Network synchronization

This guide describes network synchronization capabilities available with SR Linux. These capabilities include physical layer frequency distribution via Synchronous Ethernet and packet based distribution of time using the precision time protocol (PTP) of IEEE 1588.

SR Linux supports network synchronization on the following platforms only:

• 7220 IXR-D5, model 3HE17735AB (w/Timing)
• 7250 IXR-X1b/X3b
• 7730 SXR-series platforms: 7730 SXR-1x-44S and 7730 SXR-1d-32D

The network time architecture in the following figure shows how network synchronization is commonly distributed in a hierarchical PTP topology at the physical layer.

This architecture provides the following benefits:

• limits the need for high-quality clocks at each network element

• only requires reliable and accurate replication of the input to remain traceable to its reference

• uses reliable physical media to provide transport of the timing signal

• does not consume any bandwidth and requires limited additional processing

The synchronization network is designed so a clock always receives timing from a clock of equal or higher stratum level or quality level. This ensures that if an upstream clock has a fault condition (for example, it loses its reference and enters a holdover or free-run state) and begins to drift in frequency, the downstream clock is able to follow it. For greater reliability and robustness, most offices and nodes have at least two synchronization references that can be selected in priority order (such as primary and secondary).

Additional resiliency can be provided by the ability of the node clock to operate within prescribed network performance specifications in the absence of any reference for a specified period. A clock operating in this mode is said to hold the last known state over (or holdover) until the reference lock is once again achieved. Each level in the timing hierarchy is associated with minimum levels of network performance.

Each synchronization-capable port can be independently configured to transmit data using the node reference timing.

Specifically for synchronous Ethernet, transmission of a reference clock through a chain of Ethernet equipment requires that all equipment supports synchronous Ethernet. A single piece of equipment that is not capable of performing synchronous Ethernet breaks the chain. Ethernet frames still get through, but downstream devices should not use the recovered line timing because it is not traceable to an acceptable stratum source.