‘TRAPS’ allow a large number of devices to be monitored by the SNMP driver, without the need to poll each of those devices. Instead, the SNMP agent on a device sends an unsolicited message—a ‘TRAP’ of an unusual or unexpected event—to the driver. Operators can then decide whether to separately poll the device, or other associated device(s), to obtain further information about the cause of the alarm or event.
TRAPS do not require acknowledgment from the SNMP driver to verify the receipt of a message. SNMP v2 introduced the INFORM request, which is a trap that requires acknowledgment from the driver. SNMP v3 also uses TRAPS and the INFORM request, however, it requires additional security information to be included. TRAPS or INFORM requests sent from v1 or v2 devices are ignored by a driver configured to use only v3 TRAPS or INFORM requests.
SNMP supports several standard traps that are common to many networked devices. These are known as Generic Traps.
In addition to these Generic Traps, SNMP agents on different devices can be programmed to support any number of proprietary or enterprise Specific Traps. Such traps fall into three categories:
- Technology-specific traps
- Vendor-produced product-specific traps
- Corporate management-specific traps
In each case, the SNMP agent at the device has to be programmed to send traps. Such information is incorporated into a Management Interface Base (MIB). An MIB is a logical database that incorporates the configuration status and statistical information that is stored at the device. MIBs for technology- and product-specific traps should be supplied with the product—see the product documentation for details. The production of MIBs is beyond the scope of this guide.
Within the Geo SCADA Expert database, you define the SNMP agent’s ID and Community name. For Specific Traps, you also define the Object Identifier, and properties that define the trap-specific information that is to appear in any data packets sent by the SNMP agent. For more information, see Configuring Points on the SNMP Driver.
We recommend the use of a free tool called Getif for discovering the correct object identifiers (OIDs) for SNMP agents. You can download the tool from http://www.wtcs.org/snmp4tpc/getif.htm.
When an SNMP agent returns a trap message to the SNMP driver, the data packet that it sends contains:
- The IP address of the device on which the SNMP agent is located
- The Generic or Specific Trap Code(SNMP v1) or Object Identifier [OID] (SNMP v2 & v3)
- 0 or more binding packets that contain the Object Identifier (OID) for the MIB and the variable (which may also be the Object Identifier).