SNMP ("Simple Network Management Protocol") is a very popular computer protocol used for remotely monitoring and controlling equipment. All sorts of devices, from industrial telecommunications gear down to simple network switches and printers, can use SNMP to report their current status or receive commands. SNMP is almost always sent over standard IP.
SNMP is built on the "manager-agent" architecture, which simply means that any SNMP device is either an agent or a manager. Any device that reports its status (like a switch or printer) is an agent. All of your agents must be configured to report to an SNMP manager, which collects and present data from SNMP agents.
Precisely how you are notified depends on your specific SNMP manager and configuration. You might want a simple list showing all messages you've received. For more complex networks with a lot of messages, you'll probably benefit from a manager that filters out unimportant messages so you can focus on the few that really matter. It's also possible to receive email, text message, or phone notifications of certain SNMP messages.
Even though the name is a bit confusing, an "SNMP trap" is really just the most popular type of SNMP message. An agent sends a trap to your manager to indicate a particular problem or status.
Traps, while most common, are fairly limited. They're one-way messages from agents to managers. The manager does not respond to traps in any way, so it's possible for the occasional SNMP trap to go missing. Other message types, mostly generated by the manager, solve this problem and allow for remote controlling of agents.
If a manager wants to gather the current status of a device, it doesn't have to wait for an incoming SNMP trap (which may never come if the device has completely failed).
SNMP managers can send GET requests to "get" a value from a device. Depending on the data needed, your SNMP manager may send a GET that demands the device's on-board temperature reading, whether it has any alarm conditions to report, or just about any other information. Upon receiving a GET, an SNMP agent response with a GET response that contains the requested data.
Your manager has an overarching "bird's eye" view of your network, so it's a great place for automated decision making.
Imagine that your SNMP manager is made aware (via traps or GET responses) that several devices in one server room are overheating. Your manager can be configured to make an immediate, automated decision to activate your backup air-conditioning (HVAC) system. To do this, it must send a SET message to "set" a value on the target device (the backup HVAC, in this case). A SET-response (see the pattern?) is sent by the agent to confirm that it has received and followed the SET instructions.
There are a few other types of SNMP messages, but you understand all the basic concepts if you understand traps, GETs, and SETs.
Obviously, this hasn't been an exhaustive discussion of SNMP. I've tried to give you a reasonable introduction to the protocol without bombarding you with techno-jargon. There's a lot more information about SNMP available right here on the DPS website. I recommend:
SNMP Tutorial(read online or download the PDF version)
SNMP Troubleshooting(useful if you're stuck with a specific SNMP problem, HTML or PDF)
How to read the SNMP MIB (understand what a MIB is and how it's structured, HTML or PDF)
SNMP RTUs(specific examples of SNMP agents)
SNMP Managers (specific examples of central managers that handle SNMP and other protocols)
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