Understanding PTP

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

Precision Time Protocol (PTP) is defined in IEEE 1588 as Precision Clock Synchronization for Networked Measurements and Control Systems, and was developed to synchronize the clocks in packet-based networks that include distributed device clocks of varying precision and stability. PTP is designed specifically for industrial, networked measurement and control systems, and is optimal for use in distributed systems because it requires minimal bandwidth and little processing overhead.

Table 1. Nodes within a PTP Network

Network Element

Description

Grandmaster (GM)

A network device physically attached to the primary time 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:

  • Server mode—Distributes timing information over the network to one or more client clocks, thus allowing the client to synchronize its clock to the server clock.

  • Client mode—Synchronizes its clock to a server clock. You can enable the client mode on up to two interfaces simultaneously in order to connect to two different server clocks.

Boundary Clock (BC)

The device participates in selecting the best server clock and can act as the server clock if no better clocks are detected.

Boundary clock starts its own PTP session with a number of downstream clients. The boundary clock mitigates the number of network hops and results in packet delay variations in the packet network between the Grandmaster and client.

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.

Why PTP?

Smart grid power automation applications, such as peak-hour billing, virtual power generators, and outage monitoring and management, require extremely precise time accuracy and stability. Timing precision improves network monitoring accuracy and troubleshooting ability.

In addition to providing time accuracy and synchronization, the PTP message-based protocol can be implemented on packet-based networks, such as Ethernet networks. The benefits of using PTP in an Ethernet network include:

  • Low cost and easy setup in existing Ethernet networks

  • Limited bandwidth requirement for PTP data packets

Routers and Delays

In an IP network, routers provide a full-duplex communication path between network devices. Routers send data packets to packet destinations using IP address information contained in the packets. When the router attempts to send multiple packets simultaneously, the router buffers some packets so that they are not lost before they are sent. When the buffer is full, the router delays sending packets. This delay can cause device clocks on the network to lose synchronization with one another.

Additional delays can occur when packets entering a router are stored in its local memory while the router searches the address table to verify packet fields. This process causes variations in packet forwarding time latency, and these variations can result in asymmetrical packet delay times.

Adding PTP to a network can compensate for these latency and delay problems by correctly adjusting device clocks so that they stay synchronized with one another. PTP enables network routers to function as PTP devices, including boundary clocks (BCs) and transparent clocks (TCs).

For more information about PTP clock devices and their role in a PTP network, see the PTP Clocks section.

Message-Based Synchronization

To ensure clock synchronization, PTP requires an accurate measurement of the communication path delay between the time source (server) and the receiver (client). PTP sends messages between the server and client device to determine the delay measurement. Then, PTP measures the exact message transmit and receive times and uses these times to calculate the communication path delay.

PTP then adjusts current time information contained in network data for the calculated delay, resulting in more accurate time information.

This delay measurement principle determines path delay between devices on the network. The local clocks are adjusted for this delay using a series of messages sent between servers and clients. The one-way delay time is calculated by averaging the path delay of the transmit and receive messages. This calculation assumes a symmetrical communication path; however, routed networks do not necessarily have symmetrical communication paths, due to the various asymmetries in the network.

Using transparent clocks, PTP provides a method to measure and account for the delay in a time-interval field in network timing packets. This makes the routers temporarily transparent to the server and client nodes on the network. An end-to-end transparent clock forwards all messages on the network in the same way that a router does.

To read a detailed description of synchronization messages, see the PTP Event Message Sequences section. To learn more about how transparent clocks calculate network delays, refer to Transparent Clock, on page 7.

PTP Event Message Sequences

This section describes the PTP event message sequences that occur during synchronization.

Synchronizing with Boundary Clocks

The ordinary and boundary clocks configured for the delay request-response mechanism use the following event messages to generate and communicate timing information:

  • Sync

  • Delay_Req

  • Follow_Up

  • Delay_Resp

These messages are sent in the following sequence:

  • The server sends a Sync message to the client and notes the time (t1) at which it was sent.

  • The client receives the Sync message and notes the time of reception (t2).

  • The server conveys to the client the timestamp t1 by embedding the timestamp t1 in a Follow_Up message.

