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Automatically update port name and number for SNMP Traffic sensors when the device changes them



After a device reboot (switch, router), the port designations change. Can PRTG automatically re-recognize the ports and re-map them to the original sensors so that monitoring continues while keeping all historic data?

How do the automatic port name and number updates of PRTG work for SNMP Traffic sensors?

help ifalias ifname name port prtg router snmp switch

Created on Oct 5, 2011 12:56:08 PM by  Daniel Zobel [Product Manager]

Last change on May 6, 2019 11:45:50 AM by  Maike Guba [Paessler Support] (2,404) 2 1

6 Replies

Accepted Answer



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 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.

Note: 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.


Created on Oct 5, 2011 12:56:19 PM by  Daniel Zobel [Product Manager]

Last change on Jan 2, 2023 2:31:29 PM by  Brandy Greger [Paessler Support]



Can the ifDescr field be added to this ifAlias routine? In particular Cisco ASA firewalls will re-index the interfaces on reboots and this behavior cannot be controlled as it can be on Cisco IOS devices. Unfortunately the ASA stores any interface names or descriptions as SNMP isDescr values and not isAlias values.

This ability to re-index the sensors is great. Unfortunately it does not address the largest set of problems we have with re-indexing.

Created on Oct 10, 2011 2:51:04 PM



Dear Jim, this will be possible with todays release. Please see the Port Identification option in your PRTG device's SNMP Compatibility Options.

Created on Oct 11, 2011 2:41:23 PM by  Torsten Lindner [Paessler Support]

Last change on Oct 12, 2011 12:29:09 PM by  Daniel Zobel [Product Manager]



I can't get this to work. I have set "Automatic sensor name update if name changes in device" under "Inherit SNMP Compatibility Options" in the device group. And then i ran a Auto-discovery on the switch. I have checked that the device is inheriting settings. Also tried to run an auto-discovery on the device group.

Created on Nov 17, 2011 11:34:38 AM



"Port Name Update" works only if "Port Identification" is specified as "Use ifAlias". If specify here "Use ifDescr", then the port name is not updated (using a standard port name template).

Created on Apr 5, 2012 12:18:50 PM



Hello, On the Cisco devices there is one configuration command "snmp ifmib ifindex persist" so that OID remains the same aftert the device reboots. Have you tried it ? if you reboot the router or switch the OID will remain the same.

Created on Oct 15, 2019 10:08:09 AM

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