H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 453)

46-2
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In network management, the analysis of the log information and debugging information collected
from different devices is meaningful and valid only when network devices that generate the
information adopts the same time.
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The billing system requires that the clocks of all network devices be consistent.
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Some functions, such as restarting all network devices in a network simultaneously require that
they adopt the same time.
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When multiple systems cooperate to handle a rather complex transaction, they must adopt the
same time to ensure a correct execution order.
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To perform incremental backup operations between a backup server and a host, you must make
sure they adopt the same time.
NTP has the following advantages:
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Defining the accuracy of clocks by stratum to synchronize the clocks of all devices in a network
quickly
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Supporting access control (See Configuring Access Control Right ) and MD5 encrypted
authentication (See Configuring NTP Authentication )
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Sending protocol packets in unicast, multicast, or broadcast mode
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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.
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The local clock of the device 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 46-1 shows the implementation principle of NTP.
Device A is connected to 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:
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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.
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Device B serves as the NTP server, that is, the clock of Device A will be synchronized to that of
Device B.
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It takes one second to transfer an NTP message from Device A to Device B or from Device B to
Device A.