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.
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:
|
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.
![]() Note |
Transparent Clock (TC) is not supported. |
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-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 configuring PTP hybrid mode is described in a subsequent section in this chapter.
Time of Day (ToD) Support
The router receives GPS ToD messages in serial ASCII stream through the RS422 interface in any of the following formats:
-
NTP Type 4
-
Cisco
-
NMEA - GPZDA
![]() Note |
You can refer to the below support information in context of the current release and see relevant Release Notes for more information on supported features and hardware. |
Port States
State machine indicates the behavior of each port. The possible states are:
State |
Description |
---|---|
INIT |
Port is not ready to participate in PTP. |
LISTENING |
First state when a port becomes ready to participate in PTP: In this state, the port listens to PTP masters for a (configurable) period of time. |
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 NCS 5500 Series Router:
-
NCS55-RP does not support PTP
-
NC55-18H18F line card does not support PTP
-
SyncE is not supported on a 1GE copper SFP.
-
SyncE is not supported on 25 GE or 100 GE interfaces when they are used in 1G mode.
-
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 MACSec.
-
G.8273.2 Class-A performance is met if CFP2-DCO is configured on either Slave or Master port on the node.
-
Transparent Clock is not supported.
PTP Support Information
This table lists different types of support information related to PTP:
Transport Media |
|
Messages |
|
Transport Modes |
|
Timing Hardware Support Matrix
Feature Name |
Release Information |
Feature Description |
---|---|---|
PTP and SyncE Support on NCS-57C3-MOD-S and NCS-57C3-MOD-SE-S routers. |
Release 7.4.1 |
With this release, timing support for IEEE 1588-2008 and SyncE is extended to the following routers:
|
PTP and SyncE on breakout ports on NCS55A1-48Q6H-S, NCS55A1-24Q6H-S, and NCS-5501-SE routers. |
Release 7.2.2 |
With this release, timing support for IEEE 1588-2008 (PTPv2) telecom Profiles 8275.1 and 8275.2, and SyncE ITU-T profiles G.8261, G.8262, and G.8264 is extended to breakout ports on the following routers:
|
![]() Note |
The table also contains support details of upcoming releases. You can read this table in context of the current release and see relevant Release Notes for more information on supported features and hardware. |
This table provides a detailed information on the timing features that are supported on the following hardware PIDs.
Hardware Variant |
Features |
Cisco IOS XR Release |
Comments |
---|---|---|---|
NCS-57C3-MODS-SYS NCS-57C3-MODS-SYS |
PTP Virtual Port and APTS |
Release 7.7.1 |
|
E-SyncE |
Release 7.9.1 |
||
NCS-57B1-6D24-SYS |
PTP Virtual Port and APTS |
Release 7.7.1 |
|
NC57-24DD |
SyncE |
Release 7.5.1 |
|
NC57-18DD-SE |
SyncE |
Release 7.5.1 |
|
NCS-57C1-48Q6-SYS |
Synce |
Release 7.5.1 |
|
E-Synce |
Release 7.9.1 |
||
Default profile |
Release 7.5.1 |
||
G.8265.1 |
Release 7.5.1 |
||
G.8275.1 |
Release 7.5.1 |
||
G.8275.2 |
Release 7.5.1 |
||
RP:NC57-MOD-RP-2E with NCS573-MODS-SYS and NCS-573-MOD-SYS |
SyncE |
Release 7.4.1 |
1G clock recovery is supported from IOS XR Release 7.6.1. |
G.8275.1 |
Release 7.4.1 |
||
G.8273.2 |
Release 7.4.1 |
||
GNSS |
Release 7.4.1 |
||
NCS-57B1-5DSE-SYS NCS-57B1-6D24-SYS |
Synce |
Release 7.3.1 |
|
Esynce |
Release 7.9.1 |
||
Default profile |
Release 7.3.1 |
||
G.8265.1 |
Release 7.3.1 |
||
G.8275.1 |
Release 7.3.1 |
||
G.8275.2 |
Release 7.3.1 |
||
RP: NC55-RP2-E Line card: NC57-36H6D-S |
G.8275.1 |
Release 7.3.2 |
|
G.8273.2 |
Release 7.3.2 |
|
|
SyncE |
Release 7.3.2 |
|
|
RP:NC55-RP-E with Line cards: NC55-MOD-A-S and NC55-32T16Q4H-AT |
BITS |
Release 7.1.1 |
|
SyncE |
