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VCX V7111 VoIP Gateway User Guide
ƒ 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:
ƒ
Get Request: Specific values can be fetched using the get request to determine the
performance and state of the device. Typically, many different values and parameters
can be determined using SNMP without the overhead associated with logging into the
device, or establishing a TCP connection with the device.
ƒ Get Next Request: Enables the SNMP standard network managers to walk through all
SNMP values of a device (using 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.
ƒ Set Request: The SNMP standard provides a method of effecting an action associated
with a device (using 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 using SNMP.
ƒ Trap Message: The SNMP standard furnishes a mechanism by which devices can reach
out to a Network Manager on their own (using 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:
ƒ "mgmt " SNMP branch: Contains the standard SNMP objects usually supported (at least
in part) by all network devices.
ƒ "private " SNMP branch: Contains those ‘extended’ SNMP objects defined by network
equipment vendors.
ƒ "experimental" and "directory" SNMP branches: Also defined within the ‘internet’ root
directory, these branches are usually devoid of any meaningful data or objects.
The tree structure described above is an integral part of the SNMP standard, though the
most pertinent parts of the tree are the leaf objects of the tree that provide actual
management data regarding the device. Generally, SNMP leaf objects can be partitioned into
two similar but slightly different types that reflect the organization of the tree structure:
ƒ Discrete MIB Objects: Contain one precise piece of management data. These objects
are often distinguished from "Table"items (below) by adding a ".0" (dot-zero) extension to
their names. The operator must merely know the name of the object and no other
information.