Understanding the network.pdf - Back to Home

• DESCRIPTION—Details the role or function of the managed object and how to
use it.
• REFERENCE—This is an optional definition used when the object is derived
from a previously defined object. It points to the object that was used as a
basis for the current object.
• INDEX—When an object contains a list or tabular data values, INDEX clauses
are used to distinguish data values in different tables and rows.
• DEFVAL—Another optional definition used to provide information regarding
the default value to be used with this object.
Three main ASN.1 data types are used for formatting SMI data type definitions
within MIB objects:
• Universal—ASN.1 universal "simple" data types are unique values associated
with an ASN.1 data type. There are four SMI data types defined for SNMP v1.
The SNMP v2 SMI added one additional data type. As indicated above, simple
data types are expressed using ASN.1 keywords. All five types are described
in Table 11.4.
Table 11.4. ASN.1, (SMI v1 and SMI v2) Simple Data Types
Data Type Description
INTEGER A signed number between -2,147,483,648 and 2,147,483,648.
ASN.1 specifies no INTEGER size limit. The SMI, however, limits
it to 32 bits in length.
OCTET A sequence of octets, which are 8 bits in length. The octets can
STRINGS represent ASCI or binary data.
OBJECT A data type used for identifying the location of a data type. (We
IDENTIFER will look at object identifiers further when we discuss SNMP
MIBs).
NULL Both a data type and a data value. It is used as a placeholder or
to indicate that a data type does not contain any valid data
BIT STRING The BIT STRING data type was added in the SMI for SNMP v2. It
is a sequence of zero or more bits that represent a data value.
The value can be binary or hexadecimal.
• Tagged—ASN.1 tagged data types provide the capability to create new data
types, which are actually just specific definitions of universal (INTEGER and
OCTET STRING) ASN.1 data types. Tagged data types must be either globally
unique within the ASN.1 implementation or within a given ASN.1 module.
There are four types or classes of tags defined by ASN.1 for this purpose:
universal tags, which are defined to identify globally unique data types;