Release 7.1.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
|
G8275.1 |
Release 7.1.1 |
For the profile G8275.1 NC55-32T16Q4H-AT supports only T-BCand does not support T-GM. 25G/100G/40G is supported from IOSXR release 7.2.2 onwards. |
|
G8273.2 |
Release 7.1.1 |
Class B |
|
RP:NC55-RP2-E with Line cards: NC55-MOD-A-S and NC55-32T16Q4H-AT |
BITS |
Release 7.1.1 |
|
SyncE |
Release 7.1.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
|
G.8275.1 |
Release 7.1.1 |
For the profile G8275.1 NC55-32T16Q4H-AT supports only T-BC and does not support T-GM. 25G/100G/40G is supported from IOSXR release 7.2.2 onwards. |
|
G.8273.2 |
Release 7.1.1 |
Class B |
|
RP:NC55-RP2-E with Line card:NC55-32T16Q4H-AT |
BITS |
Release 7.1.1 |
|
SyncE |
Release 7.1.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
|
G8275.1 |
Release 7.1.1 |
For the profile G8275.1 NC55-32T16Q4H-AT supports only T-BCand does not support T-GM. 25G/100G/40G is supported from IOSXR release 7.2.2 onwards. |
|
G.8273.2 |
Release 7.1.1 |
Class C |
|
NCS-55A1-36H-SE-S |
SyncE |
Release 7.0.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
G.8265.1 |
Release 7.0.1 |
||
G.8275.1 |
Release 7.0.1 |
||
G.8275.2 |
Release 7.0.1 |
||
G.8273.2 |
Release 7.0.1 |
Class B |
|
NCS-55A1-36H-S |
SyncE |
Release 7.0.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
G.8265.1 |
Release 7.0.1 |
||
G.8275.1 |
Release 7.0.1 |
||
G.8275.2 |
Release 7.0.1 |
||
G.8273.2 |
Release 7.0.1 |
Class B |
|
NCS-55A1-24Q6H-S NCS-55A1-24Q6H-SS |
SyncE |
Release 6.6.25 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
G.8265.1 |
Release 6.6.25 |
||
G.8275.1 |
Release 6.6.25 |
||
G.8275.2 |
Release 6.6.25 |
From Release 7.7.1, support is available for PTP over IPv6 for ports 10G-25G and 40G-100G |
|
G.8273.2 |
Release 6.6.25 |
Class B |
|
NCS-55A1-48Q6H |
SyncE |
Release 6.6.25 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
G.8265.1 |
Release 6.6.25 |
||
G.8275.1 |
Release 6.6.25 |
||
G.8275.2 |
Release 6.6.25 |
||
G.8273.2 |
Release 6.6.25 |
Class B |
|
NCS-55A1-24H |
SyncE |
Release 6.5.2 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
G.8265.1 |
Release 6.5.2 |
||
G.8275.1 |
Release 6.5.2 |
||
G.8275.2 |
Release 6.5.2 |
||
G.8273.2 |
Release 6.5.2 |
Class B |
|
NCS55A2-MOD |
SyncE |
Release 6.5.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
G.8265.1 |
Release 6.5.1 |
||
G.8275.1 |
Release 6.5.1 |
||
G.8275.2 |
Release 6.5.1 |
||
G.8273.2 |
Release 6.5.1 |
Class B |
|
E-syncE |
Release 7.9.1 |
E-SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
|
RP:NC55-RP-E Linecard:NC55-MOD-A-S |
BITS |
Release 6.5.1 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
SyncE |
Release 6.5.1 |
SyncE is not supported on 100GE interfaces, when theyare used in 1G mode. |
|
G.8265.1 |
Release 6.5.1 |
||
G.8275.1 |
Release 6.5.1 |
||
G.8275.2 |
Release 6.5.1 |
This profile is supported from Release 6.5.1 for Ipv4. |
|
G.8273.2 |
Release 6.5.1 |
Class B |
|
RP:NC55-RP-E Linecard:NC55-36X100G-A-SE |
G.8273.2 |
Release 6.3.2 |
Class B |
BITS |
Release 6.3.2 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
|
SyncE |
Release 6.3.2 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. |
|
G.8265.1 |
Release 6.3.2 |
||
G.8275.1 |
Release 6.3.2 |
||
G.8275.2 |
NA |
||
G.8273.2 |
Release 6.3.2 |
Class B |
|
NCS5501-SE |
SyncE |
Release 6.3.2 |
SyncE is not supported on 25GE or 100GE interfaces, when they are used in 1G mode. SyncE is supported on 10G from ports 8 to 15, but it is not supported on these ports in 1G mode. |
G.8265.1 |
Release 6.3.2 |
||
G.8275.1 |
Release 6.3.2 |
Class B |
|
G.8275.2 |
Release 6.3.2 |
||
GNSS External |
Release 6.3.2 |
![]() Note |
Cisco NCS 5500 Series Routers support 64 PTP clients at 64 PPS sync packet rate. |
Timing features are supported on the following MPAs:
-
NC55-MPA-2TH-S
-
NC55-MPA-1TH2H-S
-
NC55-MPA-1TH2H-HD-S
-
NC55-MPA-4H-S
-
NC55-MPA-4H-HD-S
-
NC55-MPA-12T-S