18.2 Understanding SNMP Services

SNMP is based on a manager/agent architecture. The architecture of network management with SNMP includes the following elements:

  • Network Management Station (NMS)

  • Managed Device

  • Master Agent

  • Subagent

  • Management Information Base (MIB)

  • Network Management Protocol

Figure 18-1 Network Management Architecture

Network Management Station

The network management station is a workstation with one or more network management applications installed, to graphically show information about managed devices.

NMS features:

  • Provides the user interface to the entire network management system, thus providing a powerful, flexible and easy to use tool for network management

  • Allows you to perform SNMP Get, Get Next, SNMP Get Response and Set operations. NMS also allows you to capture SNMP Traps sent from managed devices on the network.

  • Monitors one or more network management applications (NMA) simultaneously. NMS has facilities to graphically show information about managed devices, table viewing, and logging.

  • Allows you to compile the MIB file using the MIB compiler present in the NMS.

Managed Devices

A managed device is any device that has SNMP installed on it. A managed device could be a host, router, bridge, hub, etc. NMS can monitor and communicate with managed devices.

The information between the NMS and the managed device is transferred through two types of agents: subagent and master agent.


The subagent gathers information about the managed device and passes the information to the master agent.

Master Agent

The master agent exchanges information between the various subagents and the NMS. The master agent runs on the same host machine as the subagents with which it communicates.

Management Information Base

SNMP exchanges network information in the form of protocol data units (PDUs). PDUs contain information about variables stored on the managed device. These variables are known as managed objects and have values and titles that are reported to the NMS. All managed objects are defined in the Management Information Base (MIB). MIB is a virtual database with a tree-like hierarchy.

SNMP Network Management Protocol

The basic functions of SNMP are listed in the following table.




Used by the manager to request information from an agent.

Get Next

Used by the manager to obtain information from an array or a table.

Get Response

Used by the queried agent to satisfy a request made by the manager.


Used by the manager to modify the value of the variable which resides on the agent’s MIB.


Used by the agent to notify the manager that a certain event has occurred.

For more information about SNMP, refer to the following Web sites: