CISCO System Management Software User Guide

CISCO System Management Software

Product Information

Specifications

• Feature name: Support for Precision Time Protocol (PTP)
• Release Information: Release 7.3.1
• Feature Description: Precision Time Protocol (PTP) is based on the IEEE 1588-2008 clock synchronization standard and enables clocks in a distributed system to be synched with highly precise clocks. PTP ensures that the best clock is selected as a timing source (the primary clock) and all other clocks are synchronized with the primary clock.

Product Usage Instructions

PTP Overview

Precision Time Protocol (PTP) is a protocol that defines a method to distribute time around a network. PTP ensures that the best available clock is selected as the source of time (the grandmaster clock) for the network and that other clocks in the network are synchronized to the grandmaster.

Configuring PTP

PTP Clocks

source. All clocks are synchronized to the grandmaster clock.
Ordinary Clock (OC)| An ordinary clock is a 1588 clock with a single PTP port that
can operate in one of the following modes:
Boundary Clock (BC)| The device participates in selecting the best master clock and
can act as the master clock if no better clocks are detected.
Boundary clock starts its own PTP session with a number of
downstream slaves. The boundary clock mitigates the number of
network hops and packet delay variations in the packet network
between the Grand Master and Slave.
Transparent Clock (TC)| A transparent clock is a device or a switch that calculates the
time it requires to forward traffic and updates the PTP time
correction field to account for the delay, making the device
transparent in terms of time calculations.

Frequency and Time Selection

The selection of the source to synchronize the device clock frequency is made by frequency synchronization, and is outside of the scope of PTP. The Announce, Sync, and Delay-request frequencies must be the same on the master and slave.

Delay Request-Response Mechanism

The Delay Request-response mechanism (defined in section 11.3 of IEEE Std 1588-2008) lets a slave port estimate the difference between its own clock- time and the clock-time of its master. The following options are supported:

• When running a port in Slave state, a router can send Delay-request messages and handle incoming Sync, Follow-up, and Delay-response messages. The timeout periods for both Sync and Delay-response messages are individually configurable.

Hybrid Mode

Frequency selection uses the algorithm described in ITU-T recommendation G.781. The ToD selection is controlled using the time-of-day priority configuration. This configuration is found under the clock interface frequency synchronization configuration mode and under the global PTP configuration mode. It controls the order for which sources are selected for ToD. Values in the range of 1 to 254 are allowed, with lower numbers indicating higher priority.

FAQ

Q: What is PTP?

A: PTP stands for Precision Time Protocol. It is a protocol that enables clocks in a distributed system to be synchronized with highly precise clocks.

Q: What is the purpose of PTP?

A: The purpose of PTP is to ensure that the best clock is selected as a timing source (the grandmaster clock) for the network and that all other clocks in the network are synchronized with the grandmaster.

Q: What are the different types of PTP clocks?

A: The different types of PTP clocks are Grandmaster (GM), Ordinary Clock (OC), Boundary Clock (BC), and Transparent Clock (TC).

Table 1: Feature History Table

Protocol (PTP) is based on the IEEE 1588-2008 clock synchronization standard and

enables clocks in a distributed system to be synched with highly precise clocks. The precision in time synchronization is achieved through packets that are transmitted and received in a session between the primary clock and secondary

clock. PTP also ensures that the best clock is selected as a timing source (the primary clock) and all other clocks are synchronized with the primary clock.

Precision Time Protocol (PTP) is a protocol that defines a method to distribute time around a network. PTP support is based on the IEEE 1588-2008 standard.
This module describes the concepts around this protocol and details the various configurations involved.

PTP Overview

The Precision Time Protocol (PTP), as defined in the IEEE 1588 standard, synchronizes with nanosecond accuracy the real-time clocks of the devices in a network. The clocks are organized into a master-slave hierarchy. PTP identifies the port that is connected to a device with the most precise clock. This clock is referred to as the master clock. All the other devices on the network synchronize their clocks with the master and are referred to as members. Constantly exchanged timing messages ensure continued synchronization.

PTP ensures that the best available clock is selected as the source of time (the grandmaster clock) for the network and that other clocks in the network are synchronized to the grandmaster.

Table 2: PTP Clocks

source. All clocks are synchronized to the grandmaster clock.
Ordinary Clock (OC)| An ordinary clock is a 1588 clock with a single PTP port that can operate in one of the following modes:

•  Master mode—Distributes timing information over the network to one or more slave clocks, thus allowing the slave to synchronize its clock to the master.

