H3C Technologies H3C S6300 Series Switches - Manual (Page 67)

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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 fall into two 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
OC. A port that neither receives nor sends synchronization time is a passive port.
Grandmaster clock
In Figure 19 , 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.
2. The master node periodically sends announce messages to member nodes. If the member nodes
do not receive announce messages from the master node, they consider the master node invalid
and start to elect another GM.
Clock source type
A clock node of a device can use one of the following clock sources: local clock source and Bits clock
source (Bits1 and Bits2) that connects the device.
• Local clock—38.88 MHz clock signals generated by a crystal oscillator inside the clock monitoring
module.
• Bits clock—Clock signals generated by a specific Bits clock device. The signals are sent to the clock
monitoring module through a specific interface on the device and then sent to all member devices
by the clock monitoring module.
The clock node determines to use which type of clock source based on the specified algorithm.
Synchronization mechanism
Based on the exchanged synchronization messages, a member node calculates the round-trip delay of
the path to the master node. The one-way delay equals half of the round-trip delay (assume the delays in
both directions are the same). Then the member node synchronizes its clock with the master clock
according to the offset between the clocks.