Essential SNMP

The Structure of Management Information

So far, we have used the term “management information” to refer to the operational parameters of SNMP-capable devices. However, we’ve said very little about what management information actually contains or how it is represented. The first step toward understanding what kind of information a device can provide is to understand how this data itself is represented within the context of SNMP. The Structure of Management Information Version 1(SMIv1, RFC 1155) does exactly that: it defines precisely how managed objects^[7] are named and specifies their associated datatypes. The Structure of Management Information Version 2 (SMIv2, RFC 2578) provides enhancements for SNMPv2. We’ll start by discussing SMIv1 and will discuss SMIv2 in the next section.

The definition of managed objects can be broken down into three attributes:

Name

The name, or object identifier(OID), uniquely defines a managed object. Names commonly appear in two forms: numeric and “human readable.” In either case, the names are long and inconvenient. In SNMP applications, a lot of work goes into helping you navigate through the namespace conveniently.

Type and syntax

A managed object’s datatype is defined using a subset of Abstract Syntax Notation One(ASN.1). ASN.1 is a way of specifying how data is represented and transmitted between managers and agents, within the context of SNMP. The nice thing about ASN.1 is that the notation is machine-independent. This means that a PC ...