Juniper ACX1000 Configuration Manual page 326

ACX Series Universal Access Router Configuration Guide
268
The Precision Time Protocol (PTP) standardized by IEEE 1588 improves the current
methods of synchronization used within a distributed network. You can use PTP across
packet-based networks including, but not limited to, Ethernet networks. Queuing and
buffering delays in the switch can cause variable delay to packets, which affects path
delay measurements. Queuing delays vary based on the network load and also depend
on the architecture of the switch or the router.
Transparent clocks measure and adjust for packet delay. The transparent clock computes
the variable delay as the PTP packets pass through the switch or the router. The switch
(QFX5100 or EX4600) or the router (ACX5048, or ACX5096 routers) acts as a transparent
clocks only and operates between the master and slave clocks in a distributed network.
Transparent clocks improve synchronization between the master and slave clocks and
ensure that the master and slave clocks are not impacted by the effects of packet delay
variation.
The transparent clock measures the residence time (the time that the packet spends
passing through the switch or the router), and adds the residence time into the correction
field of the PTP packet. The slave clock accounts for the packet delay by using both the
timestamp of when it started and the information in the correction field.
ACX5048 and ACX5096 routers support end-to-end transparent clocks. With an
end-to-end transparent clock, only the residence time is included in the correction field
of the PTP packets. The residence timestamps are sent in one packet as a one-step
process. In a two-step process, estimated timestamps are sent in one packet, and
additional packets contain updated timestamps.
NOTE: ACX5048 and ACX5096 routers support only the one-step process,
which means that the timestamps are sent in one packet.
You can enable or disable a transparent clock globally for the switch or router. With a
global configuration, the same configuration is applied to each interface. If the transparent
clock is disabled, PTP packet correction fields are not updated. If the transparent clock
is enabled, the PTP packet correction fields are updated.
PTP over Ethernet, IPv4, IPv6, unicast, and multicast for transparent clocks are supported.
NOTE: ACX5048 and ACX5096 routers do not support PTP over IPv6 for
transparent clocks.
ACX5048 and ACX5096 routers do not support the following:
Boundary clock
Ordinary clock
Transparent clock over MPLS switched path
Transparent clock with more than two VLAN tags
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