  • The client sends a Delay_Req message to the server and notes the time (t3) at which it was sent.

  • The server receives the Delay_Req message and notes the time of reception (t4).

  • The server conveys to the client the timestamp t4 by embedding it in a Delay_Resp message.

After this sequence, the client possesses all four timestamps. These timestamps can be used to compute the offset of the client clock relative to the server, and the mean propagation time of messages between the two clocks.

The offset calculation is based on the assumption that the time for the message to propagate from server to client is the same as the time required from client to server. This assumption is not always valid on an Ethernet/IP network due to asymmetrical packet delay times.

Figure 1. Detailed Steps—Boundary Clock Synchronization

Synchronizing the Local Clock

In an ideal PTP network, the server and client clock operate at the same frequency. However, drift can occur on the network. Drift is the frequency difference between the server and client clock. You can compensate for drift by using the time stamp information in the device hardware and follow-up messages (intercepted by the router) to adjust the frequency of the local clock to match the frequency of the server clock.

PTP Clocks

A PTP network is made up of PTP-enabled devices and devices that are not using PTP. The PTP-enabled devices typically consist of the following clock types.

Grandmaster Clock

Within a PTP domain, the grandmaster clock is the primary source of time for clock synchronization using PTP. The grandmaster clock usually has a very precise time source, such as a GPS or atomic clock. When the network does not require any external time reference and only needs to be synchronized internally, the grandmaster clock can free run.

Ordinary Clock

An ordinary clock is a PTP clock with a single PTP port. It functions as a node in a PTP network and can be selected by the BMCA as a server or client within a subdomain. Ordinary clocks are the most common clock type on a PTP network because they are used as end nodes on a network that is connected to devices requiring synchronization. Ordinary clocks have various interfaces to external devices.

Boundary Clock

A boundary clock in a PTP network operates in place of a standard network router. Boundary clocks have more than one PTP port, and each port provides access to a separate PTP communication path. Boundary clocks provide an interface between PTP domains. They intercept and process all PTP messages, and pass all other network traffic. The boundary clock uses the BMCA to select the best clock seen by any port. The selected port is then set as a client. The server port synchronizes the clocks connected downstream, while the client port synchronizes with the upstream server clock.

Transparent Clock

The role of transparent clocks in a PTP network is to update the time-interval field that is part of the PTP event message. This update compensates for switch delay and has an accuracy of within one picosecond.

Restrictions for PTP

PTP over MPLS is not supported.

PTP Profiles

ITU-T Telecom Profiles for PTP

Cisco IOS XR software supports ITU-T Telecom Profiles for PTP as defined in the ITU-T recommendation. A profile consists of PTP configuration options applicable only to a specific application.

Separate profiles can be defined to incorporate PTP in different scenarios based on the IEEE 1588-2008 standard. 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 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 Server or Client instead of using FSM 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. The default domain number is 4.

  • Port Numbers: Multiple ports can be configured; however, all ports must be of the same type, either Server or Client.

G.8275.1

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 with SyncE 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 Server to Client clock 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 server-only port state to enforce the port to be a server 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)

    • Telecom Time subordinate/client Clock (T-TSC)

    • Telecom Boundary Clock (T-BC)

  • 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, T-BC continues to provide its own timing information to its peers, although derived clock is not as accurate as the T-GM.

  • T-TSC: The telecom time subordinate/client 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.

Performance Requirements

The router is compliant with Class B performance requirements for T-TSC and T-BC as documented in G.8273.2.

G.8275.2

The G.8275.2 is a PTP profile for use in telecom networks where phase or time-of-day synchronization is required. It differs from G.8275.1 in that it is not required that each device in the network participates in the PTP protocol. Also, G.8275.2 uses PTP over IPv4 in unicast mode.

The G.8275.2 profile is based on the partial timing support from the network. Hence nodes using G.8275.2 are not required to be directly connected.

The G.8275.2 profile is used in mobile cellular systems that require accurate synchronization of time and phase. For example, the fourth generation (4G) of mobile telecommunications technology.

