PTP Boundary Clock Overview

PTP Boundary Clock

Boundary clocks can improve the accuracy of clock synchronization by reducing the number of 1588v2-unaware hops between the timeTransmitter and the timeReceiver. Boundary clocks can also be deployed to deliver better scale because they reduce the number of sessions and the number of packets per second on the timeTransmitter.

The boundary clock intercepts and processes all PTP messages and passes all other traffic. The best timeTransmitter clock algorithm (BTCA) is used by the boundary clock to select the best configured acceptable timeTransmitter clock that a boundary timeReceiver port can see. To configure a boundary clock, include the boundary statement at the [edit protocols ptp clock-mode] hierarchy level and at least one timeTransmitter with the master statement and at least one timeReceiver with the slave statement at the [edit protocols ptp] hierarchy level.

All PTP packets uses the best-effort queue instead of network control queue.

If clksyncd-service restart is initiated, then the show ptp lock status detail CLI command output of Clock reference state and 1pps reference state fields shows incorrect information. The following is a sample of output for show ptp lock status detail:

Figure 1 illustrates two boundary clocks in a network in which the clock flow is from the upstream node (BC-1) to the downstream node (BC-2). This figure also applies to MX Series routers and QFX Series switches.

The first boundary clock—BC-1—has four ports. Each port is configured as follows:

• BC-1 P-1 and BC-1 P-4 are boundary timeReceiver ports connected to two grandmaster clocks—OC-1 and OC-5. The grandmaster clocks are included as the clock sources in the timeReceiver port configurations. From the packets received on the timeReceiver ports, BC-1 selects the best timeTransmitter, synchronizes its clock, and generates PTP packets, which are sent over the timeTransmitter ports—BC-1 P-2 and BC-1 P-3—to the downstream timeReceiver clocks.

• BC-1 P-2, a timeTransmitter port, is connected to OC-2, an ordinary remote timeReceiver. OC-2 is included as a clock client in BC-1 P-2’s timeTransmitter configuration, and so receives PTP packets from BC-1 P-2.

• BC-1 P-3, a timeTransmitter port, is connected to BC-2 P-1, a remote boundary timeReceiver port. In this situation, the timeTransmitter port—BC-1 P-3—is included as a clock source in the configuration of the boundary timeReceiver port—BC-2 P-1. In addition, the boundary timeReceiver port—BC-2 P-1—is included as a clock client in the configuration of the timeTransmitter port—BC-1 P-3. With this configuration, the boundary timeReceiver—BC-2 P1—receives PTP packets from BC-1 P3.

The second boundary clock—BC-2—has three ports. Each port is configured as follows:

• BC-2 P-1 is a boundary timeReceiver port connected to the upstream timeTransmitter port—BC-1 P3. As described previously, BC-2 P-1 receives PTP packets from BC-1 P3. The timeTransmitter ports—BC-2 P-2 and BC-2 P-3—synchronize their time from the packets received from BC-2 P1.

• BC-2 P-2 and BC-2 P-3, boundary timeTransmitter ports, are connected to ordinary remote timeReceiver clocks—OC-3 and OC-4. OC-3 and OC-4 are included as clock clients in the configuration of the timeTransmitter ports—BC-2 P2 and BC-2 P-3. Both timeReceiver clocks receive PTP packets from the timeTransmitter boundary port to which they are connected.

In this example, the boundary clock synchronizes its clock from the packets received on its timeReceiver ports from the upstream timeTransmitter. The boundary clock then generates PTP packets, which are sent over the timeTransmitter port to downstream timeReceiver clocks. These packets are timestamped by the boundary clock by using its own time, which is synchronized to the selected upstream timeTransmitter.

Clock Clients

A clock client is the remote PTP host, which receives time from the PTP timeTransmitter and is in a timeReceiver relationship to the timeTransmitter.

Note:

The term timeReceiver is sometimes used to refer to the clock client.

A device acting as a timeTransmitter boundary clock supports the following types of downstream timeReceiver clocks:

• Automatic timeReceiver—An automatic timeReceiver is configured with an IP address, which includes the subnet mask, indicating that any remote PTP host belonging to that subnet can join the timeTransmitter clock through a unicast negotiation. To configure an automatic timeReceiver, include the subnet mask in the clock-client ip-address statement at the [edit protocols ptp master interface interface-name unicast-mode] hierarchy level.

• Manual timeReceiver—A manual timeReceiver is configured with the manual statement at the [edit protocols ptp master interface interface-name unicast-mode clock-client ip-address local-ip-address local-ip-address] hierarchy level. A manual timeReceiver does not use unicast negotiation to join the timeTransmitter clock. The manual statement overrides the unicast negotiation statement configured at the [edit protocols ptp] hierarchy level. As soon as you configure a manual timeReceiver, it starts receiving announce and synchronization packets.

• Secure timeReceiver—A secure timeReceiver is configured with an exact IP address of the remote PTP host, after which it joins a timeTransmitter clock through unicast negotiation. To configure a secure timeReceiver, include the exact IP address in the clock-client ip-address statement at the [edit protocols ptp master interface interface-name unicast-mode] hierarchy level.

Note:

You can configure a maximum number of 512 timeReceiver clocks in the following combination:

• 256 automatic timeReceiver clocks

• 256 manual and secure timeReceiver clocks—Any combination of manual and secure timeReceiver clocks is allowed as long as the combined total amounts to 256.

Platform-Specific Boundary Clock 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:

Platform	Difference
ACX Series	ACX5448 router supports PTP boundary clocks for phase and time synchronization using IEEE-1588 Precision Timing Protocol (PTP). The ACX5448 router supports the following features: PTP over IPv4 (IEEE-1588v2) PTP ordinary and boundary clocks One step clock mode operation for PTP TimeTransmitter 10Mhz and 1PPS output for measurement purpose You can configure a maximum of 512 timeReceiver clocks in any combination of automatic, manual, and secure timeReceiver clocks for ACX7100 devices on Junos OS Evolved. ACX5048 and ACX5096 routers do not support boundary clock.
MX and PTX Series	On MX240, MX480, MX960, MX2010, and MX2020 platforms when the boundary clock is switched from one MPC slot to another, PTP may go into a re-acquiring state irrespective of the PTP lock state. The clock state will transition from Initializing/Free run to Acquiring and then to Phase-Aligned upon the switchover from one slot to the other. On these platforms, while the clock recovery is in progress, the downstream nodes are notified of the change through the downgrading of the clock class. The clock class 248 is transmitted to downstream nodes. The downstream nodes can take appropriate action such as to go to the holdover state or to switch to an alternate clock path. On MX304, PTX10004, PTX10008, and PTX10016, if the time of day (TOD) counter of the timestamping units (PHY or ASIC) in the system are not synced with the global TOD counter for more than 3 seconds due to any reason, then the downstream clock class of the boundary clock is degraded to 248. Once the timestamping units are in sync with the global ToD counter, the clock class value is restored to the old valid value based on its active timeTransmitter’s clock class.