| < Prev | Back to Table of Contents | Next > |
Default Object Values (DEFVAL)
Starting with RFC 1212 and later versions, the OBJECT-TYPE macro allows specification (via the DEFVAL clause) of a default value that an instance of the object will take during initialization or row creation. As well, Variations defined in an AGENT-CAPABILITIES statement allow specification of such defaults on a per-implementation basis where the OBJECT-TYPE doesn't specify a default, or where the implementation uses a different default. MIB Smithy allow specification of default values from the Defval page of the appropriate workspace.
Figure - Default Value Property
Value Type
The allowed forms for default values depend on the data type for
the object (for example, character, hex or binary strings for OCTET
STRINGs, numeric values for Integer32). To specify a default value,
you first need to select a Value Type so that MIB Smithy knows the
proper syntax to use in the generated MIB module. The appropriate
fields for the selected type will be activated and others disabled.
To delete an existing default value from the record, select the
No Default Value option. Other options are described
further below.
String Value
The String Value option for the Value Type is used to
indicate that the default value will be a simple quoted character
string. You do not need to include the quotes in the specified value:
MIB Smithy will add them for you. Leave the Value field blank for an
empty string (i.e., "").
Hex Value
The Hex Value option for the Value Type is used to
indicate that the default value will be a hexadecimal value. Hex
string values are generally used for OCTET STRING and related data
types such as the BITS construct, although they can also be used for
integer types. BITS default values are also sometimes specified as an
Identifier List. Hex strings are specified
using two single-quote characters surrounding zero or more
pairs of hex digits (0123456789ABCDEF) followed
by the letter H. SMI rules require there to be an even
number of hex digits. Examples:
''H '00'H '7FFFFFFF'H
Binary Value
The Binary Value option for the Value Type is used to
indicate that the default value will be a binary value. Binary
strings are also generally used for OCTET STRING and other places
where a Hex Value is also appropriate. They are specified using two
single-quote characters surrounding zero or more octuplets
of binary digits (01) followed by the letter
B. SMI rules require a multiple of 8 bits digits.
Examples:
''B '00000000'B '0101010101010101'B
H and
B suffixes.
Integer Value
The Integer Value option for the Value Type is used
to indicate that the default value will be a simple integer value,
used with objects of type INTEGER, Integer32, Unsigned32, Gauge32,
TimeTicks, etc. Note that objects of type Counter, Counter32 and
Counter64 may not have default values by SMI rules, as they would be
somewhat semantically meaningless.
Identifier Value
The Identifier option for the Value Type is used to
indicate that the default value will be a simple identifier (starting
with a lowercase letter). This form is used for specifying a default
by name for enumerated INTEGER objects or to reference a named OID
for objects of type OBJECT IDENTIFIER. SMIv2 requires this form to be
used for OBJECT IDENTIFIERs, rather than the multi-component OID
Value form that SMIv1 accepts.
Identifier List Value
The Identifier List option for the Value Type is used
to indicate that the default value will take the form of a list of
list of bit names, used with objects of type BITS. This option is
available as an alternative to a hex or binary string where a MIB
author desires to represent the default value in a more symbolic
form.
To add an identifier to the list, enter it into the Identifier field and press the "List Add" button. To remove an identifier from the list, select its name and press the "List Remove" button. The order that the identifiers appear is not significant, although it's suggested that they be listed in order of increasing bit value.
OID Value
The OID Value option for the Value Type is used to
indicate that the default value will take the form of a
multi-component OBJECT IDENTIFIER (such as { 0 0 }). The
forms accepted are the same as those accepted elsewhere, as detailed
in Object Identifier Values. This form
is allowed only by SMIv1. SMIv2 requires that default values of this
form be converted to a simple Identifier form
when migrating a MIB module from SMIv1 to SMIv2.
NetworkAddress Value
The NetworkAddress Value option for the Value Type is
used to indicate that the default value will be of a form acceptable
to objects using the SMIv1 NetworkAddress type. Default values of
this type consist of two parts: an Address Type (corresponding to the
CHOICE syntax used to define NetworkAddress) and an Address Value
containing the address in an appropriate form for the Address Type.
Of course, NetworkAddress only ever defined an internet
choice (for an IP Address), so normally the only options available
will be internet and an allowed form for an IP Address,
which takes an String style value (e.g. a hex
string). It does not take a dotted numeric form like
"255.255.255.0" -- this is invalid!
Also note that NetworkAddress is obsolete. It is not an allowed type in SMIv2 (which provides the IpAddress type instead), so neither is it legal to specify a default value of this type in SMIv2.
| < Prev | Back to Table of Contents | Next > |