Features of G.8275.2 profile are:

  • Clock Selection: G.8275.2 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


    Note

    See ITU-T G.8275.2 document to determine the valid values for Clock Class parameter.

  • Port State Decision: The port states are selected based on the alternate BMCA algorithm. A port can be configured as "server-only", "client-only", or "any" mode.

  • Packet Rates:

    • Synchronization/Follow-Up—minimum is one packet-per-second and maximum of 128 packets-per-second.

    • Packet rate for Announce packets—minimum of one packet-per-second and maximum of eight packets-per-second.

    • Delay-Request/Delay-Response packets—minimum is one packet-per-second and maximum of 128 packets-per-second.

  • Transport Mechanism: G.8275.2 profile supports only IPv4 PTP transport mechanism.

  • Mode: G.8275.2 profile supports transport of data packets only in unicast mode.

  • Clock Type: G.8275.2 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. However, T-GM can be connected to a GPS or GNSS for deriving better clock information.

    • Telecom Time Subordinate/Client Clock (T-TSC) and Partial-Support Telecom Time Subordinate/Client Clocks (T-TSC-P): A client 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) and Partial-Support Telecom Boundary Clocks (T-BC-P): 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.8275.2 profile network ranges from 44 to 63. The default domain number is 44.

Configuring the G.8265.1 Profile

Configuring the Client Global Configuration: Example

Master Node 

ptp
 clock
 domain 4
 profile g.8265.1 clock-type master
 profile master
  transport ipv4
  sync frequency 16
  announce interval 1
  delay-request frequency 16
interface gi 0/1/0/0
 ptp
  profile master
  transport ipv4
  port state master-only
  ipv4 address 18.1.1.1/24

Slave Node 

ptp
clock
domain 4
profile g.8265.1 clock-type slave

profile slave
transport ipv4
sync frequency 16
announce interval 1
delay-request frequency 16
interface gi 0/1/0/0
ptp
profile slave
transport ipv4
Master ipv4 18.1.1.1
port state slave-only
ipv4 address 18.1.1.2/24

Configuring the G.8265.1 Profile

Configuring PTP on the Server: Example

ptp
clock
domain 4
profile g.8265.1 clock-type master
clock-class 84
!
profile master
transport ipv4
sync frequency 16
announce interval 1
delay-request frequency 16
!
!
RP/0/RP0/CPU0:P5# show running-config interface tenGigE 0/0/0/6
Thu Mar 15 16:50:34.071 UTC
interface TenGigE0/0/0/6
ptp
profile master
transport ipv4
!
ipv4 address 4.4.4.1 255.255.255.0 
RP/0/RP0/CPU0:P5# show running-config frequency synchronization 
Thu Mar 15 16:50:48.424 UTC
frequency synchronization
quality itu-t option 1
clock-interface timing-mode system

Configuring PTP on Client: Example

ptp
clock
domain 4
profile g.8265.1 clock-type slave
!
profile slave
transport ipv4
sync frequency 16
announce interval 1
delay-request frequency 16
!
frequency priority 1
!
RP/0/RP0/CPU0:P5# show running-config interface tenGigE 0/0/0/6
Thu Mar 15 11:16:34.371 UTC
interface TenGigE0/0/0/6
ptp
profile slave
transport ipv4
master ipv4 4.4.4.1
!
!
ipv4 address 4.4.4.2 255.255.255.0
! 
RP/0/RP0/CPU0:P5# show running-config frequency synchronization 
Thu Mar 15 11:16:46.914 UTC
frequency synchronization
quality itu-t option 1

Configuring the G.8275.1 Profile

Configuring the Global Settings: Example

ptp
clock
 domain 24
 profile g.8275.1 clock-type [T-BC | TGM | TTSC]
!
profile profile1
 transport ethernet
 sync frequency 16
 announce frequency 8
 delay-request frequency 16
!
 profile profile2
 transport ethernet
 sync frequency 16
 announce frequency 8
 delay-request frequency 16
 !
 !
     physical-layer-frequency    
 !

