Part 2: How SNMP Handles Alarm Messages

Snmp uses five basic messages (Get, GetNext, GetResponse, Set and Trap) to communicate between the manager and the agent.

The Get and GetNext messages allow the manager to request information for a specific variable. The agent, upon receiving a Get or GetNext message, will issue a GetResponse message to the manager with either the information requested or an error indication as to why the request cannot be processed.

A Set message allows the manager to request a change be made to the value of a specific variable in the case of an alarm remote that will operate a relay. The agent will then respond with a GetResponse message indicating the change has been made or an error indication as to why the change cannot be made.

The Trap message allows the agent to spontaneously inform the manager of an "important" event.

As you can see, most of the messages (Get, GetNext, and Set) are only issued by the SNMP manager. Because the Trap message is the only message capable of being initiated by an agent, it is the message used by DPS Telecom remote telemetry units (RTUs) to report alarms. This notifies the SNMP manager as soon as an alarm condition occurs, instead of waiting for the SNMP manager to ask.

The small number of commands used is only one of the reasons SNMP is simple. The other simplifying factor is its reliance on an unsupervised or connectionless communication link.

This simplicity has led directly to its widespread use, specifically in the Internet Network Management Framework. Within this framework, it is considered robust because of the independence of the managers from the agents; that is, if an agent fails, the manager will continue to function, or vice versa.

Essential SNMP: What is a Trap?

An SNMP Trap is a change-of-state (COS) message - it could mean an alarm, a clear or simply a status message. You often have to parse variable bindings to decode a Trap. To make sure the meaning of a Trap is understood, all DPS Telecom SNMP equipment transmits a unique Trap ID for both alarm and clear for each alarm point. Unlike a classic telemetry master, basic SNMP managers don't keep a standing alarm list, so it's difficult to tell what's happening in your network by looking at a list of Traps.