Nortel Networks Mediant TP-1610 SIP - Manual (Page 172)

Mediant 2000 SIP
Mediant 2000 SIP User’s Manual 172 Document #: LTRT-72504
• 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.
11.1.2 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 four main branches:
• The ‘mgmt’ SNMP branch - Contains the standard SNMP objects usually supported (at least
in part) by all network devices.
• The ‘private’ SNMP branch - Contains those ‘extended’ SNMP objects defined by network
equipment vendors.
• The ‘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).
11.1.3 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.
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.