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SNMP Traps As Event Reports

Previous Page:So every MIB file needs to describe the entire OID tree?
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The first thing you should look for in the MIB is what RFC MIBs are required to support this device. The necessary RFCs will be called out in the IMPORTS line at the beginning of the MIB.

Traps: Event Reports

For telemetry purposes, the MIB elements you're most interested in are what Traps the device can send. Traps are often described as alarms, but it's better to think of them as event reports.

When a Trap is called out in the MIB, it means that the device is configured to generate a report whenever the element listed changes state. This doesn't mean that the event is necessarily important. Many Traps are merely status messages.

In SNMP v1 MIBs, Traps are always designated with the text label TRAP-TYPE. Here's an example from the MIB for the DPS Telecom NetGuardian RTU:

dpsRTUp8005Set TRAP-TYPE
      VARIABLES { sysDescr, sysLocation, dpsRTUDateTime,
dpsRTUAPort, dpsRTUCAddress, dpsRTUADisplay, dpsRTUAPoint, dpsRTUAPntDesc, dpsRTUAState }
      DESCRIPTION "Generated when discrete point 5 is set."
      ::= 8005

Fortunately, you can ignore a lot of this gobbledygook. Here are the elements that you're interested in:

TRAP-TYPE: This tells you it's a Trap.

DESCRIPTION: This is a human-readable description of the Trap. It should give you a good basic indication of what the Trap signifies.

VARIABLES: This tells you actual information will be included in the Trap. When an actual Trap is sent, each of these variables will be paired with a numerical value that indicates its current state. A variable-and-value pair is called a variable binding.

The variables look pretty cryptic, but it's easy to find out what they mean. Each variable is a text label for an OID defined elsewhere in the MIB. You can do a Ctrl-F search for any variable term and find its definition. For example, "dpsRTUAPort" is defined in the DPS MIB like this:

      ACCESS read-only
      STATUS mandatory
      DESCRIPTION "RTU port number."
      ::= {dpsRTUAlarmEntry 1}

Trap variables are your best guide to what alarms you'll get from an SNMP device. Depending on the device, the variables can be highly detailed or they can be vague summary alarms.

Next Page:Object-Types: Data you can read and sometimes write
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