This is the configuration setting that selects which of the five timing sources the node re-transmits out its external timing output port, for feeding other equipment in the rack or site. Because this is read-write and drives an outbound signal other devices may depend on, setting it to internal(5) means every downstream device wired to that output is now synchronized to this node's free-running oscillator rather than a traceable network reference. For example, an admin changes nodeExternalTimingSource from externala(1) to portrefa(3) so a co-located DACS can derive timing from the switch's recovered trunk clock instead of a failed BITS feed.