An integer identifier for the subinterface, used in conjunction with
the command-line interface. It is provided here for cross-reference
purposes only.
The value must be unique among subinterfaces configured on the same
underlying ATM adaptation layer. The upper bound for the value is
further constrained to be identical with the maximum number of
subinterfaces supported by the lower interface.
If this object is not configured, a value will be allocated internally
and can be queried after table entry creation has succeeded.