Configuring Client Port: Example

interface GigabitEthernet0/0/0/3
 ptp
  profile profile1
  multicast target-address ethernet 01-1B-19-00-00-00
  transport ethernet
  port state slave-only
  local-priority 10
!
  frequency synchronization
  selection input
  priority 1
  wait-to-restore 0
 ! 
!

Configuring Server Port: Example

interface GigabitEthernet0/0/0/1
 ptp
  profile profile2
  multicast target-address ethernet 01-1B-19-00-00-00
  port state master-only
  transport ethernet
  sync frequency 16
  announce frequency 8
  delay-request frequency 16
 !
 frequency synchronization
 !
!

Configuring the G.8275.2 Profile

Global configuration for the telecom profile for Server clock:

ptp
 clock
  domain 44
  profile g.8275.2 clock-type T-GM
 !
 profile master
  transport ipv4
  sync frequency 64
  announce frequency 8
  unicast-grant invalid-request deny
  delay-request frequency 64
 !
!
 
interface GigabitEthernet0/0/0/11
 ptp
  profile master
 !
 ipv4 address 11.11.11.1 255.255.255.0
!

Global configuration for the telecom profile for Client clock:

ptp
 clock
  domain 44
  profile g.8275.2 clock-type T-TSC
 !
 profile slave
  transport ipv4 
  port state slave-only
  sync frequency 64
  announce frequency 8
  delay-request frequency 64
 !
 log
  servo events
  best-master-clock changes
 !        
!
interface GigabitEthernet0/0/0/12
 ptp
  profile slave
  master ipv4 10.10.10.1
  !
 !
 ipv4 address 10.10.10.2 255.255.255.0
!

Global configuration with clock type as T-Boundary Clock (T-BC) for the telecom profile:


ptp
 clock
  domain 44
  profile g.8275.2 clock-type T-BC
 !
 profile slave
  transport ipv4 
  port state slave-only
  sync frequency 64
  announce frequency 8
  unicast-grant invalid-request deny
  delay-request frequency 64
 !
 profile master
  transport ipv4
  sync frequency 64
  announce frequency 8
  unicast-grant invalid-request deny
  delay-request frequency 64
 !
 log
  servo events
  best-master-clock changes
 !        
!


interface GigabitEthernet0/0/0/11
 ptp
  profile master
 !
 ipv4 address 10.10.10.2 255.255.255.0
!

interface GigabitEthernet0/0/0/12
 ptp
  profile slave
  master ipv4 10.10.10.1 
  !
 !
 ipv4 address 10.10.10.3 255.255.255.0
!

Example: Configuring G.8275.2 in Hybrid Mode

  1. Configuring Sync2

    clock-interface sync 2 location 0/RP0/CPU0
port-parameters
gps-input tod-format cisco pps-input ttl <depending on the tod format incoming : cisco/ntp4>
!
frequency synchronization
selection input
priority 1
wait-to-restore 0
quality receive exact itu-t option 1 PRC

  2. Configuring the T-GM with GNSS as source


    Note

    If the server clock receives front panel inputs, skip to step b.

    1. Enabling GNSS

      gnss-receiver 0 location 0/RP1/CPU0
no shut
constellation auto
frequency synchronization
selection input
wait-to-restore 0
quality receive exact itu-t option 1 PRC

    2. Configuring global PTP

      ptp
          clock
          domain 44
          profile g.8275.2 clock-type T-GM      
         !
         profile 8275.2
        transport ipv4
        port state any
        sync frequency 64
        announce frequency 8
        delay-request frequency 64
         !
         physical-layer-frequency        
        !

    3. Configuring global frequency

      frequency synchronization
        quality itu-t option 1
        clock-interface timing-mode system
        !

    4. Enabling GPS for phase and frequency input

      clock-interface sync 2 location 0/RP0/CPU0
         port-parameters
         gps-input tod-format ntp4 pps-input ttl baud-rate 9600
         !
         frequency synchronization
         selection input
         priority 1
         wait-to-restore 0
         quality receive exact itu-t option 1 PRC
         !
         !

    5. Configuring PTP and SyncE output on port for T-GM

      interface HundredGigE0/0/0/1
    ptp
    profile 8275.2
    !
    frequency synchronization
    !

