Users Guide

PTP is more accurate than NTP because it uses hardware timestamping. PTP also accounts for device latency while
synchronizing time. NTP synchronizes clocks with millisecond accuracy; PTP achieves submicrosecond accuracy.
OS10 supports PTP on all platforms that support hardware time stamping.
PTP-enabled devices consist of the following clock types:
Ordinary clock A device with a single physical port is called an ordinary clock. This device could take on a master or slave
clock role.
NOTE: OS10 switch cannot function as the grandmaster clock and hence OS10 does not support the
ordinary clock configuration.
Boundary clock A device with multiple physical ports that synchronizes time from one network segment to another is
called a boundary clock. One port is a slave that synchronizes time from upstream PTP device. The other
ports are masters that distribute time to downstream devices. The best master clock algorithm (BMCA)
decides the individual state of a port, master or slave.
End-to-end
transparent clock
Calculates the residence time of the PTP event message and updates the correction field (CF) of the
event message before forwarding the message. The ports are not in any specific state.
Best master clock algorithm
PTP uses the best master clock algorithm (BMCA) to compare clocks in a network. BMCA determines the status of ports in the
network:
● Master—A clock that provides time to other clocks in the network.
● Slave—A clock that receives time from other clocks in the network.
● Passive—A port that is not a master or slave.
This algorithm determines if the newly discovered foreign clock is better than the local clock. The grandmaster field in the
Announce message contains information about the foreign master clock. Information about the local clock is present in the
default data set of the clock. The foreign and local clocks are compared based on the following attributes:
1. Priority1—(Applicable only for the system-default profile) Configurable attribute that determines the master from an ordered
set of clocks. Priority1 is the most significant of the six attributes that devices use to select a master clock. The lower the
value of priority1, the higher its priority.
2. ClockClass—Defines the traceability of a clock.
3. ClockAccuracy—Defines the accuracy of a clock.
4. OffsetScaledLogVariance—Defines the stability of a clock.
5. Priority2—Configurable attribute that determines a master among equivalent clocks. Priority2 is the fifth-most significant
attribute out of the six attributes that devices use to select a master clock. The lower the value of priority2, the higher its
priority.
6. ClockIdentity—Unique identifier that determines a master when two clocks are exactly the same. The clock with the lower
clock identity has the highest preference.
When a PTP node receives two Announce messages from the same foreign master, PTP selects the best master based on the
StepsRemoved field of the Announce message. This field indicates the number of boundary clocks between the local clock
and the grandmaster clock.
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System management