•  Slave mode—Synchronizes its clock to a master clock. You can enable the slave mode on up to two interfaces simultaneously in order to connect to two different master clocks.

Boundary Clock (BC)| The device participates in selecting the best master clock and can act as the master clock if no better

clocks are detected.

Boundary clock starts its own PTP session with a number of downstream slaves. The boundary clock mitigates the number of network hops and packet delay variations in the packet network between the Grand Master and Slave.

Transparent Clock (TC)| A transparent clock is a device or a switch that

calculates the time it requires to forward traffic and updates the PTP time correction field to account for the delay, making the device transparent in terms of time calculations.

PTP consists of two parts

• The port State machine and Best Master Clock Algorithm: This provides a method to determine state of the ports in the network that will remain passive (neither master nor slave), run as a master (providing time to other clocks in the network), or run as slaves (receiving time from other clocks in the network).
• Delay-Request/Response mechanism and a Peer-delay mechanism: This provides a mechanisms for slave ports to calculate the difference between the time of their own clocks and the time of their master clock.

Frequency and Time Selection
The selection of the source to synchronize the device clock frequency is made by frequency synchronization, and is outside of the scope of PTP. The Announce, Sync, and Delay-request frequencies must be the same on the master and slave.

Delay Request-Response Mechanism
The Delay Request-response mechanism (defined in section 11.3 of IEEE Std 1588-2008) lets a slave port estimate the difference between its own clock- time and the clock-time of its master. The following options are supported:

• One-step mechanism – The timestamp for a Sync message is sent in the Sync message itself.
• Two-step mechanism – The timestamp for a Sync message is sent later in a Follow-up message.

When running a port in Slave state, a router can send Delay-request messages and handle incoming Sync, Follow-up, and Delay-response messages. The timeout periods for both Sync and Delay-response messages are individually configurable.

Hybrid Mode
Your router allows the ability to select separate sources for frequency and time-of-day (ToD). Frequency selection can be between any source of frequency available to the router, such as: BITS, GPS, SyncE or IEEE 1588 PTP. The ToD selection is between the source selected for frequency and PTP, if available (ToD selection is from GPS, or PTP). This is known as hybrid mode, where a physical frequency source (BITS or SyncE) is used to provide frequency synchronization, while PTP is used to provide ToD synchronization.
Frequency selection uses the algorithm described in ITU-T recommendation G.781. The ToD selection is controlled using the time-of-day priority configuration. This configuration is found under the clock interface frequency synchronization configuration mode and under the global PTP configuration mode. It controls the order for which sources are selected for ToD. Values in the range of 1 to 254 are allowed, with lower numbers indicating higher priority.
The steps involved in section Configuring PTP Hybrid Mode of the topic G.8275.1, on page 12.

Time of Day (ToD) Support
The router receives GPS ToD messages in serial ASCII stream through the RS422 interface in one of the following configurable formats:

• NTP Type 4
• Cisco

Port States for PTP
State machine indicates the behavior of each port. The possible states are:

this state, the port listens to PTP masters for a (configurable) period of time.
State| Description
---|---
PRE-MASTER| Port is ready to enter the MASTER state.
MASTER| Port provides timestamps for any Slave or boundary clocks that are listening.
UNCALIBRATED| Port receives timestamps from a Master clock but, the router’s clock is not yet synchronized to the Master.
SLAVE| Port receives timestamps from a Master clock and the router’s clock is synchronized to the Master.
PASSIVE| Port is aware of a better clock than the one it would advertise if it was in MASTER state and is not a Slave clock to that Master clock.

Restrictions for PTP

The following PTP restrictions apply to the Cisco 8000 Series Router:

• Sync2 interface is supported only if 10 MHz, 1 Pulse per Second (PPS) and time-of-day (ToD) ports are configured.

• PTP is not supported with global MACSec.

• PTP is not supported with MACSec on the same interface.
  However, PTP is supported if MACSec is not configured on interface.

• PTP is not supported with global MACSec-FIPS-Post.
  MACSec-FIPS-Post is not available per interface.

• Transparent Clock is not supported. One-step clock is supported. It can receive follow-up PTP packets, that is, it can support a two-step peer primary but it cannot send follow-up PTP packets.

