Essential SNMP

Extensions to the SMI in Version 2

SMIv2 extends the SMI object tree by adding the snmpV2 branch to the internet subtree, adding several new datatypes, and making a number of other changes. Figure 2-3 shows how the snmpV2 objects fit into the bigger picture; the OID for this new branch is 1.3.6.1.6.3.1.1, or iso.org.dod.internet.snmpV2.snmpModules.snmpMIB.snmpMIBObjects. SMIv2 also defines some new datatypes, which are summarized in Table 2-2.

Figure 2-3.  SMIv2 registration tree for SNMPv2

Table 2-2. New Datatypes for SMIv2

Datatype	Description
Integer32	Same as an INTEGER.
Counter32	Same as a Counter.
Gauge32	Same as a Gauge.
Unsigned32	Represents decimal values in the range of 0 to 232 - 1 inclusive.
Counter64	Similar to Counter32, but its maximum value is 18,446,744,073,709,551,615. Counter64 is ideal for situations in which a Counter32 may wrap back to 0 in a short amount of time.
BITS	An enumeration of nonnegative named bits.

The definition of an object in SMIv2 has changed slightly from SMIv1. There are some new optional fields, giving you more control over how an object is accessed, allowing you to augment a table by adding more columns, and letting you give better descriptions. Here’s the syntax of an object definition for SMIv2. The changed parts are in bold:

<name> OBJECT-TYPE
    SYNTAX <datatype>
    UnitsParts <Optional, see below>
            MAX-ACCESS <See below> ...