Each master clock has its own configuration for IP address, packet rate, and messaging timeouts, and for statistics, alarms, and events. Each available master clock advertises its presence and information using Announce messages. If both master clocks are available, the slave clock uses the Best Master Clock Algorithm (BMCA) to dynamically compare the information in the Announce messages of each master clock to determine to which of the two master clocks it should synchronize. This master clock is known as the best master. After the slave clock has determined which is the best master, it may begin to negotiate with it for unicast synchronization communication.
The configured setting for the profile command determines the precedence order for selecting the best master clock algorithm. The 7705 SAR supports the following profile settings: ieee1588-2008, itu-telecom-freq, g8275dot1-2014, g8275dot2-2016, iec-61850-9-3-2016, and c37dot238-2017. For information about the g8275dot1-2014 and g8275dot2-2016 profile parameters, see ITU-T G.8275.1 and G.8275.2. For information about the iec-61850-9-3-2016 and c37dot238-2017 profile parameters, see IEC/IEEE 61850-9-3 and C37.238-2017.
If the profile setting for the clock is ieee1588-2008, iec-61850-9-3-2016, or c37dot238-2017, the precedence order for the best master selection algorithm is as follows:
priority1 (user-configurable on the master clock side)
clock class
clock accuracy
PTP variance (offsetScaledLogVariance)
priority2 (user-configurable on the master clock side)
clock identity
distance (number of boundary clocks)
If the profile setting for the clock is itu-telecom-freq (ITU-T G.8265.1 profile), the precedence order for the best master selection algorithm is as follows:
clock class
peer ID
If the profile setting for the clock is g8275dot1-2014 or g8275dot2-2016, the precedence order for the best master selection algorithm is as follows if the grand master clock is connected to a primary reference time clock (PRTC) in locked mode:
clock class
clock accuracy
PTP variance (offsetScaledLogVariance)
priority2 (user-configurable on the master clock side)
localPriority
steps removed from the grand master
port identities
port numbers
If the profile setting for the clock is g8275dot1-2014 or g8275dot2-2016, the precedence order for the best master selection algorithm is as follows if the grand master clock is in holdover and out of holdover specification, or is without a time reference since startup:
clock class
clock accuracy
PTP variance (offsetScaledLogVariance)
priority2 (user-configurable on the master clock side)
localPriority
clock identity
steps removed from the grand master
port identities
port numbers
Figure: Messaging Sequence Between the PTP Slave Clock and PTP Master Clocks shows an example of the messaging sequence between the PTP slave clock and the two PTP master clocks.
