948 CHAPTER 77: NTP CONFIGURATION
■ To implement certain functions, such as scheduled restart of all devices within
the network, all devices must be consistent in timekeeping.
■ When multiple systems process a complex event in cooperation, these systems
must use that same reference clock to ensure the correct execution sequence.
■ For increment backup between a backup server and clients, timekeeping must
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
time among all devices within the network.
■ NTP supports access control and MD5 authentication.
■ NTP can unicast, multicast or broadcast protocol messages.
How NTP Works 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
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
to that of Switch B.
■ It takes 1 second for an NTP message to travel from one switch to the other.
Figure 277 Basic work flow of NTP
IP network
IP network
IP network
IP network
Switch BSwitch A
Switch B
Switch A
Switch B
Switch A
Switch B
Switch 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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