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Clock Synchronization

Clock synchronization aims to coordinate otherwise independent clocks.

Clock Synchronization Overview

Configuring external clock synchronization and automatic clock selection requires making clock selection, quality level, and priority considerations. The clock source selection algorithm is used to pick the two best upstream clock sources from among the various sources on the basis of system configuration and execution criteria such as quality level, priority, and hardware restrictions.

You can configure several options for external clock synchronization.

The devices support external clock synchronization using Synchronous Ethernet. Synchronous Ethernet is a physical layer technology that functions regardless of the network load and supports hop-by-hop frequency transfer, where all interfaces on the trail must support Synchronous Ethernet.

Currently, two types of clocking modes are supported, the distributing clocking mode and the centralized clocking mode.

The Switch Control Board (SCB) supports distributed clocking mode. The Enhanced Switch Control Board—SCBE—supports centralized clocking mode and has one external clock interface.

The Enhanced Switch Control Board—SCBE2—supports centralized clocking mode and has two external clock interfaces external-0/0 and external-1/0. Note that the external-0/0 interface refers to the external interface on the SCB in slot 0 and the external 1/0 interface refers to the external interface on the SCB slot 1.

On SCBE2, you can configure the external synchronization options only on the external interface on the active SCB. Therefore, if the active SCB is in slot 0, then you can configure the external-0/0 interface only. If the active SCB is in slot 1, then you can configure the external-1/0 interface only.

The Enhanced Switch Control Board—SCBE3—provides improved fabric performance and bandwidth capabilities and supports centralized clocking mode. SCBE3 does not support BITS E1/T1 external interfaces.

On SCBE3, you can configure the external synchronization options only on the external interface on the active SCB. Therefore, if the active SCB is in slot 0, then you can configure the external-0/0 GPS interface only. If the active SCB is in slot 1, then you can configure the external-1/0 GPS interface only.

Platform-Specific Clock Synchronization Behavior

Use Feature Explorer to confirm platform and release support for specific features.

Use the following table to review platform-specific behaviors for your platform:

Table 1: Platform-Specific Behavior

Platform

Difference

MX Series

  • MX 10003 routers support external clock synchronization and automatic clock selection for Synchronous Ethernet, T1 or E1 line timing sources, and external inputs.

  • In a non-redundant configuration the SCBE3-MX provides fabric bandwidth of up to 1 Tbps per slot (four fabric planes) and 1.5 Tbps per slot fabric bandwidth when all six fabric planes are used (with MPC10E line cards). The SCBE3-MX is supported on the MX960, MX480 and MX240 devices starting from Junos 18.4R1.

PTX Series

  • The PTX Series Packet Transport Routers support an external synchronization interface that can be configured to synchronize the internal Stratum 3 clock on the CCG to an external source, and then synchronize the chassis interface clock to that source.