It has been a while since I last did a MIBwalk of an MX67, but the following OIDs appear to show the up/down status on each of the 5 ports for the MX. 1.3.6.1.2.1.2.2.1.8.1 to 1.3.6.1.2.1.2.2.1.8.5 where a value of 1 = UP, and 2 = DOWN. There do not appear to be any counters for time in an MX that I have found, sadly. Uptime counters exist in Meraki switches, just not in MX's or AP's. For a dual connected device if you were watching these values, you would see if the adjacent device was disconnected. But this may not tell you there is an upstream issue causing data not to transmit. But there are similar OIDs that show traffic is passing on the interfaces, if you see a counter is no longer incrementing that would likely be your best option for monitoring issues. This is provided you don't mind using SNMP and getting into the weeds a little bit. To play with SNMP use SNMPview 2.5 and/or Paessler SNMP Tester software. Both are easy to setup.
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