Essential SNMP, 2nd Edition

SNMP Operations

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. Each of the following SNMP operations has a standard PDU format:

• get

• getnext

• getbulk (SNMPv2 and SNMPv3)

• set

• getresponse

• trap

• notification (SNMPv2 and SNMPv3)

• inform (SNMPv2 and SNMPv3)

• report (SNMPv2 and SNMPv3)

In addition to running actual command-line tools, we will also provide packet dumps of the SNMP operations. For those of you who like looking at packet dumps, this will give you an inside look at what the packet structure is for each command. The packet dumps themselves were taken using the command-line version of Ethereal (http://www.ethereal.com). Let's take a look at each operation now. All of the get and set operations were captured with the following command:

$ /usr/sbin/tethereal -i lo -x -V -F libpcap -f "port 161"

Traps and notifications were captured with this command:

$ /usr/sbin/tethereal -i lo -x -V -F libpcap -f "port 162"

The get Operation

The get request is initiated by the NMS, which sends the request to the agent. The agent receives the request and processes it to the 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 ...