. When the MX is in NAT mode, VRRP heartbeats are not sent over the WAN; there is no guarantee that the WAN interfaces can communicate with each other. Instead, we use a mechanism called "connection monitor" to determine the WAN state of the device.
For more in-depth information regarding the VRRP Mechanics on the MX, please see the NAT HA Failover Behaviordocumentation.
Connection monitor is an uplink monitoring engine built into every MX Security Appliance. The mechanics of the engine are described inthis article. When all uplinks of a Primary MX are marked as failed by connection monitor, that MX will stop sending VRRP heartbeat packets, which will initiate a Warm Spare failover. Once at least one uplink on the Primary returns to a working state, the Primary resumes sending heartbeat packets and the Secondary relinquishes the Active role back to the Primary.