SNMP Reference Guide

All SNMP systems consist of one or more managed systems that provide data through an SNMP agent to a management
system. The management system provides a user interface to view data from the managed systems. The management
system and managed systems communicate over a network (typically through User Datagram Protocol/Internet Protocol
[UDP/IP]).
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.
Management Information Base Object Identifiers
Each data class within a Management Information Base (MIB) is defined by an Object Identifier (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.
The assignment of values for each OID prefix component can be illustrated by using a tree structure. The following is an
example of an OID assignment:
Table 14. ROOT
CCITT(0)
ISO(1)
ORG(3)
DOD(6)
INTERNET(1)
MGMT(2)
MIB(1)
EXPERIMENTA
L(3)
PRIVATE(4)
20