3Com VCX V7122, User Manual, Page: 365 / 488

V7122 Gateway User Guide
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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 “Tabl”’ 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.
ƒ 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).
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.
Typically, when a MIB is compiled into the system, the manager creates new folders or
directories that correspond to the objects. These folders or directories can typically be
viewed with a MIB Browser, which is a traditional SNMP management tool incorporated into
virtually all Network Management Systems.
The act of compiling the MIB allows the manager to know about the special objects
supported by the agent and access these objects as part of the standard object set.
Carrier Grade Alarm System
The basic alarm system has been extended to a carrier-grade alarm system. A carrier-grade
alarm system provides a reliable alarm reporting mechanism that takes into account EMS
outages, network outages, and transport mechanism such as SNMP over UDP.