Understanding SNMP Messages

GET and SET are two types of messages defined by SNMP. GET and SET messages are sent by an NMS to a master agent. You can use one of the message types, or both with the Administration Server.

SNMP exchanges network information in the form of Protocol Data Units (PDUs). These units contain information about variables stored on the managed device, such as the web server. These variables, also known as managed objects, have values and titles that are reported to the NMS as necessary. Protocol data units sent by the server to the NMS are known as “traps.” The use of GET, SET, and “trap” messages are illustrated in the following examples.

NMS-initiated Communication. The NMS requests information from the server or changes the value of a variable store in the server’s MIB. For example:

1. The NMS sends a message to the Administration Server master agent. The message can be a request for data (a GET message), or an instruction to set a variable in the MIB (a SET message).

2. The master agent forwards the message to the appropriate subagent.

3. The subagent retrieves the data or changes the variable in the MIB.

4. The subagent reports data or status to the master agent, and the master agent forwards the message back (a GET message) to the NMS.

5. The NMS displays the data textually or graphically through its network management application.

Server-initiated Communication. The server subagent sends a message or “trap” to the NMS when a significant event has occurred. For example:

1. The subagent informs the master agent that the server has stopped.

2. The master agent sends a message or “trap” reporting the event to the NMS.

3. The NMS displays the information textually or graphically through its network management application.