  3. Configuring G.8275.2 on T-BC

    1. Configuring global SyncE

      frequency synchronization
     quality itu-t option 1
     clock-interface timing-mode system
     !

    2. Configuring global PTP

      ptp
clock
domain 44
profile g.8275.2 clock-type T-BC
!
profile 8275.2
  transport ipv4
  port state any
  sync frequency 64
  announce frequency 8
  delay-request frequency 64
!
physical-layer-frequency  <-- This is a mandatory command -->
!

    3. Configuring Client port on Hybrid BC

      interface HundredGigE0/0/0/0
ptp
profile 8275.2
!
frequency synchronization
selection input
priority 1
wait-to-restore 0
!
!

    4. Configuring Server port on Hybrid BC

      interface HundredGigE0/0/0/1
ptp
profile 8275.2
!
frequency synchronization
!
!

  4. Configuring G8275.2 on T-TSC

    1. Configuring global SyncE

      frequency synchronization
     quality itu-t option 1
     clock-interface timing-mode system
     !

    2. Configuring global PTP

      ptp
clock
domain 44
profile g.8275.2 clock-type T-TSC
!
profile 8275.2
  transport ipv4
  port state any
  sync frequency 64
  announce frequency 8
  delay-request frequency 64
!
physical-layer-frequency  <-- This is a mandatory command -->
!

    3. Configuring Client port on Hybrid BC

      interface HundredGigE0/0/0/0
ptp
profile 8275.2
!
frequency synchronization
selection input
priority 1
wait-to-restore 0
!
!

Example: Configuring G.8275.2 in Non-Hybrid Mode

  1. Configuring Sync2

    clock-interface sync 2 location 0/RP0/CPU0
port-parameters
gps-input tod-format cisco pps-input ttl <depending on the tod format incoming : cisco/ntp4>
!
frequency synchronization
selection input
priority 1
wait-to-restore 0
quality receive exact itu-t option 1 PRC

  2. Configuring the T-GM with GNSS as source


    Note

    If the server clock receives front panel inputs, skip to step b.

    1. Enabling GNSS

      gnss-receiver 0 location 0/RP1/CPU0
frequency synchronization
selection input
wait-to-restore 0
quality receive exact itu-t option 1 PRC

    2. Configuring global PTP

      ptp
          clock
          domain 44
          profile g.8275.2 clock-type T-GM      
         !
         profile 8275.2
        transport ipv4
        port state any
        sync frequency 64
        announce frequency 8
        delay-request frequency 64
         !       
        !

    3. Enabling GPS for phase and frequency input

      clock-interface sync 2 location 0/RP0/CPU0
         port-parameters
         gps-input tod-format ntp4 pps-input ttl baud-rate 9600
         !
         
         selection input
         priority 1
         wait-to-restore 0
         quality receive exact itu-t option 1 PRC
         !
         !

    4. Configuring PTP and SyncE output on port for T-GM

      interface HundredGigE0/0/0/1
    ptp
    profile 8275.2
    !
    !

  3. Configuring G.8275.2 on T-BC

    1. Configuring global PTP

      ptp
clock
domain 44
profile g.8275.2 clock-type T-BC
!
profile 8275.2
  transport ipv4
  port state any
  sync frequency 64
  announce frequency 8
  delay-request frequency 64
!

    2. Configuring Client port on Hybrid BC

      interface HundredGigE0/0/0/0
ptp
profile 8275.2
!
selection input
priority 1
wait-to-restore 0
!
!

    3. Configuring Server port on Hybrid BC

      interface HundredGigE0/0/0/1
ptp
profile 8275.2
!
!

  4. Configuring G8275.2 on T-TSC

    1. Configuring global PTP

      ptp
clock
domain 44
profile g.8275.2 clock-type T-TSC
!
profile 8275.2
  transport ipv4
  port state any
  sync frequency 64
  announce frequency 8
  delay-request frequency 64
!

    2. Configuring Client port on Hybrid BC

      interface HundredGigE0/0/0/0
ptp
profile 8275.2
!
selection input
priority 1
wait-to-restore 0
!
!

