H3C Technologies H3C SR8800 - Manual (Page 64)

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Configuring NTP
NTP overview
Defined in RFC 1305, the Network Time Protocol (NTP) synchronizes timekeeping among distributed
time servers and clients. NTP runs over the User Datagram Protocol (UDP), using UDP port 123.
The purpose of using NTP is to keep consistent timekeeping among all clock-dependent devices within
the network so that the devices can provide diverse applications based on the consistent time.
For a local system running NTP, its time can be synchronized by other reference sources and can be used
as a reference source to synchronize other clocks.
NTP applications
An administrator can by no means keep synchronized time among all the devices within a network by
changing the system clock on each station, because this is a huge amount of workload and cannot
guarantee the clock precision. NTP, however, allows quick clock synchronization within the entire
network while it ensures a high clock precision.
NTP is used when all devices within the network must be consistent in timekeeping, for example:
• In analysis of the log information and debugging information collected from different devices in
network management, time must be used as reference basis.
• All devices must use the same reference clock in a charging system.
• 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 19 shows the basic work flow of NTP. Device A and Device 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 Device A and Device B, the clock of Device A is set
to 10:00:00 am while that of Device B is set to 11:00:00 am.
• Device B is used as the NTP time server, namely Device A synchronizes its clock to that of Device B.
• It takes 1 second for an NTP message to travel from one device to the other.