Information About Generalized Precision Time Protocol
Generalized PTP is an IEEE 802.1AS standard, that provides a mechanism to synchronize the clocks of the bridges and end-point devices in a network. Generalized PTP defines the mechanism to elect the grandmaster clock (BMCA) among the time-aware bridges and talker and listener. The grandmaster is the root of the timing hierarchy that gets established in the time-aware network and distributes time to the nodes below to enable synchronization.
Time synchronization also requires determining the link delay and switch delays in the network nodes. A generalized PTP switch is an IEEE 1588 boundary clock, which also determines the link delay using the peer-to-peer delay mechanism. The delays that are computed are included in the correction field of the PTP messages and relayed to the endpoints. The talker and listener use this generalized PTP time as a shared clock reference, which is used to relay and recover the media clock. Generalized PTP currently defines only domain 0, which is what the generalized PTP switch supports.
The peer-to-peer delay mechanism runs on Spanning Tree Protocol-blocked (STP-blocked) ports as well. No other PTP messages are sent over blocked ports.
In a PTP domain, BMCA organizes clocks and ports into an hierarchical fashion, which includes clocks and port states:
Grandmaster (GM or GMC)
Boundary Clock (BC)