An value that identifies the primary reference source that the network clock available from this source is traceable to if configured as a source of network clock for NCDP. (See American National Standards Institute, ANSI T1.101, 'Synchronization Interface for Digital Networks' and Bell Communications Research, GR-436-CORE, 'Digital Network Synchronization Plan' for more detailed information regarding Primary Reference Sources.). If a source of network clock is an onboard oscillator local to the device it is 'internal' otherwise it is 'external'. If the value of this object is used as a tiebreaker by the NCDP algorithm,'external' wins over 'internal'. All 'external' sources of network clock are assumed to be traceable to the same PRS by the NCDP protocol entity.