PTP Configuration on Asterfusion Enterprise SONiC Distribution Switch

The Precision Time Protocol (PTP) is used to synchronize clocks across network devices with sub-microsecond accuracy. Unlike NTP, which relies on software timestamps, PTP uses hardware timestamping to achieve much higher precision, making it widely adopted in applications such as broadcasting, financial trading, industrial automation, and 5G transport networks.
In a PTP system, devices can be either PTP-aware (Ordinary Clocks, Boundary Clocks, and Transparent Clocks) or non-PTP network elements such as switches and routers.
PTP works as a distributed protocol where clocks exchange synchronization messages and form a master-slave hierarchy. At the top of this hierarchy is the Grandmaster Clock, which provides the reference time for the entire domain. Other clocks adjust their local time based on synchronization messages from their upstream master. All synchronization takes place within a logical scope called a PTP domain.

Nodes within a PTP domain are referred to as clock nodes, and the interfaces on these nodes that run the PTP protocol are called PTP interfaces. The PTP protocol defines the following three basic types of clock nodes:

• OC (Ordinary Clock)

An Ordinary Clock has only one PTP interface participating in time synchronization within a PTP domain. This interface typically acts as a Slave, synchronizing time from an upstream clock node. When the OC acts as a time source, the same interface can also function as a Master, distributing time to downstream clock nodes.

• BC (Boundary Clock)

A Boundary Clock has multiple PTP interfaces participating in time synchronization within the same PTP domain. One interface acts as a Slave, synchronizing time from an upstream clock node. The remaining interfaces act as Master interfaces, distributing the synchronized time to downstream clock nodes. When serving as a time source, a BC can distribute time simultaneously through multiple Master interfaces.

• TC (Transparent Clock)

A Transparent Clock also has multiple PTP interfaces, but unlike OCs and BCs, it does not synchronize its own time through any interface. Each interface neither acts as a Master nor a Slave; they simply forward PTP messages while correcting for the residence time (the delay introduced by the device itself).

The Precision Time Protocol (PTP) synchronization process consists of two main phases:

• Master-Slave Hierarchy Establishment

Within a PTP domain, all clock nodes are organized in a hierarchical structure. The reference time for the entire domain is provided by the Grandmaster Clock (GM), which represents the highest-level clock in the hierarchy.

Clock nodes exchange Announce messages containing information such as clock priority, clock class, and time accuracy. Based on this information, the nodes collectively elect one clock as the Grandmaster for the domain. Through this process, the master-slave relationships between nodes are established, and the master or slave role of each interface on the nodes is determined.

• Clock Synchronization

Once the hierarchy is established, synchronization between the master and slave clocks is performed as follows:

• • The master sends a Sync message to the slave and records the send timestamp.
  • The slave receives the Sync message and records the receive timestamp.
  • The slave sends a Delay_Request message to the master and records the send timestamp.
  • The master receives the Delay_Request message and records the receive timestamp.
  • The master sends a Delay_Response message to the slave.
  • The slave calculates the path delay and offsets using all collected timestamps and adjusts its local clock to align with the master clock.

SM-TLV (SMPTE Management TLV) is a set of management information extended by SMPTE for PTP (IEEE 1588v2), used for time synchronization and status monitoring in broadcast media networks.

In 5G ORAN (Open Radio Access Network), the fronthaul network requires sub-microsecond time synchronization to ensure strict alignment of RU (Radio Unit), DU (Distributed Unit), and CU (Central Unit) in radio scheduling and waveform processing.
In this scenario, switches typically operate in Layer 2 IEEE 1588v2 Transparent Clock (BC) mode and adopts a two-step time synchronization mechanism to ensure high-precision timing.

This network supports IP-based transmission of multiple high-definition video, audio, and ancillary data streams, requiring precise audio-video synchronization, low latency, and high timing accuracy. PTP (Precision Time Protocol) provides a unified clock to achieve end-to-end synchronization in accordance with the SMPTE 2110 standard. The network is configured with both primary and backup clock sources, allowing automatic switchover to the backup clock in the event of a primary clock failure, ensuring continuous and stable synchronization.

As an example, the following configuration illustrates two Spine switches. The Leaf switch configuration follows the same pattern.