368 CHAPTER 35: NTP CONFIGURATION
as the stratum number increases. A stratum 16 clock is in the unsynchronized
state and cannot serve as a reference clock.
■ The local clock of a Switch 4210 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 109 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:
■ 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.
■ Device B serves as the NTP server, that is, the clock of Device A will be
synchronized to that of Device B.
■ It takes one second to transfer an NTP message from Device A to Device B or
from Device B to Device A.
Figure 109 Implementation principle of NTP
The procedure of synchronizing the system clock is as follows:
■ Device A sends an NTP message to Device B, with a timestamp 10:00:00 am
(T
1
) identifying when it is sent.
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.
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