PTP Support Information

This table lists different types of support information related to PTP:

Transport Media| •  UDP over IPv4

•  UDP over IPv6

•  Ethernet

---|---
Messages| •  Signaling

•  Announce

•  Sync

•  Follow-up

•  Delay-request

•  Delay-response

•  Management

---|---
Transport Modes| •  Unicast: This is the default mode. All packets are sent as unicast messages. Unicast is applicable only for PTP over IP profiles.

•  Multicast: All packets are sent as multicast messages. Multicast is the only mode for PTP over ethernet profiles.

Timing Profile and Class Support Matrix

This table provides a detailed information on the timing features that are supported on the Cisco 8000 series routers and line cards.
Table 3: Timing Profile and Class Support Matrix

Hardware Module| Supported Profile| Supported G.8273.2 Class| Cisco IOS XR Release
---|---|---|---
•  88-LC0-36FH-M line card

•  8202-32FH-M router

| G.8273.2| Class C| Release 7.5.2
G.8275.1| NA
•  88-LC0-36FH line card

•  88-LC0-34H14FH line

card

•  8201-32FH router

| G.8273.2| Class C| Release 7.3.3
G.8275.1| NA
•  8201 router

•  8202 router

•  8800-LC-48H line card

•  8800-LC-36FH line

card

| G.8273.2| Class C| Release 7.3.1
G.8275.1| NA
G.8265.1| NA
G.8263| NA

Configuring PTP Delay Asymmetry

Table 4: Feature History Table

(PTP) paths can impact PTP accuracy and in turn impact clock settings for all devices in a network. This feature allows you to configure the static asymmetry

such that the delay is accounted for and the PTP synchronization

remains accurate.

The delay-symmetry command is introduced for this feature.

Configure PTP delay asymmetry to offset the static delays on a PTP path that occur due to different route selection for forward and reverse PTP traffic. Delays can also be due to any node having different delay for ingress or egress path. These delays can impact PTP accuracy due to the asymmetry in PTP. With this feature, you can enable a higher degree of accuracy in the PTP server performance leading to better synchronization between real-time clocks of the devices in a network.
Configuration of this delay asymmetry provides an option to configure static delays on a client clock for every server clock. You can configure this delay value in microseconds and nanoseconds. Configured PTP delay asymmetry is also synchronized with the Servo algorithm.

Note

• If you configure multiple PTP delay asymmetries for the same PTP profile, the latest PTP delay asymmetry that you configure is applied to the PTP profile.
• For G8275.1 and G8275.2 PTP profiles, PTP delay asymmetry is supported for both, client port and dynamic port that act as a client.
• Fixed delay can be measured by using any test and measurement tool. Fixed delay can be compensated by using the positive or negative values. For example, if the fixed delay is +10 nanoseconds, configure – 10 nanoseconds to compensate the fixed delay.

A positive value indicates that the server-to-client propagation time is longer than the client-to-server propagation time, and conversely for negative values.

Supported PTP Profiles
The following PTP profiles support the configuration of PTP delay asymmetry:

• PTP over IP (G8275.2 or default profile)
• PTP over L2 (G8275.1)

Restrictions

• PTP delay asymmetry can be configured only on the PTP port of the grandmaster clock, which can either be a boundary clock or an ordinary clock.
• PTP delay asymmetry is supported for delay compensation of fixed cables and not for variable delay in the network.
• PTP delay asymmetry can be configured within the range of 3 microseconds and -3 microseconds or 3000 nanoseconds and -3000 nanoseconds.

Configuration
To configure PTP delay asymmetry:

1. Configure an interface with PTP.
2. Configure PTP delay asymmetry on the client side.

Configuration Example

ITU-T Telecom Profiles for PTP

Cisco IOS XR software supports ITU-T Telecom Profiles for PTP as defined in the ITU-T recommendations. A profile is a specific selection of PTP configuration options that are selected to meet the requirements of a particular application.
PTP lets you define separate profiles to adapt itself for use in different scenarios. A telecom profile differs in several ways from the default behavior defined in the IEEE 1588-2008 standard and the key differences are mentioned in the subsequent sections.
The following sections describe the ITU-T Telecom Profiles that are supported for PTP.

