Are you taking into acount the time it needs to detect the failure? When you unplug the cable on the MX side, it immediately detects link down and can use the other links. When you unplug the cable on the CPE side then the MX will not detect a link down since it is connected to the switch. You have to wait until the ICMP, HTTP and DNS test start to fail. I will paste the following sentence of the documentation: When both the HTTP and ICMP tests have been unsuccessful for a period of time that exceeds 300 seconds, the uplink will be failed over. Therefore, it can take approximately five minutes for failover to occur in the event of a soft failure (where the physical link is still up but provides no internet access).
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