Understanding Precision Time Protocol
The IEEE 1588 standard describes the use of PTP for fault-tolerant synchronization of network real-time clocks.
The clocks in a PTP network are organized into a master-slave hierarchy. The grandmaster clock is called the Best Master Clock (BMC), and is the root of the master-slave clock hierarchy. PTP uses the BMC algorithm to identify the switch port that is connected to a device that has the most accurate clock which becomes the master clock.
The master clock receives its clock source from the Global Positioning System (GPS) time. The slaves are the other network devices that synchronize their clocks to the master clock. The parent is the clock to which the member-slave clocks synchronize. Timing messages between the master and slave clocks ensure continued synchronization.
Synchronization behavior depends on the PTP clock setting mode that you configure on the switch. The mode can be boundary, end-to-end transparent, or forward.
A switch clock in boundary mode participates in the selection of the most accurate master clock. If more accurate clocks are not detected, that switch clock becomes the master clock. If a more accurate clock is found among the slave clocks, then the switch synchronizes to that clock and becomes a slave clock. After initial synchronization, the switch and the connected devices exchange timing messages to correct the changes caused by clock offsets and network delays.
A switch clock in end-to-end transparent mode synchronizes all switch ports with the master clock. This switch does not participate in master clock selection and uses the default PTP clock mode on all ports.
A switch clock in forward mode allows incoming PTP packets to pass-through the switch as normal multicast traffic.
When the switch is in PTP forward mode, PTP configuration is not available except when changing PTP mode to another mode. You can only configure per-port PTP when the switch is in boundary mode.
You can enable PTP pass-through processing by performing a global configuration on the switch. After PTP pass-through is enabled, all PTP messages are passed to and from the expansion module ports in the VLAN on which the packets are received.
The PTP pass-through feature is not compatible with the Virtual Routing and Forwarding (VRF), Policy Based Routing (PBR), and PVLAN (Private Virtual Local Area Network) features.