3Com Switch 4800G 24-Port Configuration Manual page 948

948 C 77: NTP C
HAPTER ONFIGURATION
How NTP Works
To implement certain functions, such as scheduled restart of all devices within
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the network, all devices must be consistent in timekeeping.
When multiple systems process a complex event in cooperation, these systems
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must use that same reference clock to ensure the correct execution sequence.
For increment backup between a backup server and clients, timekeeping must
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be synchronized between the backup server and all the clients.
Advantages of NTP:
NTP uses a stratum to describe the clock precision, and is able to synchronize
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time among all devices within the network.
NTP supports access control and MD5 authentication.
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NTP can unicast, multicast or broadcast protocol messages.
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Figure 277 shows the basic work flow of NTP. Switch A and Switch B are
interconnected over a network. They have their own independent system clocks,
which need to be automatically synchronized through NTP. For an easy
understanding, we assume that:
Prior to system clock synchronization between Switch A and Switch B, the
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clock of Switch A is set to 10:00:00 am while that of Switch B is set to
11:00:00 am.
Switch B is used as the NTP time server, namely Switch A synchronizes its clock
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to that of Switch B.
It takes 1 second for an NTP message to travel from one switch to the other.
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Figure 277 Basic work flow of NTP
NTP message 10:00:00 am
Switch A
1.
Switch A
2.
NTP message
Switch A
3.
NTP message received at 10:00:03 am
Switch A
4.
IP network
NTP message 10:00:00 am
IP network
10:00:00 am 11:00:01 am
IP network
IP network
Switch B
11:00:01 am
Switch B
11:00:02 am
Switch B
Switch B