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Figure 18 Clock nodes in a PTP domain
Besides the three basic types of clock nodes, PTP introduces some hybrid clock nodes. For example,
a TC+OC has multiple PTP ports in a PTP domain: one port is the OC type, and the others are the
TC type. A TC+OC forwards PTP messages through TC-type ports and performs delay corrections.
In addition, it synchronizes time through its OC-type port. TC+OCs include these types: E2ETC+OC
and P2PTC+OC.
Master-member/subordinate relationship
The master-member/subordinate relationship is defined as follows:
• Master/Member node—A master node sends a synchronization message, and a member
node receives the synchronization message.
• Master/Member clock—The clock on a master node is a master clock, and that on a member
node is a member clock.
• Master/Subordinate port—A master port sends a synchronization message, and a
subordinate port receives the synchronization message. The master and subordinate ports can
be on a BC or an OC. A port that neither receives nor sends synchronization time is a passive
port.
Grandmaster clock
In Figure 18, all clock nodes are organized together and ultimately derive their time from a clock
known as the "grandmaster clock (GM)." The GM clock source synchronizes its time to the entire
PTP domain through PTP messages exchanged among the clock nodes.
A GM can be manually configured, or it can be elected through the Best Master Clock (BMC)
algorithm as follows:
1. By exchanging announce messages containing the priorities, time class, and time accuracy of
GMs, clock nodes in a PTP domain elect a GM. The master nodes, member nodes, master
ports, and subordinate ports are specified during the process. Then a loop-free, interconnected
spanning tree with the GM as the root is generated for the PTP domain.
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