User`s manual

Mediant 1000
H.323 User's Manual 346 Document #: LTRT-83401
 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.
 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.
15.1.2 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 the following four main branches:
 "mgmt" SNMP branch: Contains the standard SNMP objects usually supported (at
least in part) by all network devices.
 "private" SNMP branch: Contains those ‘extended’ SNMP objects defined by
network equipment vendors.
 "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:
 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.
 Table MIB Objects: Contain multiple sections of management data. These objects
are distinguished from ‘Discrete’ items (above) by requiring a "." (dot) extension to
their names that uniquely distinguishes the particular value being referenced. The "."
(dot) extension is the "instance" number of an SNMP object. For "Discrete" objects,
this instance number is zero. For "Table" objects, this instance number is the index
into the SNMP table. SNMP tables are special types of SNMP objects which allow
parallel arrays of information to be supported. Tables are distinguished from scalar
objects, so that tables can grow without bounds. For example, SNMP defines the
"ifDescr" object (as a standard SNMP object) that indicates the text description of
each interface supported by a particular device. Since network devices can be
configured with more than one interface, this object can only be represented as an
array.
By convention, SNMP objects are always grouped in an "Entry" directory, within an object
with a "Table" suffix. (The ifDescr object described above resides in the "ifEntry" directory
contained in the "ifTable" directory).
15.1.3 SNMP Extensibility Feature
One of the principal components of an SNMP manager is a MIB Compiler which allows
new MIB objects to be added to the management system. When a MIB is compiled into an
SNMP manager, the manager is made "aware" of new objects that are supported by
agents on the network. The concept is similar to adding a new schema to a database.