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Trap - A message generated asynchronously by network devices. It is an unsolicited
message from an agent to the manager.
Each of these message types fulfills a particular requirement of Network Managers:
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Get Request - Specific values can be fetched via the ‘get’ request to determine the
performance and state of the device. Typically, many different values and parameters
can be determined via SNMP without the overhead associated with logging into the
device, or establishing a TCP connection with the device.
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Get Next Request - Enables the SNMP standard network managers to ‘walk’ through all
SNMP values of a device (via the ‘get-next’ request) to determine all names and values
that an operant device supports. This is accomplished by beginning with the first SNMP
object to be fetched, fetching the next name with a ‘get-next’, and repeating this
operation.
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Set Request - The SNMP standard provides a method of effecting an action associated
with a device (via the ‘set’ request) to accomplish activities such as disabling interfaces,
disconnecting users, clearing registers, etc. This provides a way of configuring and
controlling network devices via SNMP.
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Trap Message - The SNMP standard furnishes a mechanism by which devices can
‘reach out’ to a Network Manager on their own (via a ‘trap’ message) to notify or alert the
manager of a problem with the device. This typically requires each device on the network
to be configured to issue SNMP traps to one or more network devices that are awaiting
these traps.
The above message types are all encoded into messages referred to as Protocol Data Units
(PDUs) that are interchanged between SNMP devices.
SNMP MIB Objects
The SNMP MIB is arranged in a tree-structured fashion, similar in many ways to a disk
directory structure of files. The top level SNMP branch begins with the ISO ‘internet’
directory, which contains four main branches:
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The ‘mgmt’ SNMP branch - Contains the standard SNMP objects usually supported (at
least in part) by all network devices.
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The ‘private’ SNMP branch - Contains those ‘extended’ SNMP objects defined by
network equipment vendors.
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The ‘experimental’ and ‘directory’ SNMP branches - Also defined within the ‘internet’ root
directory, these branches are usually devoid of any meaningful data or objects.