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Network Clocking Overview
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The 1588-2008 standard defines other clocking devices that are not described here.
Note
When a shut/no shut is carried on the loopback interface, the PTP port is deleted and recreated. This
Note
causes the PTP counters to reset.
Transparent Clocking
A transparent clock is a network device such as a switch that calculates the time it requires to forward
traffic and updates the PTP time correction field to account for the delay, making the device transparent
in terms of timing calculations. The transparent clock ports have no state because the transparent clock
does not need to synchronize to the grandmaster clock.
There are two kinds of transparent clocks:
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The Cisco ME 3600X 24CX Series Switch does not currently support peer-to-peer transparent clock
Note
mode.
For information on how to configure the Cisco ME 3600X 24CX Series Switch as a transparent clock,
see
Clock Synchronization
PTP master devices periodically launch an exchange of messages with slave devices to help each slave
clock recompute the offset between its clock and the master clock. Periodic clock synchronization
mitigates any drift between the master and slave clocks.
Cisco ME-3600X-24CX Series Switch Chassis Configuration Guide
4-2
Ordinary clock—An ordinary clock is a 1588 clock with a single PTP port that can serve in one of
the following roles:
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Master mode—Distributes timing information over the network to one or more slave clocks,
thus allowing the slave to synchronize its clock to the master.
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Slave mode—Synchronizes its clock to a master clock. You can enable slave clocking on up to
two interfaces simultaneously in order to connect to two different master clocks.
Boundary clock—The device participates in selecting the best master clock and can act as the master
clock if no better clocks are detected.
Transparent clock—A transparent clock is a device or a switch that calculates the time it requires to
forward traffic and updates the PTP time correction field to account for the delay, making the device
transparent in terms of time calculations.
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.
Peer-to-peer transparent clock— Measures the residence time of a PTP message and computes the
link delay between each port and a similarly equipped port on another node that shares the link. For
a packet, this incoming link delay is added to the residence time in the correction field of the PTP
message or an associated follow-up message.
Configuring a Transparent Clock, page
Chapter 4
10-11.
Configuring Clocking and Timing
OL-27620-01
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