clock quality attributes. If all the domains have equal priority and the threshold values less than master clock
attributes or if the threshold values are greater than the master clock attributes, BMCA is used to select the
master clock.
Grandmaster capability feature controls the switch’s ability of propagating its clock to other devices that it is
connected to. When the switch receives announce messages on an interface, it checks the clock class threshold
and clock accuracy threshold values. If the values of these parameters are within the predefined limits, then
the switch acts as per PTP standards specified in IEEE 1588v2. If the switch does not receive announce
messages from external sources or if the parameters of the announce messages received are not within the
predefined limits, the port state will be changed to listening mode. On a switch with no slave ports, the state
of all the PTP enabled ports is rendered as listening and on a switch with one slave port, the BMCA is used
to determine states on all PTP enabled ports. Convergence time prevents timing loops at the PTP level when
grandmaster capability is disabled on a switch. If the slave port is not selected on the switch, all the ports on
the switch will be in listening state for a minimum interval specified in the convergence time. The convergence
time range is from 3 to 2600 seconds and the default value is 3 seconds.
The interface cost applies to each PTP enabled port if the switch has more than one path to grandmaster clock.
The port with the least cost value is elected as slave and the rest of the ports will remain as passive ports.
The clock identity is a unique 8-octet array presented in the form of a character array based on the switch
MAC address. The clock identity is determined from MAC according to the IEEE1588v2-2008 specifications.
The clock ID is a combination of bytes in a VLAN MAC address as defined in IEEE1588v2.
PTP Device Types
The following clocks are common PTP devices:
Ordinary clock
Communicates with the network based on a single physical port, similar to an end host. An ordinary
clock can function as a grandmaster clock.
Boundary clock
Typically has several physical ports, with each port behaving like a port of an ordinary clock. However,
each port shares the local clock, and the clock data sets are common to all ports. Each port decides its
individual state, either master (synchronizing other ports connected to it) or slave (synchronizing to a
downstream port), based on the best clock available to it through all of the other ports on the boundary
clock. Messages that are related to synchronization and establishing the master-slave hierarchy terminate
in the protocol engine of a boundary clock and are not forwarded.
Transparent clock
Forwards all PTP messages like an ordinary switch or router but measures the residence time of a packet
in the switch (the time that the packet takes to traverse the transparent clock) and in some cases the link
delay of the ingress port for the packet. The ports have no state because the transparent clock does not
need to synchronize to the grandmaster clock.
There are two kinds of transparent clocks:
End-to-end transparent clock
Measures the residence time of a PTP message and accumulates the times in the correction field of
the PTP message or an associated follow-up message.
Cisco Nexus 3548 Switch NX-OS System Management Configuration Guide, Release 7.x
8
Configuring PTP
PTP Device Types
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