23.8. Serving NTP Time With PTP

NTP to PTP synchronization in the opposite direction is also possible. When ntpd is used to synchronize the system clock, ptp4l can be configured with the priority1 option (or other clock options included in the best master clock algorithm) to be the grandmaster clock and distribute the time from the system clock via PTP:

~]# cat /etc/ptp4l.conf
[global]
priority1 127
[eth3]
# ptp4l -f /etc/ptp4l.conf

With hardware time stamping, phc2sys needs to be used to synchronize the PTP hardware clock to the system clock:

~]# phc2sys -c eth3 -s CLOCK_REALTIME -w

To prevent quick changes in the PTP clock's frequency, the synchronization to the system clock can be loosened by using smaller P (proportional) and I (integral) constants of the PI servo:

~]# phc2sys -c eth3 -s CLOCK_REALTIME -w -P 0.01 -I 0.0001