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IEEE 1588v2 PTP Boundary Clock Overview

IEEE 1588v2 PTP Boundary Clock

An IEEE 1588v2 boundary clock has multiple network connections and can act as a source (master) and a destination (slave or client) for synchronization messages. It synchronizes itself to a best master clock through a slave port and supports synchronization of remote clock clients to it on master ports. Boundary clocks can improve the accuracy of clock synchronization by reducing the number of 1588v2-unaware hops between the master and the client. 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 master.

The boundary clock intercepts and processes all PTP messages and passes all other traffic. The best master clock algorithm (BMCA) is used by the boundary clock to select the best configured acceptable master clock that a boundary slave 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 master with the master statement and at least one slave with the slave statement at the [edit protocols ptp] hierarchy level.

Figure 1 illustrates two ACX Series boundary clocks in a network in which the clock flow is from the upstream node (BC-1) to the downstream node (BC-2).

Figure 1: Boundary Clocks in a Network

• BC-1 P-1 and BC-1 P-4 are boundary slave ports connected to two grandmaster clocks—OC-1 and OC-5. The grandmasters are included as the clock sources in the slave port configurations. From the packets received on the slave ports, BC-1 selects the best master, synchronizes its clock, and generates PTP packets, which are sent over the master ports—BC-1 P-2 and BC-1 P-3—to the downstream clients.
• BC-1 P-2, a master port, is connected to OC-2, an ordinary remote slave. OC-2 is included as a clock client in BC-1 P-2’s master configuration, and so receives PTP packets from BC-1 P-2.
• BC-1 P-3, a master port, is connected to BC-2 P-1, a remote boundary slave port. In this situation, the master port—BC-1 P-3—is included as a clock source in the configuration of the boundary slave port—BC-2 P-1. In addition, the boundary slave port—BC-2 P-1—is included as a clock client in the configuration of the master port—BC-1 P-3. With this configuration, the boundary slave—BC-2 P1—receives PTP packets from BC-1 P3.

• BC-2 P-1 is a boundary slave port connected to the upstream master port—BC-1 P3. As described previously, BC-2 P-1 receives PTP packets from BC-1 P3. The master 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 master ports, are connected to ordinary remote slaves—OC-3 and OC-4. OC-3 and OC-4 are included as clock clients in the configuration of the master ports—BC-2 P2 and BC-2 P-3. Both slaves receive PTP packets from the master boundary port to which they are connected.

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

Clock Clients

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

Note: The term slave is sometimes used to refer to the clock client.

An ACX Series router acting as a master boundary clock supports the following types of downstream clients:

• Automatic client—An automatic client is configured with an IP address, which includes the subnet mask, indicating that any remote PTP host belonging to that subnet can join the master clock through a unicast negotiation. To configure an automatic client, 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 client—A manual client 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 client does not use unicast negotiation to join the master clock. The manual statement overrides the unicast negotiation statement configured at the [edit protocols ptp] hierarchy level. As soon as you configure a manual client, it starts receiving announce and synchronization packets.
• Secure client—A secure client is configured with an exact IP address of the remote PTP host, after which it joins a master clock through unicast negotiation. To configure a secure client, 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 the maximum number of clients (512 ) in the following combination: Automatic clients 256. Manual and secure clients 256—Any combination of manual and secure clients is allowed as long as the combined total amounts to 256.