Monitoring devices with SNMP

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Using SNMP to monitor devices

SNMP (Simple Network Management Protocol) is a protocol designed for communicating with devices for the purposes of monitoring and management.

Elements can be monitoried either by, making requests (active), or by receiving “Trap” notifications (SNMP) from the monitored host


An SNMP request is made up of:

  • A IP address
  • an OID corresponding to a MIB object
  • A port number (161 or 162)
  • a community string
  • a version (v1, v2c or v3)

example :, . (MIB  SNMPv2-SMI), 161, public, v2c

How SNMP works

Using SNMP requires preparation. You must enable SNMP on the target machine in order to query or generate traps. In addition, you need research time, to investigate what information can be obtained from your chosen device via SNMP, and which MIBs are supported.

OID : An SNMP agent is installed on the machine to monitor ( almost always ‘built-in’). This agent will fetch the requested information. To assist the agent, we specify  through the OID what information we want the agent to retreive.

MIB : A management information base (MIB) is a structured information database housed on an entity (switch, router, server or other device)


There are TWO ways to monitor devices using SNMP

Active (polling)

In this method of monitoring, the server (in our case the ServiceNavBox) polls at regular intervals the components to be monitored. Most of our “plugins” use this method of monitoring. By default, the protocol used is UDP port 161.

Passive (Traps)

Here, it is the monitored device that sends at regular intervals (or more commonly, when a noteworthy internal event occurs) metrics and messages to a central monitoring platform. By default, the protocol used is UDP  port 162.

Our product supports SNMP traps: See how to configure trap support on your ServiceNavBox



ServiceNav has many templates that use the SNMP protocol and additionally provides the ability for customers to create their own using our generic templates.

GLOBAL-SNMP-OID Performs SNMP collection of information, identified by an OID, on a device.
Global-SNMP-OID-Status Checks an SNMP OID against a list of statuses. It is possible to specify output text for each value that the OID may take.
Global-SNMP-Operation Performs one or two mathematical operations using multiple OIDs and returns one or two results. The check triggers an alert if the result of the first calculation exceeds set thresholds.
Global-SNMP-Table-Rate Checks an entire SNMP table by collecting two values for each index, and dividing them to obtain a rate.

Alerts if at least one of the rates obtained exceeds the indicated threshold.

The use of a white / black list is possible to check only certain indexes of the table.

Collects for each index of the table the following performance data: calculated rate, value of the numerator multiplied by a factor.

In Alert cases, displays the list of indexes affected, the rate, and the units of the value derived after multiplication by the factor.

The OIDs to be checked for the table, the values, the white / black lists, the unit of the performance data, the multiplication factor, the suffixes of the data of the performance (absolute value, performance) are all configurable.

Global-SNMP-Table-Status Collects the status of an SNMP table from the status of each instance, taking into account a whitelist or blacklist. The output texts are configurable.

UK ServiceNav Product Development Manager; my priority is to be needful of the particular requirements of all ‘English-speaking’ markets where ServiceNav is sold. I have over 20 years experience of the IT monitoring field - covering a wide variety of products and technologies.

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