It is quite common to deploy the network adapter in master clock mode as a secondary/backup master when the primary master is usually a dedicated PTP Grandmaster clock device having a time source reference such as. GPS.
The Best Master Clock Algorithm (BMC) uses a hierarchical selection algorithm to select the active master clock based on properties in the periodic Announce messages sent by all master clocks. Clock properties are considered in the following order:
- BMC priority 1.
- BMC priority 2.
- Unique identifier (tie breaker).
In the ptp_master_freerun.cfg file the following clock properties are configurable (default values are shown):
# ptp_clock_priority1 128 # ptp_clock_priority2 128
Priority1 and priority2 values are in the range 0-255 (0 is the highest priority, 255 the lowest).
sfptpdmaster config file.