SNMP Trap Introduction

An SNMP trap is a notification event sent by a managed device over a network when a change-of-state (COS) event occurs. Some events that will cause a device to send SNMP traps include power outages, security breaches, and other major events. However, devices will also send traps for simple status events, such as doors opening and closing. These traps are sent across the network in the same manner, and are given no priority when using a standard SNMP manager.

MIB Flow Chart

Trap Messages From SNMP Devices Managed by One SNMP Application Simple Network Management Protocol (SNMP) is used when sending trap communications across a network to the device manager. In some cases, SNMP alleviates managers of the responsibility of requesting information from every device along a network individually. Instead, managed devices send unsolicited notification in the form of autonomous traps to one common SNMP network monitoring application.

Managers then receive the SNMP trap, and can take action based upon the event described by the trap. Managers cannot send a trap message back to a device, however, as trap communication only occurs from device to network manager. The management application must notify the proper person of the event.

SNMP Capture Results

Autonomous Traps Notify You of All Critical Alarm Events SNMP trap requests can fall under two categories, polled or autonomous. When an SNMP manager operates using polled SNMP traps, it will periodically request updates from all managed devices. This is done according a specific time frame, such as every half hour, or every five minutes. When traps are autonomous, they are automatically sent to the manager any time a COS event occurs. Rather than updating the manager on the status of a door every few minutes, autonomous SNMP traps are sent every time a door opens or closes.

An SNMP Alarm can be Identified Through Trap ID Granularity or Through Variable Bindings SNMP traps sent from devices generally conform to 1 of 2 major trap systems: granular or variable bindings. When a trap message is assembled in the granular format, each individual trap is given a trap identifier rather than a Variable Binding Style (VBS). This identifier is a number that is recognized by the SNMP manager to indicate a particular state change, such as a specific door opening. T/Mon makes use of this message format.

Other SNMP managers assemble their traps in the VBS format, which assign each trap the same trap ID number and store alarm data in the variable bindings of the trap. The messages differentiate by giving a different detail message for each trap, such as indicating a door is open, or a battery charge is low.