The nbsCmmcSysProtoIndex of nbsCmmcSysProtoTable corresponding
to the current operational family and rate of this port's
protocol.
The value 1 is reserved for 'custom' aka 'userDefined'.
If a PortNominalBitRate set is received, PortProtoAdmin
will automatically change to 1.
The value 2 is reserved for 'bypass' aka 'transparent'.
Not supported value: 0