User manual

Chapter 4. SNMP
38
The MIB includes the definitions of relevant managed objects (MIB variables) for the specific
node. Various MIB's can be defined for various management purposes, types of equipment,
etc. The management data itself is a collection of integer, string and MIB address variables
that contain all the information necessary to manage the node.
A leaf object's definition includes the range of instances (values) and the "access" rights:
Read-only Instances of an object can be read, but cannot be set.
Read-write Instances of an object can be read or set.
Write-only Instances of an object can be set, but cannot be read.
Not accessible Instances of an object cannot be read, nor set.
4.4 MIB Structure
The MIB has an inverted tree-like structure (root over leaves), with each definition of a
managed instance forming one leaf, located at the end of a branch of that tree. Each "leaf" in
the MIB is reached by a unique path, therefore by numbering the branching points, starting
with the top, each leaf can be uniquely defined by a sequence of numbers. The formal
description of the managed objects and the MIB structure is provided in a special
standardized format, called Abstract Syntax Notation 1, or ASN.1 (pronounced A-S-N dot
one).
Since the general collection of MIB's can also be organized in a similar structure, under the
supervision of the Internet Activities Board (IAB), any parameter included in a MIB that is
recognized by the IAB is uniquely defined.
To provide the flexibility necessary in a global structure, MIB's are classified in various
classes (branches), one of them being the experimental branch, another being the
management (mgmt) branch, and yet another the group of private (enterprise-specific) branch.
Under the private enterprise-specific branch of MIB's, each enterprise (manufacturer) can be
assigned a number, which is its enterprise number. The assigned number designates the top of
an enterprise-specific sub-tree of non-standard MIB's.
Enterprise-specific MIB's are published and distributed by their creators, who are responsible
for their contents.
The MIB supported by the FMUX04 SNMP Agent follows RFC 1158 (MIB-II standard).
4.5 SNMP Communities
To enable the delimitation of management domains, SNMP uses "communities". Each
community is identified by a name, which is an alphanumeric string of up to 255 characters
defined by the user. Any SNMP entity (this term includes both managed nodes and
management stations) is assigned by its user a community name. In parallel, the user defines
for each SNMP entity a list of the communities which are authorized to communicate with it,
and the access rights associated with each community (this is the SNMP community name
table of the entity).