G.8265.1

G. 8265.1 profile fulfills specific frequency-distribution requirements in telecom networks. Features of G.8265.1 profile are:

• Clock advertisement: G.8265.1 profile specifies changes to values used in Announce messages for advertising PTP clocks. The clock class value is used to advertise the quality level of the clock, while the other values are not used.
• Clock Selection: G.8265.1 profile also defines an alternate Best Master Clock Algorithm (BMCA) to select port states and clocks is defined for the profile. This profile also requires to receive Sync messages (and optionally, Delay-Response messages) to qualify a clock for selection.
• Port State Decision: The ports are statically configured to be Master or Slave instead of using state machines to dynamically set port states.
• Packet Rates: The packet rates higher than rates specified in the IEEE 1588-2008 standard are used. They are:
  • Sync/Follow-Up Packets: Rates from 128 packets-per-second to 16 seconds-per-packet.
  • Delay-Request/Delay-Response Packets: Rates from 128 packets-per-second to 16 seconds-per-packet.
  • Announce Packets: Rates from 8 packets-per-second to 64 packets-per-second.
• Transport Mechanism: G.8265.1 profile only supports IPv4 PTP transport mechanism.
• Mode: G.8265.1 profile supports transport of data packets only in unicast mode.
• Clock Type: G.8265.1 profile only supports Ordinary Clock-type (a clock with only one PTP port).
• Domain Numbers: The domain numbers that can be used in a G.8265.1 profile network ranges from 4 to 23.
• Port Numbers: All PTP port numbers can only be one (1) because all clocks in this profile network are Ordinary Clocks.
• G.8261 class-specification standard is supported.

G. 8265.1 profile defines an alternate algorithm to select between different master clocks based on the local priority given to each master clock and their quality levels (QL). This profile also defines Packet Timing Signal Fail (PTSF) conditions to identify the master clocks that do not qualify for selection. They are:

• PTSF-lossSync condition: Raised for master clocks that do not receive a reliable stream of Sync and Delay-Resp messages. Cisco IOS XR software requests Sync and Delay-Resp grants for each configured master clock to track the master clock with this condition.
• PTSF-lossAnnounce condition: Raised for master clocks that do not receive a reliable stream of Announce messages.
• PTSF-unusable condition: Raised for master clocks that receives a reliable stream of Announce, Sync, and Delay-Resp messages, but not usable by slave clocks. Cisco IOS XR software does not use this condition.

Configuring Global G.8265.1 Master Profile
The following configuration describes the steps involved to create a global configuration profile for a PTP interface that can then be assigned to any interface as required. It uses G.8265.1 profile as an example:

Verification
To display the configured PTP profile details, use show run ptp command.

Configuring Global G.8265.1 Slave Profile
The following configuration describes the steps involved to create a global configuration profile for a PTP
interface that can then be assigned to any interface as required. It uses G.8265.1 profile as an example:

Verification
To display the configured PTP profile details, use show run ptp command.

Configuring PTP Master Interface
The following configuration describes the steps involved to configure a PTP interface to be a Master.

G.8263 Standard

Table 5: Feature History Table

Support for ITU-T G.8263 standard for secondary clock with ITU-T

G.8265.1 profile

| Release 7.3.1| ITU-T G.8263 is a performance

compliance standard for secondary clocks configured with ITU-T

G.8265.1 profiles. These clocks drive frequency synchronization based on the PTP packets received at the secondary devices, from traceable primary devices.

G.8263 is the performance compliance standard for the clocks with G.8265.1 profile configured. These clocks drive frequency synchronization based on the PTP packets that are received at the secondary devices from traceable primary device. To handle excess PDV in the network, special servo mode is enabled by configuring the network-type high-pdv command in the PTP configuration.

Configuration
To configure high PDV mode on the slave clock, use the following steps:

1. Configure telecom profile G.8265.1 and clock-type as slave.
2. Configure network type as high PDV.

Configuration Example

Table 6: Feature History Table

architecture defined in ITU-T G.8275 for systems requiring accurate phase and time synchronisation, phase or

time-of-day synchronization is

required, and where each network device participates in the PTP protocol. Support of this capability is extended on the Cisco 8000 Series router, in this release.

