Overview of SNMP Agents

SNMP uses the term manager for the client application that accesses the data about a managed device or system. The manager usually runs on a system that is different from the managed system. The term agent is used for the program that implements the protocol stack for servicing the requests from the manager. The SNMP agent typically runs on the managed device. The agent offers services on a designated TCP/IP port. The default SNMP port is 161.

Information about the target device is contained in a Management Information Base (MIB). MIBs are used by agents and managers so that both programs have knowledge of the data available. The MIB tells the manager about the device's functions and data. The MIB also tells the manager how to address or access that information in the form of managed objects. To access this management information, the manager issues requests to the agent. The requests contain identifiers for the MIB's data objects that are of interest to the manager. If the request can be successfully completed, the agent returns a response that contains the values for the required data objects.

Most SNMP agents support the basic SNMP protocol stack, and some minimal MIBs. However, to make management of a device more effective, additional MIBs must be supported on the managed device. The additional MIBs are provided by device vendors to provide management information about custom features of the managed device.

A MIB that is added to an SNMP agent is commonly known as an extension because the MIB extends the capabilities of the agent. An agent that can accept extensions is extensible. The System Management Agent (SMA), described in this manual, is an extensible agent. The extensions to the System Management Agent are called extension modules.