As I understand it, the failover will not occur because the link from your MX to your WAN ISP is still active and even though the ISP link is down, the MX67C will not failover. For the failover to occur, 3 things must ALL fail. 1.) DNS lookup, 2.) ICMP/HTTP failure and 3.) ARP failure to gateway. Since the ISP upstream link is failing, not the local Ethernet link between MX and Say Cable Modem, the failover will not trigger.  See link: Connection Monitoring for WAN Failover - Cisco Meraki "Note: An MX will only failover to a backup cellular connection if all three tests (internet, DNS, and ARP) are marked as failed." I have not found a way to disable the ARP monitoring before failover, so this make the failover feature very less usable! This is as of Current version: MX 18.107.2. ... View more