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

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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 monitoring continues while keeping all historic data?

How do PRTG's automatic port name and number updates 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 [Paessler Support]

Last change on Jan 4, 2013 1:45:50 PM by  Daniel Zobel [Paessler Support]



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This article applies to PRTG Network Monitor 13 or later, as well as partly to v9 and v12

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 1.3.6.1.2.1.31.1.1.1.18.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.

Note: Sensor name updates are only applied if the respective option is enabled in the SNMP Compatibility options.


Scenario 1: PRTG Just Started or Device Was Reset

If PRTG was just 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

In usual operation, PRTG can also detect changes to the device’s ifAlias* values.

ifAlias Known and Changed

If PRTG has ifAlias* values for this device and it was changed, PRTG will update the Sensor Name and the ifAlias* (the ifAlias* was user edited)

ifAlias Not Known or Unchanged

If PRTG does not find ifAlias* values in its records or if a found ifAlias* value was not 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 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 changing port order.


* Note

If Use ifDescr or ifName is set in the Port Identification setting of the PRTG device's SNMP Compatibility Options, PRTG will use the ifDescr resp. ifName field instead of the ifAlias field in this scenario.

See Also

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

Last change on Nov 17, 2014 6:10:12 PM by  Gerald Schoch [Paessler Support]



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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 by  Jim Kirby (181) 2 1



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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 [Paessler Support]



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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 by  Eirik (0) 1



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"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 by  landy (0)



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