Understanding Transparent Clocks in Precision Time Protocol

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 QFX5100, EX4600, ACX5048, ACX5096, ACX6360-OR, and PTX10001-20C devices act as transparent clocks only and operate between the primary and client clocks in a distributed network. Transparent clocks improve synchronization between the primary and client clocks and ensure that the primary and client 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 client clock accounts for the packet delay by using both the timestamp of when it started and the information in the correction field.

ACX5048 , ACX5096, ACX6360-OR, and PTX10001-20C devices 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, which is not supported on ACX6360-OR, and PTX10001-20C devices, estimated timestamps are sent in one packet, and additional packets contain updated timestamps.

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.

On QFX5100, EX4600, and EX4400 switches, PTP over Ethernet, IPv4, IPv6, unicast, and multicast for transparent clocks are supported. EX4400 switches also support IRB and LAG.

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

ACX6360-OR and PTX10001-20C devices do not support the following:

• Boundary, ordinary, primary, and client clocks

• Transparent clock over MPLS switched path

• Transparent clock with more than two VLAN tags

• PTP over Ethernet

• PTP over IPv4

• PTP multicast mode

• Configuration of unicast and broadcast modes.

  Unicast mode is enabled by default.

• Transparent clock in transponder mode

• PTP while MACSec is enabled

• Two-step process