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Snmp monitor



I am not sure what happened as of late, but it seems that my snmp traffic sensor is losing connection. I am connecting this sensor to an off site router's WAN interface using it's public ip address. It seems to connect and then looses connection after a couple minutes in PRTG and keeps doing that over and over. PRTG sits on a machine behind a load balanced (Nat'd) firewall and we do have a vpn tunnel to this remote router. The access rule we have on the remote router does allow for access to port 161 from any source ip and I can see in the log the source ip on the incoming log only shows one of our ip's from our load balanced network. For whatever reason the one internet connection it won't work over, so when it starts using that line PRTG starts disconnecting. If I connect to it's private address since we have the vpn tunnel it works fine, but it obviously isn't as reliable as the sensor is dependent on the vpn tunnel being up. This is seemed to work in the past and kind of just started a month or so ago for unknown reasons. I am trying to see if this is a limitation of PRTG or if maybe it is an issue with trying to connect to the public address of the remote router while we have a vpn tunnel to that network.

load-balance prtg snmp-sensor

Created on Jun 18, 2014 4:39:05 PM

4 Replies




there are no limitations in PRTG to monitoring public interfaces on router via SNMP. However, as SNMP uses UDP, this can be tricky through the WAN, as UDP is not connection-based, so packets can be lost.
Do you see the same failures with our SNMP Tester (https://www.paessler.com/tools/snmptester)? Please run it on the PRTG Host, and perform a "Scan available standard interfaces" against the target device, in repeated manner every 60 seconds. What does the Tester show at a moment, when the sensors fail in PRTG?

best regards.

Created on Jun 20, 2014 1:54:39 PM by  Torsten Lindner [Paessler Support]



I get the same thing, no response. So basically as I mentioned above I am load balancing 2 internet connection on my firewall and I ran a packet capture on both WAN interfaces where the snmp requests come from. The one internet connection is fine and I can clearly see the snmp get-request and the get-response from the router on port 161. When load balancing occurs and I end up going out the other WAN interface the packet capture only shows the get request, never see the get-response. It seems that the router we are monitoring will only send snmp responses over the one line, no idea why, but it just doesn't seem to making it to our firewall for whatever reason according to the packet capture.

Created on Jun 20, 2014 2:30:27 PM



I'm afraid it's something then where PRTG or we can do very little about. Maybe monitoring it from the LAN-Interface would be an option with a Remote Probe?

Created on Jun 20, 2014 2:54:38 PM by  Torsten Lindner [Paessler Support]



Yeah, if I use the lan ip of the router since we have a vpn tunnel it works just fine. However, it did work in the past, not sure what could of changed. I am going to try bringing down the vpn tunnel later on today and see if that solves the issue.

Created on Jun 20, 2014 3:30:33 PM

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