This article applies to PRTG Network Monitor version 19 or later

Auto-Discovery with SNMP Traffic Sensors

The auto-discovery always creates SNMP Traffic sensors for all interfaces whose SNMP traffic counters are above 0. As a result, every interface that has had traffic since the last restart of the monitored device is added, even if it is currently disconnected or disabled.

For more information, see NICs not as intended after autodiscovery and Auto-Discovery adds disabled interface.

Conversely, interfaces that have seen no traffic (their SNMP traffic counters are still 0) will not be added automatically when running an auto-discovery. See also Cisco Switching auto discovery ports.

The reason for this behavior is that we do not want to miss interfaces that are relevant (still in use), but might be disconnected due to a failure or disabled by mistake at the time that the auto-discovery takes place.

Possible Solution

If you do not want the auto-discovery to add SNMP Traffic sensors for deactivated or disabled interfaces, for example, you need to restart the device or its SNMP traffic counters before running the auto-discovery.

Note: Resetting SNMP traffic counters too often is not advisable as it will result in loss of data or erratic readings for existing SNMP Traffic sensors that are monitoring said device.
Also be aware that for Cisco devices, the SNMP traffic counters are not the same as the interface counters displayed within IOS. For more detailed information, see Auto-Discovery adds disabled interface.

More

• Enable Connection State Handling by default during autodiscovery
• Network adapter template
• PRTG Manual: SNMP Traffic Sensor
• PRTG Manual: Auto-Discovery