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I use Nagios to monitor about 30 Cisco 2960 switches (check_snmp_load, check_snmp_int). I only check load and one or two backbone interfaces of every switch. I get a lot of false positives caused by lack of an answer from devices. I suppose SNMP UDP packets get lost somewhere but I have not been able to troubleshoot it. I checked sh int summary on every switch and there is no information about dropped packets (IQD nor OQD).

The same Nagios instance monitors dozens of other devices (linux servers, iDRACs/iLOs, SAN switches, tape libraries etc.) and there is always everything alright with those.

The only difference in infrastructure connections between Nagios and 2960s / other devices is that 2960s are connected through quite old FC infrastructure. Is it possible it is a matter of these FC cables? How can I check it (without purchasing expensive network measurements)? I already used iperf but it did not show any problems.

What else can I do to troubleshoot this issue?

Update: My SNMP configuration on 2960s is quite simple:

#sh run | include snmp
snmp-server community commstr RO
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Dry different software? I don't know nagios that well, but I have never had any problems with Cacti for example. – jwbensley Apr 23 '13 at 14:17
@Mike I would be interested to see what your snmp configuration looks like on your 2960. Can you run a 'show run | include snmp' (remove anything you think is not necessary) – hfranco Apr 29 '13 at 20:16

on the switch you can enable

deb snmp packet

then you will see every snmp packet going in/out your switch. on the nagios server use tcpdump or tskark (wireshark) for network sniffing. if you monitor both at the same time you should be able to say whether the problem is caused by packet loss or not.

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