This is usually because the device doesn't respond to SNMP or WMI when making a change, such as polling times, custom properties, etc.. I usually use the report down interfaces, because it provides a single place to see Unknown Interfaces. Usually when there are Unknown interfaces it is either the Communication (SNMP/WMI) is broken or the Interface no longer lives on that Node. Either way, they point me to fixing those issues and keeping the environment clean and up to date. There is also Alerts you can configure for this, but is more complicated.
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