Slow Tracking

Under normal configured conditions, any change in offset triggers an immediate reaction in the servo. With the Slow Tracking feature enabled, the servo corrects the phase offset based on the configured value. If the phase offset exceeds the acceptable range, servo goes into Holdover state. In such a condition, the Slow Tracking feature becomes inactive and the servo corrects itself to the latest offset and goes into Phase locked state. Slow Tracking becomes active again.


Note

  • The supported slow tracking rate range is from 8-894 nanoseconds per second and must be in multiples of 8.

  • This feature is active only when servo is in Phase locked mode.

 
Router:# config
ptp
clock
domain 24
profile g.8275.1 clock-type T-BC
!
profile profile1
multicast target-address ethernet 01-1B-19-00-00-00
transport ethernet
sync frequency 16
clock operation one-step
announce frequency 8
delay-request frequency 16
!
physical-layer-frequency
servo-slow-tracking 16
!

PTP Holdover Traceability Suppression

Table 2. Feature History Table

Feature Name

Release Information

Feature Description

PTP Holdover Traceability Suppression

Release 7.4.1

When a device which is configured as a Boundary clock (T-BC) loses synchronization with a quality Primary clock, to ensure that the downstream nodes continue to receive the configured clock class for a specified duration, and it’s traceable you can configure this feature.

When the device loses synchronization with a quality Primary clock, to ensure that the downstream nodes continue to receive the configured clock class, and it’s traceable you can configure this feature.

This feature enables the device which is configured as a boundary clock (T-BC) with PTP Profiles G.8275.1 or G.8275.2 to send out the configured clock-class as holdover clock-class and the time traceability flag to be set as TRUE for the specified duration. This is to ensure the down-stream nodes do not have an impact as this is a deviation from prescribed G.8275.1 ITU-T standards.


Note

  • There are PTP flaps during switchovers or ISSU as the PTP holdover timer is running on the active RSP.

  • Once the configured holdover override duration has lapsed and the device is unable to receive a quality Primary clock within this duration, the device sends the prescribed default clock class of 165, and the traceability flag will be set as FALSE to advertise loss of clock to downstream nodes.

Configuring PTP Holdover traceability suppression

This section describes how to configure the PTP holdover traceability suppression feature:

Router# config
Router(config)# ptp
Router(config-ptp)# holdover-spec-duration 1000
Router(config-ptp)# holdover-spec-clock-class 135
Router(config-ptp)# uncalibrated-traceable-override
Router(config-ptp)# holdover-spec-traceable-override

IEEE Default Profile

The IEEE 1588 standard defines one profile, the default profile A telecom profile defines:

  • Restrictions on network technology

  • Required PTP options

  • Allowed PTP options

  • Forbidden PTP options

The IEEE 1588 Default Profile can be configured only over IP and MPLS networks.

The Default Profile requires the following PTP options:

  • The standard BMCA, with both priority fields set to 128.

  • All management messages implemented

  • Domain number zero

Example: Hybrid Default Profile

Global PTP Configuration:


ptp
 clock
  domain 0
 exit
 profile slave
  transport ipv4
  sync frequency 32
  announce frequency 2
  delay-request frequency 32
 exit
 profile master
  transport ipv4
 exit
 uncalibrated-clock-class 255 unless-from-holdover
 freerun-clock-class 255
 startup-clock-class 255
 physical-layer-frequency  <-- This is a mandatory command -->
exit

PTP 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: 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 (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, and is described in the Configuring Frequency Synchronization module in this document. The ToD selection is controlled using the time-of-day priority configuration. This configuration is found under the source 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.

Configuring PTP Hybrid Mode


Note

You must configure the PTP hybrid mode when using the G.8275.1 PTP profile.

Configure hybrid mode by selecting PTP for the time-of-day (ToD) and another source for the frequency. This task summaries the hybrid configuration. See the other PTP configuration modules for more detailed information regarding the PTP configurations. For more information on SyncE configurations, see the Configuring Ethernet Interfaces section in the Interface and Hardware Component Configuration Guide for Cisco NCS 560 Series Routers.

