When a Boundary Clock is the Grandmaster in a PTP Network

In a Precision Time Protocol (PTP) network, clock hierarchy and synchronization accuracy are paramount. Typically, a dedicated grandmaster clock serves as the ultimate time source, with ordinary clocks synchronizing downstream. However, in certain network designs, a boundary clock—traditionally used to segment networks—can assume the role of grandmaster. This article explores the circumstances under which a boundary clock becomes the grandmaster, the implications on time synchronization, and best practices for network designers.

Understanding PTP Clock Roles

PTP (defined in IEEE 1588) is built on a hierarchical model:

- Grandmaster Clock:

The primary time source in the network. It is usually connected to a highly accurate reference (e.g., GNSS or an atomic clock) and distributes time to all other devices.

- Ordinary Clocks:

These are end devices (or nodes) with a single PTP port that function solely as time receivers.

- Boundary Clocks:

Boundary clocks (BCs) have multiple ports. They act as both slaves (receiving time from an upstream source) and masters (distributing time to downstream devices). They are often used to reduce network traffic by segmenting the timing domain.

- Transparent Clocks:

These devices correct for network-induced delays as PTP messages pass through, but they do not create new time references.

In a typical configuration, a boundary clock relays timing information from a grandmaster to other devices. However, if the network conditions or configuration changes such that no higher-quality time source is available, a boundary clock can assume the grandmaster role.

When Does a Boundary Clock Become the Grandmaster?

A boundary clock may become the grandmaster when its internal configuration or the network topology causes it to be elected by the Best Master Clock Algorithm (BMCA). Several factors influence this decision:

1. Shared vs. Separate Time Sources

Boundary clocks can internally use one of two approaches:

- Shared Hardware Clock:

When the boundary clock's master and slave functions share the same high-precision hardware clock, the device's announce messages typically set the _stepsRemoved_ value to zero. This signals that there is no added delay between its internal processes.

- Separate Timing Subsystems:

If the boundary clock receives time from an external source (such as GNSS) for its grandmaster function and then recovers that time via a separate internal path (often with a phase-locked loop), additional delay is introduced. In this case, a _stepsRemoved_ value of one is appropriate to account for the internal propagation delay.

Douglas Arnold from Meinberg explains that "if the GM function and BC subsystems share a hardware clock, the stepsRemoved should be set to ZERO. If the BC subsystem recovers time through an internal link, then the stepsRemoved should be ONE".

2. BMCA Criteria and Network Topology

The BMCA algorithm evaluates each clock's properties—including priority, clock class, accuracy, and variance—to determine the best master. In a network where the boundary clock's attributes (after considering _stepsRemoved_) outperform available ordinary clocks, the BMCA may select the boundary clock as the grandmaster. This situation can occur in networks with:

- High Network Segmentation: Where boundary clocks are deployed to isolate segments, reducing the burden on the original grandmaster.

- Redundancy Requirements: Where multiple potential grandmasters are available and the boundary clock, by virtue of its configuration, emerges with the best overall quality.

3. Configuration and Vendor Choices

IEEE 1588 leaves some details to vendor implementation. Some manufacturers design boundary clocks with flexible configuration options that allow network engineers to tune parameters (such as _stepsRemoved_) to suit the network's needs. As a result, in some deployments a boundary clock might be configured to become the grandmaster if it can guarantee minimal timing error across its downstream ports.

Implications for Synchronization Accuracy

When a boundary clock becomes the grandmaster:

- Reduced Network Hops: The boundary clock "recreates" the time reference locally for downstream devices. This segmentation can improve synchronization accuracy by minimizing the accumulation of delay errors.

- Controlled Delay Compensation: By setting the correct _stepsRemoved_ value, the boundary clock informs downstream devices about the internal delay, allowing them to compensate accurately.

- Potential Trade-Offs: While beneficial in segmented networks, excessive reliance on boundary clocks can introduce error if many cascaded BCs are used. Network designers must balance scalability with accuracy.

Best Practices for Network Designers

To ensure optimal time synchronization when a boundary clock becomes the grandmaster, consider the following:

- Evaluate Internal Time Sources: Choose boundary clocks that share a high-precision hardware clock with minimal internal delay. Set _stepsRemoved_ to zero when possible.

- Monitor BMCA Behavior: Regularly check announce messages and clock attributes to verify that the boundary clock is performing as expected when it assumes the grandmaster role.

- Minimize Cascading: Limit the number of boundary clock "hops" in your network to reduce cumulative delay.

- Vendor Configuration Options: Understand your equipment's implementation details. Some vendors allow fine-tuning of PTP parameters—leverage these to optimize your network's synchronization performance.

Conclusion

In PTP networks, boundary clocks are versatile devices that typically segment the network and reduce traffic to the grandmaster. However, when network topology, configuration, and BMCA criteria align, a boundary clock can be elected as the grandmaster. By understanding how factors like shared hardware clocks, internal delay (reflected in _stepsRemoved_), and vendor-specific configurations affect the BMCA, network engineers can design robust, accurate timing networks. Proper configuration not only ensures synchronization accuracy but also enhances the network's scalability and redundancy.

Whether you're troubleshooting PTP networks or designing new systems, knowing when a boundary clock takes the lead is key to achieving precision time synchronization.