Precision Time Protocol Overview
Increase in bandwidth requirements on wireless backhaul networks and the need to reduce costs and to improve flexibility have triggered the need for a packet-based backhaul infrastructure. Traditional metro deployments do not cater to the delivery of synchronization services, and this leaves operators with no other choice than to keep older parallel infrastructure. Physical layer–based Synchronous Ethernet and packet-based Precision Time Protocol (PTP) enable routers and switches to deliver synchronization services that meet the requirements of today’s mobile network, as well as Long Term Evolution (LTE)–based infrastructures. Physical layer–based technologies function regardless of network load, whereas packet-based technologies require careful architecture and capacity planning. For information about Synchronous Ethernet, see Synchronous Ethernet Overview.
PTP, also known as IEEE 1588v2, is a packet-based technology that enables the operator to deliver synchronization services on packet-based mobile backhaul networks. IEEE 1588 PTP (Version 2) clock synchronization standard is a highly precise protocol for time synchronization that synchronizes clocks in a distributed system. The time synchronization is achieved through packets that are transmitted and received in a session between a master clock and a slave clock.
The system clocks can be categorized based on the role of the node in the network. They are broadly categorized into ordinary clocks and boundary clocks. The master clock and the slave clock are known as ordinary clocks. The boundary clock can operate as either a master clock or a slave clock. The following list explains these clocks in detail:
Master clock—The master clock transmits the messages to the PTP clients (also called slave node or boundary node). This allows the clients to establish their relative time distance and offset from the master clock (which is the reference point) for phase synchronization. Delivery mechanism to the clients is either unicast or multicast packets over Ethernet or UDP.
Slave clock—Located in the PTP client (also called slave or slave node), the slave clock performs clock and time recovery operations based on the received and requested timestamps from the master clock.
Boundary clock—The boundary clock operates as a combination of the master and slave clocks. The boundary clock endpoint acts as a slave clock to the master clock, and also acts as the master to all the slaves reporting to the boundary endpoint.
Table 1 summarizes the first Junos OS release that supports PTP on various Juniper Networks devices:
Table 1: Precision Time Protocol Support
Juniper Networks Devices
Junos OS Release
MX80 Universal Routing Platforms with model number MX80-P
MX-MPC2E-3D-P (MPC2E P) on MX240, MX480, and MX960 routers
MX-MPC2E-3D-P (MPC2E P) on MX2010 and MX2020 routers
MX-MPC2E- 3D-NG (MPC2E NG)
MPC4E-3D-32XGE-SFPP on MX240, MX480, MX960, MX2010, MX2020
MPC4E-3D-2CGE-8XGE on MX240, MX480, MX960, MX2010, MX2020
MPC3E-3D-NG-Q on MX240, MX480, MX960, MX2010, MX2020
MPC3E-3D-NG on MX240, MX480, MX960, MX2010, MX2020
Following enhanced MPCs support PTP (1588v2):
Ethernet Modular Interface Cards (MICs) on MX240, MX480, and MX960 routers
Ethernet Modular Interface Cards (MICs) on MX2010 and MX2020 routers
On MX240, MX480, MX960, MX2010, and MX2020 routers, the following Enhanced MPCs (MPCEs) support PTP (1588v2) under express licensing only:
For more information about obtaining a license, contact JTAC.
ACX Series Universal Metro Routers
MPC6E, MPC7E, MPC8E, MPC9E, MPC2E NG, and MPC3E NG on MX2008.
Fixed port PIC (6xQSFPP) and modular MIC (JNP-MIC1) on MX10003 routers
Fixed port PICs (4xQSFP28 and 8xSFPP) on MX204 routers
MPC7E-10G and MPC7E-MRATE on MX240, MX480, MX960, MX2010, MX2020
MPC8E and MPC9E on MX2010, MX2020
You can configure timestamping either at the physical layer or at the nonphysical layer on the 10-Gigabit Ethernet and 100-Gigabit Ethernet ports. Juniper Networks recommends that you configure timestamping at the physical layer if the port supports IEEE 1588 timestamping, which is also known as PHY timestamping.
On 10-Gigabit Ethernet ports, PHY timestamping and WAN-PHY framing are mutually exclusive—that is, you cannot configure PHY timestamping on a 10-Gigabit Ethernet port if you have configured WAN-PHY framing mode on that port. This is applicable only for MPC5E and MPC6E with 24x10XGE MIC.
PHY timestamping is not supported on the enhanced MPCs MPC1E, MPC2E, and MPC4E. Only hardware timestamping is supported on these MPCs. Therefore, a packet delay variation (also known as jitter) of up to 1 microsecond is observed on these MPCs for a very small percentage of packets occasionally. Hardware timestamping is typically timestamping either at FPGA or similar device.
Unified in-service software upgrade (unified ISSU) is currently not supported when clock synchronization is configured for PTP on the MICs and Enhanced MPCEs on MX240, MX480, MX960, MX2010, and MX2020 routers.
To switch between the PTP and Synchronous Ethernet modes, you must first deactivate the configuration for the current mode and then commit the configuration. Wait for a short period of 30 seconds, configure the new mode and its related parameters, and then commit the configuration.
G.8275.1 Telecom Profile
Profiles were introduced in IEEE 1588-2008 to define a combination of options and attribute values, aimed at supporting a given application. G.8275.1 is a PTP profile for telecom applications requiring accurate phase and time synchronization. It supports the architecture defined in ITU-T G.8275 to enable the distribution of phase and time with full timing support and is based on the second version of PTP defined in [IEEE 1588].
If you don’t configure a profile, the device operates in IEEE1588v2 profile which is the default profile.
ACX Series routers do not support G.8275.1 Telecom Profile.
The following sections give a brief overview about the types of clocks supported in the G.8275.1 profile and about the Alternate BMCA:
Types of Clocks Supported in the G.8275.1 Profile
There are two types of clocks supported in this profile, the ordinary clock and the boundary clock.
There are two types of ordinary clocks:
One that can be only a grandmaster clock (T-GM)
One that can be only a slave clock (a slave-only ordinary clock or T-TSC)
There are two types of boundary clocks:
One that can be only a grandmaster clock (T-GM )
One that can become a master clock and a slave clock to another PTP clock (T-BC)
MX Series routers support the TSC and TBC clock types.
The G.8275.1 profile uses an alternate Best Master Clock Algorithm (BMCA). The alternate BMCA allows:
A new per-port attribute named notSlave. The notSlave port attribute is implemented using the protocols ptp master stanza configuration.
Multiple active grandmasters.
Per-port attribute local-priority to be used as a tie-breaker in the dataset comparison algorithm.
PTP over Link Aggregation Group
Junos Supports PTP over LAG based on the recommendation in ITU-T-G.8275.1. For each aggregated Ethernet link configured as PTP master or slave, you can specify one member link of the aggregated Ethernet bundle as primary and another as secondary. PTP switches over to the secondary member in the aggregated Ethernet bundle when the primary aggregated Ethernet link is down. For providing both link-level and FPC-level redundancy, the primary and secondary interfaces of the aggregated Ethernet bundle must be configured on separate line cards. If both primary and secondary are configured on the same line card, it would provide only link-level redundancy.
PTP master streams are created on the FPC on which the primary interface is present. Announce and sync packets are transmitted on this active PTP aggregated Ethernet link. The line card on the PTP slave containing this active PTP aggregated Ethernet link will receive announce and sync packets from the remote master.
This table summarizes the first Junos OS release that supports PTP over LAG on various Juniper Networks devices:
Table 2: PTP over LAG Support
Juniper Network Devices
PTP over IPv4
PTP over Ethernet