To configure PTP Hybrid mode:

  1. Configure Global Frequency Synchronization

    RP/0/RP0/CPU0:router(config)# frequency synchronization
    RP/0/RP0/CPU0:router(config)# commit
    RP/0/RP0/CPU0:router(config)# quality itu-t option [1 | 2]

  2. Configure Frequency Synchronization in Interface.

    RP/0/RP0/CPU0:router(config)# interface GigabitEthernet 0/0/0/0
RP/0/RP0/CPU0:router(config-if)# frequency synchronization
RP/0/RP0/CPU0:router(config-if-freqsync)# selection input
RP/0/RP0/CPU0:router(config-if-freqsync)# time-of-day-priority 100
RP/0/RP0/CPU0:router(config-if-freqsync)# commit

  3. Configure Global PTP

    RP/0/RP0/CPU0:router(config)# ptp
RP/0/RP0/CPU0:router(config-ptp)# time-of-day priority 1
RP/0/RP0/CPU0:router(config)# commit

  4. Configure Client Port

    RP/0/RP0/CPU0:router(config)# interface GigabitEthernet0/0/0/2 RP/0/RP0/CPU0:router(config-if)# ptp
RP/0/RP0/CPU0:router(config-if)# profile slave
RP/0/RP0/CPU0:router(config-if)# multicast target-address ethernet 01-1B-19-00-00-00 
RP/0/RP0/CPU0:router(config-if)# transport ethernet sync frequency 16
RP/0/RP0/CPU0:router(config-if)# announce frequency 8
RP/0/RP0/CPU0:router(config-if)# delay-request frequency 16
RP/0/RP0/CPU0:router(config-if)# frequency synchronization
RP/0/RP0/CPU0:router(config-if-freqsync)# selection input
RP/0/RP0/CPU0:router(config-if-freqsync)# priority 1
RP/0/RP0/CPU0:router(config-if-freqsync)# wait-to-restore 0

  5. Configure Server Port

    RP/0/RP0/CPU0:router(config)# interface GigabitEthernet0/0/0/3 
RP/0/RP0/CPU0:router(config)# ptp
RP/0/RP0/CPU0:router(config)# profile master
RP/0/RP0/CPU0:router(config)# multicast target-address ethernet 01-1B-19-00-00-00 
RP/0/RP0/CPU0:router(config)# port state master-only 
RP/0/RP0/CPU0:router(config)# transport ethernet
RP/0/RP0/CPU0:router(config)# sync frequency 16
RP/0/RP0/CPU0:router(config)# announce frequency 8
RP/0/RP0/CPU0:router(config)# delay-request frequency 16
RP/0/RP0/CPU0:router(config)# frequency synchronization
RP/0/RP0/CPU0:router(config-if-freqsync)# exit

Verifying the PTP Hybrid Mode Configurations

Use the following show commands to verify the configurations:

  • show ptp platform servo 

    RP/0/RP0/CPU0:ios# show ptp platform servo 
Tue Mar  5 07:08:00.134 UTC
Servo status: Running
Servo stat_index: 2
Device status: PHASE_LOCKED
Servo Mode: Hybrid
Servo log level: 0
Phase Alignment Accuracy: 0 ns
Sync timestamp updated: 8631
Sync timestamp discarded: 0
Delay timestamp updated: 8631
Delay timestamp discarded: 0
Previous Received Timestamp T1: 22521.011765183  T2: 22521.011766745  T3: 22521.018061685  T4: 22521.018063247  
Last Received Timestamp T1: 22521.073747183  T2: 22521.073748745  T3: 22521.080054957  T4: 22521.080056515  
Offset from master:  0 secs, 2 nsecs
Mean path delay   :  0 secs, 1560 nsecs
setTime():1  stepTime():1 adjustFreq():0 
Last setTime: 21984.000000000 flag:0  Last stepTime:-276573300 Last adjustFreq:0 
RP/0/RP1/CPU0:ios#

  • show running-config ptp 

    RP/0/RP0/CPU0:router# show running-config ptp
ptp
clock
domain 24
profile g.8275.1 clock-type T-BC
!
profile slave
transport ethernet
sync frequency 16
announce frequency 8
delay-request frequency 16
!
profile master
transport ethernet
sync frequency 16
announce frequency 8
delay-request frequency 16
!
!
physical-layer frequency
!

