The maximum number of VPI Bits that may
currently be used at this ATM interface.
The value is the minimum of
atmInterfaceMaxActiveVpiBits, and the
atmInterfaceMaxActiveVpiBits of the interface's
UNI/NNI peer.
If the interface does not negotiate with
its peer to determine the number of VPI Bits
that can be used on the interface, then the
value of this object must equal
atmInterfaceMaxActiveVpiBits.