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Troubleshooting SNMP monitoring

Configuring SNMP for network monitoring can be a headache. This document describes some common problems and fixes.

Configuring SNMP agent

SNMP software running on a network device is referred to as a 'SNMP agent'.

Although every network device has a different configuration process the following steps are usually required. Instructions are for SNMP versions 1 and 2. If you're using SNMP v3 we'll assume you're already an expert :)

SNMP version

Set SNMP version to either v1 or v2c. If no option exists for 'v2c' then try 'v2'.

SNMP community

The SNMP community acts like a password. Default is often set to 'public' and we strongly recommend this is changed.

Read Only or Read Write permissions

Opsview requires only 'read only' access (RO). We recommend you don't set greater level of permissions than you actually need.

Access Control

Most devices have some form of access control. Usually this involves specifying the IP address of your monitoring server. Beware if you're using Network Address Translation (NAT) on your network.

Configuring Opsview

In Opsview, SNMP settings are contained in the Host configuration form. From left navigator select Configuration → Hosts and edit a host. In the host configuration, you need to select the SNMP tab.

  • Set correct SNMP version. Most plugins will use SNMP version 2 and then fallback to 1, but it will be quicker and more correct to choose the correct version
  • Set correct community. There will be more security options if SNMP version 3 is selected.

'Query host' option allows you to confirm settings are correct as it queries the host with the credentials and returns back information about the device and the interfaces on it.

Configuring firewall

If a firewall sits between monitoring server and SNMP agent you must ensure SNMP traffic can pass.

Opsview to SNMP agent

Communication from Opsview to SNMP agent is via UDP port 161

SNMP agent to Opsview

If you're intending to process SNMP traps it will be necessary to allow communication from SNMP agent to Opsview server. SNMP traps use UDP port 162.

Common problems

Nagios plugin responds with 'Agent not responding, tried SNMP v1 and v2c'

Essentially this means plugin could not communicate with with SNMP agent. We recommend you check settings in Opsview and on device. Also check your firewall settings.

Intermittent 'time out' problems with Nagios plugin

If you're experiencing intermittent 'time out' errors with Nagios plugin try the following:

  • Increase value of 'time out' argument for plugin. Most plugins support this option and it will force the plugin to wait longer for a response from SNMP agent.
  • It is possible to overload the SNMP agent with requests. If you increase polling interval or reduce number of service checks against device it may help.
  • Check firewall logs for indication that it may be dropping SNMP traffic - assuming there is a firewall between Opsview and SNMP agent.

SNMP tools

Net-SNMP package should be installed as a pre-requisite package for Opsview software. This package includes some helpful command line tools for diagnosing SNMP issues. More information can be found on Net-SNMP website:

http://net-snmp.sourceforge.net/docs/man/

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