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Simple Network Management Protocol (SNMP)

Simple Network Management Protocol (SNMP) is an application–layer protocol defined by the Internet Architecture Board (IAB) in RFC1157 for exchanging management information between network devices. It is a part of Transmission Control Protocol⁄Internet Protocol (TCP⁄IP) protocol suite.

SNMP is one of the widely accepted protocols to manage and monitor network elements. Most of the professional–grade network elements come with bundled SNMP agent. These agents have to be enabled and configured to communicate with the network management system (NMS).

SNMP agent’s key functions

  • Collects management information about its local environment
  • Stores and retrieves management information as defined in the MIB.
  • Signals an event to the manager.
  • Acts as a proxy for some non–SNMP manageable network node.

Every SNMP agent maintains an information database describing the managed device parameters. The SNMP manager uses this database to request the agent for specific information and further translates the information as needed for the Network Management System (NMS). This commonly shared database between the Agent and the Manager is called Management Information Base (MIB).

Typically these MIB contains standard set of statistical and control values defined for hardware nodes on a network. SNMP also allows the extension of these standard values with values specific to a particular agent through the use of private MIBs.

In short, MIB files are the set of questions that a SNMP Manager can ask the agent. Agent collects these data locally and stores it, as defined in the MIB. So, the SNMP Manager should be aware of these standard and private questions for every type of agent.

Basic commands of SNMP

The simplicity in information exchange has made the SNMP as widely accepted protocol. The main reason being concise set of commands, here are they listed below:

  • GET: The GET operation is a request sent by the manager to the managed device. It is performed to retrieve one or more values from the managed device.
  • GET NEXT: This operation is similar to the GET. The significant difference is that the GET NEXT operation retrieves the value of the next OID in the MIB tree.
  • GET BULK: The GETBULK operation is used to retrieve voluminous data from large MIB table.
  • SET: This operation is used by the managers to modify or assign the value of the Managed device.
  • TRAPS: Unlike the above commands which are initiated from the SNMP Manager, TRAPS are initiated by the Agents. It is a signal to the SNMP Manager by the Agent on the occurrence of an event.
  • INFORM: This command is similar to the TRAP initiated by the Agent, additionally INFORM includes confirmation from the SNMP manager on receiving the message.
  • RESPONSE: It is the command used to carry back the value(s) or signal of actions directed by the SNMP Manager.

SNMP traps enable an agent to notify the SNMP manager of significant events by an unsolicited SNMP message. SNMP Trap protocols include current sys-Up-Time value, an OID identifying the type of trap and optional variable bindings. Destination addressing for SNMP traps is determined in an application-specific manner typically through trap configuration variables in the MIB. The format of the trap message was changed in SNMPv2 and the protocol data units was renamed SNMPv2-Trap. 

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