1-2
z The clock stratum determines the accuracy, which ranges from 1 to 16. The stratum of a reference
clock ranges from 1 to 15. The clock accuracy decreases as the stratum number increases. A
stratum 16 clock is in the unsynchronized state and cannot serve as a reference clock.
z The local clock of an S3100 Ethernet switch cannot be set as a reference clock. It can serve as a
reference clock source to synchronize the clock of other devices only after it is synchronized.
Implementation Principle of NTP
Figure 1-1 shows the implementation principle of NTP.
Ethernet switch A (Device A) is connected to Ethernet switch B (Device B) through Ethernet ports. Both
having their own system clocks, they need to synchronize the clocks of each other through NTP. To help
you to understand the implementation principle, we suppose that:
z Before the system clocks of Device A and Device B are synchronized, the clock of Device A is set
to 10:00:00 am, and the clock of Device B is set to 11:00:00 am.
z Device B serves as the NTP server, that is, the clock of Device A will be synchronized to that of
Device B.
z It takes one second to transfer an NTP message from Device A to Device B or from Device B to
Device A.
Figure 1-1 Implementation principle of NTP
IP network
IP network
IP network
IP network
Device BDevice A
Device B
Device A
Device B
Device A
Device B
Device A
10:00:00 am 11:00:01 am
10:00:00 am
NTP message 10:00:00 am 11:00:01 am 11:00:02 am
NTP message
NTP message
NTP message received at 10:00:03 am
1.
3.
2.
4.
The procedure of synchronizing the system clock is as follows:
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