Monitoring traffic by ifDescr OID

what device are you monitoring?

I monitor Linux boxes. IfIndex constantly changes in Linux dirung reboots. Is this issue solved in last versions of PRTG that it can track the changed IfIndexes automatically?

Under certain conditions, PRTG is able to Automatically update port name and number for SNMP Traffic sensors when the device changes them.

I try to get Scenario 2 in https://kb.paessler.com/knowledgebase/en/topic/25893-automatically-update-port-name-and-number-for-snmp-traffic-sensors-when-the-device-changes-them Nothing helps all traffic sensors are in alarm state - PRTG does not handle ifIndex change properly.

I do the following. 1) Create new device in PRTG 2) Create a sensor 3) Delete interface on the physical device 4) Recreate interface and check that ifIndex changed 5) Check if sensor is OK.

So nothing helps - the sensor is always in Error state after ifIndex change. Also Interface number (in Sensor Settings) changes from e.g. 140:test1 to 140:_. I repeat steps 1-5 many times with different options in Device settings:

- Port Name Template ([port]) [ifalias]; Port Name Update Keep port names (use this if you edit the names in PRTG); Port Identification Use ifAlias (recommended). When create a sensor I got in Sensor settings: Interface number 134:_ . So I guess that is because device do not use ifAlias.

- Port Name Template ([port]) [ifalias]; Port Name Update Automatic sensor name update if name changes in device; Port Identification Use ifDescr. When create a sensor I got in Sensor settings: Interface number 134:test1 . So I guess that PRTG and device successfully work with ifDescr, so I use it further.

- Port Name Template ([port]) [ifalias]; Port Name Update Keep port names (use this if you edit the names in PRTG); Port Identification Use ifDescr. When create a sensor I got in Sensor settings: Interface number 134:test1 .

- Port Name Template ([port]) [ifalias]; Port Name Update Keep port names (use this if you edit the names in PRTG); Port Identification Use ifDescr. When create a sensor I got in Sensor settings: Interface number 134:test1 .

- Port Name Template ([port]) [ifdescr]; Port Name Update Keep port names (use this if you edit the names in PRTG); Port Identification Use ifDescr. When create a sensor I got in Sensor settings: Interface number 134:test1 .

- Port Name Template [ifdescr]; Port Name Update Keep port names (use this if you edit the names in PRTG); Port Identification Use ifDescr. When create a sensor I got in Sensor settings: Interface number 134:test1 .

After all these actions I try to get the Scenario 1 https://kb.paessler.com/knowledgebase/en/topic/25893-automatically-update-port-name-and-number-for-snmp-traffic-sensors-when-the-device-changes-them . I restarted snmpd to flush SNMP uptime (each PRTG device has a SNMP Uptime sensor). Nothing changed on a device with any configuration - all traffic sensors are in alarm state - PRTG does not handle ifIndex change! I also checked by snmpwalking than linux's net-snmp package does not recognize ifAlias, but does ifDescr, ifName.

PRTG Network Monitor 9.2.0.2141

The Port-Name Template is only important for the Sensors Name in PRTG. It is not used for the identification of an interface (in case the interface changed on the monitored device). For this only the Port Identification -setting is important. However, if your Linux targets do not offer ifAlias (or not use it) you have of course to use the ifDescr. Otherwise, it may also very well be that your Linux targets don't work according to the standard there, and so this can't be solved with PRTG.

I used other parameters just to make sure that they do not change the behavior. Now I understand that that was the case. I am not sure how ifDesc should work according to the standard, but I found out that ifDescr fileld changes properly to reflect ifIndex change. It is easy to check with snmpwalk, e.g. for ppp0 ifIndex = 15:

root@linux:~# snmpwalk -v 2c -c public 127.0.0.1 | grep ifIndex
IF-MIB::ifIndex.1 = INTEGER: 1
IF-MIB::ifIndex.2 = INTEGER: 2
IF-MIB::ifIndex.3 = INTEGER: 3
IF-MIB::ifIndex.15 = INTEGER: 15

root@linux:~# snmpwalk -v 2c -c public 127.0.0.1 | grep ifDescr
IF-MIB::ifDescr.1 = STRING: lo
IF-MIB::ifDescr.2 = STRING: eth0
IF-MIB::ifDescr.3 = STRING: eth1
IF-MIB::ifDescr.15 = STRING: ppp0

After ppp0 reconnect it changes ifIndex to 17. ifDescr changes accordingly:

root@linux:~# snmpwalk -v 2c -c public 127.0.0.1 | grep ifIndex
IF-MIB::ifIndex.1 = INTEGER: 1
IF-MIB::ifIndex.2 = INTEGER: 2
IF-MIB::ifIndex.3 = INTEGER: 3
IF-MIB::ifIndex.17 = INTEGER: 17

root@linux:~# snmpwalk -v 2c -c public 127.0.0.1 | grep ifDescr
IF-MIB::ifDescr.1 = STRING: lo
IF-MIB::ifDescr.2 = STRING: eth0
IF-MIB::ifDescr.3 = STRING: eth1
IF-MIB::ifDescr.17 = STRING: ppp0

PRTG is always in Error state after ifIndex change. Interface number in PRTG changes from 15:ppp0 to 15:_. This problem has existed beginning from PRTG Traffic Grapher and still exists in PRTG v9.2. That is strange nobody has reported this problem for Linux yet because it is so easy to check...