  • show running-config frequency synchronization 

    RP/0/RP0/CPU0:router# show running-config frequency synchronization
Tue Feb 6 06:36:26.472 UTC
frequency synchronization
quality itu-t option 1
clock-interface timing-mode system
!

  • show frequency synchronization interface brief 

    RP/0/RP0/CPU0:P3# show frequency synchronization interface brief

Tue Feb 6 06:37:49.234 UTC
Flags: > - Up D - Down S - Assigned for selection
d - SSM Disabled x - Peer timed out i - Init state
s - Output squelched
Fl Interface QLrcv QLuse Pri QLsnd Output driven by
==== ======================== ===== ===== === ===== ========================
>S GigabitEthernet0/0/0/2 PRC PRC 1 DNU GigabitEthernet0/0/0/2
>x GigabitEthernet0/0/0/3 Fail n/a 100 PRC GigabitEthernet0/0/0/2
>x GigabitEthernet0/0/0/4 Fail n/a 100 PRC GigabitEthernet0/0/0/2
RP/0/RP0/CPU0:P3#

Configuring PTP Delay Asymmetry

Table 3. Feature History Table

Feature Name

Release Information

Description

PTP Delay Asymmetry

Release 7.3.1

Any delays on Precision Time Protocol (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

/* Configure an interface with PTP. */
Router# configure
Router(config)# interface HundredGigE 0/1/0/0
Router(config-if)# ptp

/* Configure PTP delay asymmetry on the client side. */
Router(config-if-ptp)# delay-asymmetry 3 microseconds
Router(config-if-ptp)# commit

Running Configuration

interface preconfigure HundredGigE 0/1/0/0
 ptp
  delay-asymmetry 3 microseconds

Verification

To verify if PTP delay asymmetry is applied, use the show ptp foreign-masters command: 

Router# show ptp foreign-masters 
Sun Nov 1 10:19:21.874 UTC
Interface HundredGigE0/1/0/0 (PTP port number 1)
IPv4, Address 209.165.200.225, Unicast
Configured priority: 1
Configured clock class: None
Configured delay asymmetry: 3 microseconds <- configured variable delay asymmetry value
Announce granted: every 2 seconds, 300 seconds
Sync granted: 16 per-second, 300 seconds
Delay-resp granted: 16 per-second, 300 seconds
Qualified for 2 minutes, 45 seconds
Clock ID: 80e01dfffe8ab73f
Received clock properties:
Domain: 0, Priority1: 128, Priority2: 128, Class: 6
Accuracy: 0x22, Offset scaled log variance: 0xcd70
Steps-removed: 1, Time source: GPS, Timescale: PTP
Frequency-traceable, Time-traceable
Current UTC offset: 37 seconds (valid)
Parent properties:
Clock ID: 80e01dfffe8ab73f
Port number: 1

To validate the approximate compensated delay value, use the show ptp platform servo command: 

Router# show ptp platform servo
Mon Jun 27 22:32:44.912 UTC
Servo status: Running
Servo stat_index: 2
Device status: PHASE_LOCKED
Servo Mode: Hybrid
Servo log level: 0
Phase Alignment Accuracy: -2 ns
Sync timestamp updated: 18838
Sync timestamp discarded: 0
Delay timestamp updated: 18837
Delay timestamp discarded: 0
Previous Received Timestamp T1: 1657002314.031435081  T2: 1657002314.031436686  T3: 1657002314.026815770  T4: 1657002314.026814372  
Last Received Timestamp T1: 1657002314.031435081  T2: 1657002314.031436686  T3: 1657002314.088857790  T4: 1657002314.088856392  
Offset from master:  0 secs, 1502 nsecs    <<--compensated value shows 1.5 microseconds because the asymmetry configured under the interface is
3 microseconds.->>
Mean path delay   :  0 secs, 103 nsecs
setTime():0  stepTime():0 adjustFreq():2 
Last setTime: 0.000000000 flag:0  Last stepTime:0 Last adjustFreq:-5093