PTP is a time synchronization protocol for nodes distributed across a network. Its hardware timestamp feature provides greater accuracy than other time synchronization protocols such as the Network Time Protocol (NTP).

A PTP system can consist of a combination of PTP and non-PTP devices. PTP devices include ordinary clocks, boundary clocks, and transparent clocks. Non-PTP devices include ordinary network switches, routers, and other infrastructure devices.

PTP is a distributed protocol that specifies how real-time PTP clocks in the system synchronize with each other. These clocks are organized into a master-slave synchronization hierarchy with the grandmaster clock, which is the clock at the top of the hierarchy, determining the reference time for the entire system. Synchronization is achieved by exchanging PTP timing messages, with the members using the timing information to adjust their clocks to the time of their master in the hierarchy. PTP operates within a logical scope called a PTP domain.

The following clocks are common PTP devices:

Ordinary clock

Communicates with the network based on a single physical port, similar to an end host. An ordinary clock can function as a grandmaster clock.

Boundary clock

Typically has several physical ports, with each port behaving like a port of an ordinary clock. However, each port shares the local clock, and the clock data sets are common to all ports. Each port decides its individual state, either master (synchronizing other ports connected to it) or slave (synchronizing to a downstream port), based on the best clock available to it through all of the other ports on the boundary clock. Messages that are related to synchronization and establishing the master-slave hierarchy terminate in the protocol engine of a boundary clock and are not forwarded.

Transparent clock

Forwards all PTP messages like an ordinary switch or router but measures the residence time of a packet in the switch (the time that the packet takes to traverse the transparent clock) and in some cases the link delay of the ingress port for the packet. The ports have no state because the transparent clock does not need to synchronize to the grandmaster clock.

There are two kinds of transparent clocks:

End-to-end transparent clock

Measures the residence time of a PTP message and accumulates the times in the correction field of the PTP message or an associated follow-up message.

Peer-to-peer transparent clock

Measures the residence time of a PTP message and computes the link delay between each port and a similarly equipped port on another node that shares the link. For a packet, this incoming link delay is added to the residence time in the correction field of the PTP message or an associated follow-up message.

Note	PTP operates only in boundary clock mode. We recommend that you deploy a Grand Master Clock (10 MHz) upstream. The servers contain clocks that require synchronization and are connected to the switch. End-to-end transparent clock and peer-to-peer transparent clock modes are not supported.

In a properly synchronized PTP network, when any PTP node goes down and comes up, the PTP clock is synchronized to its primary time source (GM). During this process, the local node has significant correction and it tries to correct its local clock. At that time, the node can send incorrect time to the downstream nodes and cause issues for all downstream nodes. The Time Distribution (TD) hold feature, introduced in Cisco NX-OS Release 10.5(1)F, resolves this issue by ensuring that the node is properly synchronized to its primary source and distributes time to the downstream nodes during boot up.

The TD hold feature holds the time distribution until a Boundary Clock (BC) node locks to the primary time source and settles down to the target correction value. The TD hold enabled node receives all PTP packets, does the normal state change, and synchronizes time, but it does not send any PTP packets out.

Note

If all nodes reboot at the same time (with a difference of few seconds), each node will be in active hold time, which sometimes results in no nodes having secondary port. This leads to the BMC taking a long time to find the best clock. Hence, the user needs to take this into account when enabling this feature.

The PTP process consists of two phases: establishing the master-slave hierarchy and synchronizing the clocks.

Within a PTP domain, each port of an ordinary or boundary clock follows this process to determine its state:

• Examines the contents of all received announce messages (issued by ports in the master state)

• Compares the data sets of the foreign master (in the announce message) and the local clock for priority, clock class, accuracy, and so on

• Determines its own state as either master or slave

After the master-slave hierarchy has been established, the clocks are synchronized as follows:

• The master sends a synchronization message to the slave and notes the time it was sent.

• The slave receives the synchronization message and notes the time that it was received. For every synchronization message, there is a follow-up message. The number of sync messages should be equal to the number of follow-up messages.

• The slave sends a delay-request message to the master and notes the time it was sent.

• The master receives the delay-request message and notes the time it was received.

• The master sends a delay-response message to the slave. The number of delay request messages should be equal to the number of delay response messages.

• The slave uses these timestamps to adjust its clock to the time of its master.

Stateful restarts are not supported for PTP.

• For Cisco Nexus 3600 Series switches, PTP clock correction is expected to be in the 3-digit range, from 100 to 999 nanoseconds.

• PTP operates only in boundary clock mode. End-to-end transparent clock and peer-to-peer transparent clock modes are not supported.

• PTP supports transport over User Datagram Protocol (UDP). Transport over Ethernet is not supported.

• PTP supports only multicast communication. Negotiated unicast communication is not supported.

• PTP is limited to a single domain per network.

• Forwarding PTP management packets is not supported.

• PTP-capable ports do not identify PTP packets and do not time-stamp or redirect those packets unless you enable PTP on those ports.

• 1 pulse per second (1 PPS) input is not supported.

• PTP over IPv6 is not supported.

• Cisco Nexus switches should be synchronized from the neighboring master using a synchronization log interval that ranges from –2 to –5.

• Beginning with Cisco NX-OS Release 10.5(1)F, the following attributes are added to the PTP high-correction notification:

  • lastHighCorrectionMPD

  • maxHighCorrectionTime

  • maxHighCorrectionValue

  • maxHighCorrectionMPD

• Beginning with Cisco NX-OS Release 10.5(1)F, the PTP Time Distribution (TD) hold feature is introduced. This feature allows for holding the time distribution until a Boundary Clock node locks to the primary time source and settles down to the target correction value.