Software switch command line arguments

All command line arguments for tsnctld relate to clock synchronization. We recommend you to read the page about clock synchronization first, it contains an introduction to synchronization strategies, as well as example commands. This page focuses more on the technical details and is intended for those who are already familiar with gPTP.

Under the hood, tsnctrld achieves clock synchronization by starting and monitoring executables from the chrony and linuxptp packages.

-d / --disable-clock-sync

Skips clock synchronization altogether. Use this flag on all instances of tsnctrld if you want to set up your own clock synchronization. Not providing this flag would cause tsnctrld’s clock synchronization to interfere with your own, likely causing time skips. When you use this flag it is then your responsibility that CLOCK_REALTIME and CLOCK_TAI are synchronized across all devices that run tsnctrld.

Due to this option disabling clock synchronization, it is incompatible with other options that modify clock synchronization, namely -i / --nic, -g / --grandmanster, and -n / --ntp

-n / --nic

When you run tsnctrld, you must either provide this option or -d / --disable-clock-sync. When you provide this option, you must follow it up with one or more network interface card (NIC) names, the NICs you want to run gPTP on.

tsnctrld launches ptp4l to synchronize the NIC’s PHCs with other devices and launches phc2sys to synchronize the PHCs with each other and CLOCK_REALTIME (and therefore CLOCK_TAI). CLOCK_TAI is hardcoded to be 37 seconds ahead of CLOCK_REALTIME, there is no automated way to adjust this offset in case of a leap second. ptp4l uses the default gPTP.cfg configuration.

-g / --grandmaster

Configures this device’s clocks to have a PTP priority1 value of 10 instead of the default of 248, which makes this device’s clocks a lot more likely to be elected grandmaster.

If you didn’t also provide the -g / --grandmaster flag, tsnctld will run chronyd (an NTP daemon) in one-time sync mode to synchronize the hadware clocks and software clocks on this device with the world clocks a single time at startup.

-n / --ntp

When you provide this flag you must also provide the -g / --grandmaster flag. With this flag provided, instead of running chronyd in one-time sync mode, it is being run continously, disciplining CLOCK_REALTIME using NTP with a very light slew of max. 694ppm (about 1 minute per 24 hours). Also, instead of the PHCs disciplining CLOCK_REALTIME, CLOCK_REALTIME now disciplines the PHCs. This is why it’s important that the device running tsnctrld this mode becomes the gPTP grandmaster, since its PHCs will not respect the timestamps of any other device’s clock.

-m / --monitor-external-ptp

Can only be used when using the --disable-clock-sync flag. This is useful if you want to set up your own clock synchronization with ptp4l, but still want to let tsnctrld collect the performance monitoring metrics into the NETCONF datastore.

Further reading

Read the page about clock synchronization to learn more about the general concepts of how these command line arguments are supposed to be used.