The Precision Time Protocol (PTP), as defined in the IEEE 1588 standard, is designed for precise time synchronization across networked devices. It operates by having Foreign Masters (FMs) broadcast timing information to interfaces within the network. The selection of the Grandmaster (GM), the primary reference clock, is determined by the Best Master Clock Algorithm (BMCA). Devices synchronize their clocks to the GM through a process known as the Delay Request-Response Mechanism (DRRM), wherein ports that are directly synchronizing with the GM enter a secondary state.

Historically, ports in a passive state—those that receive timing messages from FMs but aren’t actively syncing to the GM—didn’t participate in DRRM, which meant they didn’t synchronize their clocks.

Starting Cisco IOS XR Software Release 24.2.1, DRRM has been extended to include passive ports, enabling them to engage in the exchange of delay request and response packets. This enhancement allows for the calculation of PTP phase differences between the clocks on passive ports and the GM.

This calculated PTP phase difference provides a valuable insight into the timing characteristics of other foreign masters in the network by using the grandmaster as a reference point. It can be utilized on any boundary clock or slave clock that has connections to at least one other foreign master.

You can access these measurements and the calculated PTP phase differences using show commands through the router's CLI. Also, the information can be retrieved programmatically through operational data models in YANG, providing flexibility in how you can access and utilize this synchronization data.

Phase difference for clock ACDE48FFFE234567, steps removed 1, receiving-port 1, received on interface GigabitEthernet0/2/0/3 is 40ns, configured threshold is 30ns. Raising phase difference alarm.

Starting Cisco IOS XR Software Release 24.2.1, the servo mechanism now has the ability to detect unusable clocks due to packet timing signal fail by analyzing timestamps from foreign masters. This enhancement allows the system to identify foreign masters with unstable timing as unsuitable for use. A platform supports multiple masters, such a master can be excluded from the BMCA selection process while remaining under observation for potential recovery. Even after a master is deemed unusable, the DRRM continues to operate and timestamps from it are still provided to the servo. This ongoing monitoring enables PTP to detect and respond to any improvements in the primary's timing, allowing it to be reconsidered as usable.

Foreign master with clock ID ACDE48FFFE234567, steps removed 1, receiving-port 1, received on interface GigabitEthernet0/2/0/4 is now PTSF-unusable and disqualified from selection.

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. The default domain number is 4.

• Port Numbers: All PTP port numbers can only be one (1) because all clocks in this profile network are Ordinary Clocks.

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.

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.

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.

Effective Cisco IOS XR Software Release 7.8.1, the G.8275.2 is capable of using PTP over IPv6 as well. You can now use the transport ipv6 command to set this encapsulation type.

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 fifth generation (5G) of mobile telecommunications technology.

Note

G.8275.2 profile is supported on Cisco NCS 5500 Series Routers. However, the performance standards of this profile is not aligned with any of the ITU-T standards because performance specifications for G.8275.2 profile has not yet been made available by ITU-T. For more information on hardware that supports G.8275.2 profile configurations, refer to Supported Hardware section in this chapter.

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 is configured to a master-only port state to enforce the port to be a master for unicast transport 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.

  • 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.8275.2 profile network ranges from 44 to 63. The default domain number is 44.