SNMP Reference Guide

Introduction 33
The management system and a managed system communicate by means of a
common data schema. SNMP MIB files define the structure, type, and values
of the SNMP data. While MIBs can be standardized or enterprise specific,
most operating systems supply SNMP agents for the standard MIB-I and
MIB-II schemas. MIB-I defines a base set of standard management information
for systems implementing the Internet Protocol (IP) suite. MIB-II defines
characteristics of the system, characteristics of network interfaces, and
characteristics of components of the IP on the system. In addition to the
standard MIBs, many hardware vendors have defined MIBs that provide
management data specific to their systems and peripheral devices.
Monitored data can be retrieved through SNMP using the Get command.
Typically, this command requires the host name or IP address of the target
machine as well as the OID of the data to retrieve. Exact details are dependent
on the operating system and the development tools being used to create the
management application. The Get command has a variant known as GetNext.
SNMP MIB OIDs
Each data class within an MIB is defined by an OID. OIDs are unique across
all MIBs. An OID consists of a series of digits separated by periods. The OID
functions in a similar fashion to a phone number. The phone number
011-512-471-0000 uniquely identifies a single phone. The phone number can
be broken down into a number of components to uniquely identify a phone.
The first component, 011, is the country code for the United States. The
second component, 512, identifies the area code for central Texas. The third
component, 471, is the phone exchange for a large state university in the city
of Austin. The final component, 0000, is the main switchboard.
There are two main differences between the phone number example and an
actual OID. The first difference is that there are many more components in
an OID, up to 128. The combination of these components is called an OID
prefix. The second difference is that OIDs support the concept of indexes or
keys. The OID prefix specifies the data class but does not specify an instance of
the data within the class. Indexes can be used to identify the instances of a data
class. These indexes are referred to as the OID suffix.