SNMP Manager Programmer's Guide

Constructing and Interpreting Packets

SNMP Manager Programmer’s Guide–134249

5-5

Variable Bindings

A variable binding is a pair of values describing a MIB object: the object’s instance

identifier and its value. The value is null in Get and GetNext requests, has a manager-

assigned value in Set requests, and has an agent- or subagent-assigned value in response

packets. Trap packets may or may not have variable bindings associated with them.

Variable bindings always appear at the end of packets, organized into a list as Figure 5-4

indicates.

This subsection provides an overview of instance identifiers and their role in retrieving

MIB objects in serial fashion, known as lexicographic order. It then describes how

variable bindings are associated with packets.

Instance Identifiers

The instance identifier unambiguously identifies every object in the SNMP object

hierarchy. Instance identifiers consist of an object’s OID plus a suffix of one or more

additional digits identifying the object’s instance.

Objects that are not organized into tables have only one instance. The SNMP

convention for referring to these objects is to use a suffix of zero:

oid.0

When you omit a suffix from the OID of any MIB object that is a variable binding in a

Manager Services packet, 0 is assumed.

Objects that appear in tables can have multiple instances. Individual instances of tabular

objects are differentiated by using a suffix that provides the index value of the entry in

which the object appears:

oid.entry-index-value

Figure 5-4. Variable Bindings

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instance-id-1 instance-id-n