The administrative timing source for the ERX system. The ERX
periodically monitors the status of three timing sources, primary,
secondary and tertiary. If the systems current timing source fails, the
system will automatically downgrade to the next timing source. If the
system is configured to automatically upgrade
(usdERXSysTimingDisableAutoUpgrade is set to false(2)) the system will
switch back to the higher timing source when it becomes available.
A timing source failure can be detected by comparing the operational and
admin timing sources. If they are not equal, the system has swapped
timing sources because the administratively set value is in the error
state.