This article applies as of PRTG 22, as well as partly to PRTG 9 and PRTG 12
PRTG and ifAlias
When PRTG scans a switch or router with an SNMP Traffic sensor, it stores the interface number and the ifAlias value (OID 220.127.116.11.18.104.22.168.1.1.18.x) for every port. The internal format is number:IfAlias. Some devices do not provide an ifAlias value. In this case, only the port number is stored. The Interface Number and ifAlias are shown in the sensor’s Settings tab as number:ifAlias.
How automatic port name and number updates work
With every scanning interval of an SNMP Traffic sensor, PRTG requests the router or switch device’s uptime and the port’s ifAlias* (if available) in addition to the current traffic values. With the uptime value, PRTG can identify a device reset. If PRTG scans the sensor for the first time after a PRTG (re)start, this is treated as a device reset as well.
Scenario 1: PRTG just started or device was reset
If PRTG has just been started or a device reset has been determined (reading the device uptime), PRTG tries to update the interface designations.
ifAlias known to PRTG
If PRTG has ifAlias* values stored for this device, it will scan all device interfaces and search for a matching ifAlias*.
- If found and unique, the Interface Number is updated (the device has changed the port order on reset).
- If not found, ifAlias* and Sensor Name are updated (alias values are not set correctly or were edited while PRTG was not running).
ifAlias not known to PRTG
If PRTG does not find ifAlias* values in its records, only the Sensor Name is updated.
Scenario 2: Device was not reset
When operating in the usual way, PRTG can also detect changes to the device’s ifAlias* values.
ifAlias known and changed
If PRTG has ifAlias* values for this device and they have been changed, PRTG will update the Sensor Name and the ifAlias* (the ifAlias* was edited by a user).
ifAlias not known or unchanged
If PRTG does not find ifAlias* values in its records, or if a found ifAlias* value has not been changed, PRTG will do nothing.
Scenario 3: No action possible
If the port order is changed and the ifAlias* values were edited while PRTG was not running, there is no way to identify the port that a sensor belongs to. In this case, PRTG keeps the current Interface Number and updates the ifAlias* and Sensor Name to the new values. Without ifAlias* values, PRTG cannot detect a changing port order.
If Use ifDescr or ifName is set in the Port Identification setting of the device's SNMP Compatibility Options, PRTG uses the ifDescr respectively ifName field instead of the ifAlias value in this scenario.