Operation Manual – NTP
H3C S5500-EI Series Ethernet Switches
Chapter 1 NTP Configuration
1-3
IP network
IP network
IP network
IP network
Switch B
Switch 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.
Figure 1-1
Basic work flow of NTP
The process of system clock synchronization is as follows:
z
Switch A sends Switch B an NTP message, which is timestamped when it leaves
Switch A. The time stamp is 10:00:00 am (T1).
z
When this NTP message arrives at Switch B, it is timestamped by Switch B. The
timestamp is 11:00:01 am (T2).
z
When the NTP message leaves Switch B, Switch B timestamps it. The timestamp
is 11:00:02 am (T3).
z
When Switch A receives the NTP message, the local time of Switch A is 10:00:03
am (T4).
Up to now, Switch A has sufficient information to calculate the following two important
parameters:
z
The roundtrip delay of NTP message: Delay = (T
4
–T
1
) – (T
3
-T
2
) = 2 seconds.
z
Time difference between Switch A and Switch B: Offset = ((T
2
-T
1
) + (T
3
-T
4
))/2 = 1
hour.
Based on these parameters, Switch A can synchronize its own clock to the clock of
Switch B.
This is only a rough description of the work mechanism of NTP. For details, refer to RFC
1305.
