The following KB describes what to do and check if you face an issue with M320 devices with active "External A-B LOS" alarms.

The following alarms might appear on the M320 devices with a pretty old code as [12.3R3-S4.10].

user@router> show chassis alarms

4 alarms currently active

Alarm time Class Description

2023-07-24 21:11:10 UTC Major CB 1 External-B LOS

2023-07-24 21:11:10 UTC Major CB 1 External-A LOS

2023-07-24 21:11:07 UTC Major CB 0 External-B LOS

2023-07-24 21:10:51 UTC Major CB 0 External-A LOS

If you check the logs and it was a one-time event, you can ask the customer to reboot both CBs, one at a time to avoid traffic disruption but if the alarm comes back; here is the following matching PR:

https://prsearch.juniper.net/problemreport/PR1152035

This is just a cosmetic log shown in pretty old (EOS) Junos codes.