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Object Identifier Values
In basic terms, an OID value consists of two or more integers
(called subidentifiers) separated by a dot
("."). Due to the Basic Encoding Rules (the
part of ASN.1 that defines how values are encoded for transmission
"on the wire"), the first subidentifier must be
0, 1 or 2. The second
subidentifier must be between 0 and 39 if
the first subidentifier is 0 or 1.
Otherwise, the only further restrictions imposed by SNMP are that (1)
there is a limit of 128 subidentifiers in an OID value, and (2) that
each subidentifier is restricted to the range
0..4294967295.
In the SMI/MIB module language, OID values can either be
"assigned" or "registered", depending on the type
of record or macro used to define the OID. "Registered"
OIDs are OIDs that are unique to a particular definition, such that
no two records may register the same OID. All of the SMI macros
register OIDs. The ASN.1 Value Assignment form
(<descriptor> OBJECT IDENTIFIER ::= { ... }) is
only an assignment. Assignments do not necessarily uniquely identify
anything, and can in fact have the same value as a registered OID
(although many compilers don't support this).
MIB Smithy recognizes many forms for object identifiers, owing to
the several ways in which they can be specified in the SMI language.
OID values may be relative, where the first part of the OID value is
the same as Value Reference specifying a starting point or prefix in
the OID tree (typically specified as a lowercase identifier) and
followed by one or more integer subidentifiers, or absolute, where
every integer subidentifier is present all the way up to one of the
three roots 0, 1 or 2.
Further, each subidentifier (in both absolute and relative forms)
can be a simple numeric value or a name-and-number value, where the
integer subidentifier is given in parentheses following a lowercase
identifier (as in org(3)). The following examples of OID
values are all equivalent:
system.1 SNMPv2-MIB:system.1 SNMPv2-MIB:mib-2.system(1).1 1.3.6.1.2.1.1 iso.org(3).6.1.2.1.1 iso(1).org(3).6.1.2.1.system(1)
When saving or previewing a MIB module, MIB Smithy will format OID values using whichever form was entered during creation. Some properties, such as a Conformance Group Name, provide a simple entry box where normally a single identifier is expected but OID values can be used (with caution due to compatibility issues such usage creates). Most OID values, such as those assigned to an OBJECT-TYPE, provide a multi-part form and filters to make changes easy, as shown below.
Figure - Object Identifier Value Property
The entire OID value can, at your discretion, be entered into the
Subidentifier(s) field. However, the Module Filter and Parent
Identifier comboboxes can be used to quickly select a desired parent.
The Parent Identifier combobox will list all records that have OID
values that can be used as parents for the record, subject to the
Module Filter selection, which can be used to filter the Parent
Identifier list to only those records that are in the selected
module. Checking the "Include module reference in value"
option will preserve the selected module name when saving or
previewing the module (as in ... ::= { SNMPv2-MIB.system 1
}). This can be used in cases where a module references two or
more external (imported) records having the same name. This feature
of the SMI language is not widely supported so it should not be used
unless absolutely necessary.
{ braces
} are expected.
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