An integer identifier for the Frame Relay 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 Frame Relay interface.
If this object is not configured, or is configured with a
value of -1, a nonzero value will be allocated internally
and can be retrieved from this object after table entry creation
has succeeded.
A value of zero for this object is reserved for future use.