We’ve discussed how SNMP organizes information, but we’ve left out how we actually go about gathering management information. Now, we’re going to take a look under the hood to see how SNMP does its thing.
The Protocol Data Unit (PDU) is the message format that managers and agents use to send and receive information. There is a standard PDU format for each of the following SNMP operations:
get-bulk (SNMPv2 and SNMPv3)
notification (SNMPv2 and SNMPv3)
inform (SNMPv2 and SNMPv3)
report (SNMPv2 and SNMPv3)
Let’s take a look at each of these operations.
The get request is initiated by the NMS, which sends the request to the agent. The agent receives the request and processes it to best of its ability. Some devices that are under heavy load, such as routers, may not be able to respond to the request and will have to drop it. If the agent is successful in gathering the requested information, it sends a get-response back to the NMS, where it is processed. This process is illustrated in Figure 2-5.
Figure 2-5. get request sequence
How did the agent know what the NMS was looking for? One of the items in the get request is a variable binding. A variable binding, or varbind, is a list of MIB objects that allows a request’s recipient to see what the originator wants to know. Variable bindings can be thought of as ...