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 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	Beginning with Cisco NX-OS Release 7.3(0)D1(1) release, the generalized-PTP clock mode is introduced to support AVB feature.

Note	PTP operates only in boundary clock mode. Cisco recommends deployment of a Grand Master Clock (10 MHz) upstream, with servers containing clocks requiring synchronization connected to the switch. End-to-end transparent clock and peer-to-peer transparent clock modes are not supported.

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. Hence, 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.

The network-monitoring tool Pong leverages the PTP’s time synchronization infrastructure to diagnose the health of the network. Pong measures port-to-port delays and is similar to the network-monitoring utility Ping but provides for a greater depth of network diagnostics. Make sure to increase the interface MTU when you attempt pong to a destination that is several hops away.

Clocks are resources that need to be shared across different processes and across different VDCs. Multiple time synchronization protocols (such as NTP and PTP) might be running in the system, and multiple instances of the same protocol might be running in different VDCs. The clock manager allows you to specify the protocol and a VDC running that protocol to control the various clocks in the system. For information on configuring the clock manager, see the Cisco Nexus 7000 Series NX-OS Fundamentals Configuration Guide.

Stateful restarts are supported for PTP. After a reboot or a supervisor switchover, the running configuration is applied. For more information on high availability, see the Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide.