Simple Network Management Protocol (SNMP) is a standard protocol language that computers use to send commands to each other and report important information.
SNMP's purpose in monitoring and management systems is to provide network devices with a common language for exchanging data. Today, many computer devices support it, enabling them to work together.
The SNMP model is based on the manager/agent network management architecture. The managers collect and process data about devices on the network. The agents are any type of device component connected to the managed devices in the network (in remote monitoring systems, SNMP agents are usually the RTUs).
There are three versions of SNMP: v1, v2c, and v3. SNMPv3 is the most secure version when compared with the older versions.
SNMP "agents" are remote devices out in the network. They can be printers, managed switches, alarm remotes, generators, servers, and lots of other things. These agents report problems and receive commands from a central "manager". This is known as the "manager-agent model".
The SNMP agent listens to requests coming from the SNMP manager on the User Datagram Protocol (UDP) port 161.
The manager side of the model describes the device, computer or program that builds reports based on status reports sent by the agents. It ensures that devices are still active, because you may not know if a quiet device is offline or simply doesn't have any alarms to report.
A manager may wait for the agent to send data at a regular, pre-set interval, or it may proactively ask for data using a series of five basic messages: TRAP, GET, GET-NEXT, GET-RESPONSE, and SET.
For example, a manager may ask an agent for data with a "get" message, the agent will send back a "get-response". The manager might only need that one piece of data, or it can then send a "get-next" message (and then another, and then another) to request a full status update.
The SNMP manager listens to Trap messages coming from the agent on port UDP 162.
Seeing real devices in real-world examples is incredibly helpful when learning a new protocol like SNMP. Here is a small collection of examples of typical SNMP deployments including legacy integration. The managers and RTUs and specific, but the concepts are universal:
A SNMP Trap is one of the 5 basic message types used in SNMP protocol (although more types have been added since version 1 of SNMP). What makes an SNMP trap unique from all other message types is the fact that it is the only method that can be directly initiated by an SNMP agent in the field.
All other core SNMP message types are either initiated by the SNMP manager or issued in response to an SNMP manager's message. This is what makes a trap so important and the most common SNMP message in most networks. A trap is an SNMP agent's way of notifying the manager that there is a problem.
The Management Information Base, or MIB, is an ASCII text file that catalogs SNMP network elements as a list of data objects, similar to a dictionary of the SNMP language. Every object that is referred to in an SNMP message must be listed in the MIB.
When an SNMP device sends a Trap or other message, it identifies each data object in the message with a number string called an object identifier, or OID. The MIB provides a text label called for each OID. The SNMP manager then uses the MIB to decode the OID numbers into a human-readable display.
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This guidebook has been created to give you the information you need to successfully implement SNMP-based alarm monitoring in your network.
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