G.8275.1 profile fulfills the time-of-day and phase synchronization requirements in telecom networks with all network devices participating in the PTP protocol. G.8275.1 profile provides better frequency stability for the time-of-day and phase synchronization.
Features of G.8275.1 profile are:

• Synchronization Model: G.8275.1 profile adopts hop-by-hop synchronization model. Each network device in the path from master to slave synchronizes its local clock to upstream devices and provides synchronization to downstream devices.
• Clock Selection: G.8275.1 profile also defines an alternate BMCA that selects a clock for synchronization and port state for the local ports of all devices in the network is defined for the profile. The parameters defined as a part of the BMCA are:
• Clock Class
• Clock Accuracy
• Offset Scaled Log Variance
• Priority 2
• Clock Identity
• Steps Removed
• Port Identity
• notSlave flag
• Local Priority
• Port State Decision: The port states are selected based on the alternate BMCA algorithm. A port is configured to a master-only port state to enforce the port to be a master for multicast transport mode.
• Packet Rates: The nominal packet rate for Announce packets is 8 packets per second and 16 packets per second for Sync/Follow-Up and Delay-Request/Delay-Response packets.
• Transport Mechanism: G.8275.1 profile only supports Ethernet PTP transport mechanism.
• Mode: G.8275.1 profile supports transport of data packets only in multicast mode. The forwarding is done based on forwardable or non-forwardable multicast MAC address.
• Clock Type: G.8275.1 profile supports the following clock types:
• Telecom Grandmaster (T-GM): Provides timing for other network devices and does not synchronize its local clock to other network devices.
• Telecom Time Slave Clock (T-TSC): A slave clock synchronizes its local clock to another PTP clock, but does not provide PTP synchronization to any other network devices.
• Telecom Boundary Clock (T-BC): Synchronizes its local clock to a T-GM or an upstream T-BC clock and provides timing information to downstream T-BC or T-TSC clocks.
• Domain Numbers: The domain numbers that can be used in a G.8275.1 profile network ranges from 24 to 43. The default domain number is 24.

The G.8275.1 supports the following:

• T-GM: The telecom grandmaster (T-GM) provides timing to all other devices on the network. It does not synchronize its local clock with any other network element other than the Primary Reference Time Clock (PRTC).
• T-BC: The telecom boundary clock (T-BC) synchronizes its local clock to a T-GM or an upstream T-BC, and provides timing information to downstream T-BCs or T-TSCs. If at a given point in time there are no higher-quality clocks available to a T-BC to synchronize to, it may act as a grandmaster.
• T-TSC: The telecom time slave clock (T-TSC) synchronizes its local clock to another PTP clock (in most cases, the T-BC), and does not provide synchronization through PTP to any other device.

The following figure describes a sample G.8275.1 topology.

Figure 1: A Sample G.8275.1 Topology

Configuring Global G.8275.1 Profile
The following configuration describes the steps involved to create a global PTP configuration profile that can
be applied at an interface level. It uses G.8275.1 profile as an example:

Verification
To display the configured PTP profile details, use show run ptp command.

Configuring PTP Master Interface
The below configuration describes the steps involved to configure a PTP interface to be a Master.

To verify the port state details, use show run interface interface-name command.

Configuring PTP Slave Interface
This procedure describes the steps involved to configure a PTP interface to be a Slave.

Verification
To verify the port state details, use show run interface interface-name command.

Configuring PTP Hybrid Mode
This procedure describes the steps involved to configure router in a hybrid mode. You configure hybrid mode by selecting PTP for phase and time-of-day (ToD) and another source for the frequency.

Note
G.8275.1 PTP profile supports only the hybrid mode. It is mandatory to have hybrid mode for G8275.1 profile for T-BC and T-TSC clock types. By default, the hybrid mode is used, regardless of the physical-layer-frequency configuration.

To configure PTP Hybrid mode

1. Configure Global Frequency Synchronization

2. Configure Frequency Synchronization for an Interface. The time-of-day-priority setting specifies that SyncE to be used as a ToD source if there is no source available with a lower priority.

Verification

ITU-T Telecom Profile Examples:

G.8265.1 Profile Configuration Examples

Master Global Configuration:

Note
For G.8265.1 PTP master clock, either SyncE or BITS reference source configuration is recommended. Otherwise, the device uses its own internal clock.

Slave Global Configuration:

G.8275.1 Profile Configuration Examples

Master Global Configuration:

Note For T-GM clocks, sync2 clock interface has to be configured and it should be UP.

Configuring With Clock Type as Slave Clock (T-TSC)

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