H3C S9500 Series Operation Manual page 1338

Operation Manual – NTP
H3C S9500 Series Routing Switches
NTP
1.
LS _A
2.
LS_A
3.
LS _A
4.
LS_A
Figure 1-1 Basic operating principle of NTP
In the figure above, Switch A and Switch B are connected through the Ethernet port.
They have independent system clocks. Before implement automatic clock
synchronization on both switches, we assume that:
Before synchronizing the system clocks on Switch A and B, the clock on Switch A
is set to 10:00:00am, and that on B is set to 11:00:00am.
Switch B serves as an NTP time server. That is, Switch A synchronizes the local
clock with the clock of B.
It takes one second to transmit a data packet from either A or B to the opposite
end.
The system clocks are synchronized as follows:
Switch A sends an NTP packet to Switch B. The packet carries the timestamp
10:00:00am (T
When the NTP packet arrives at Switch B, Switch B adds a local timestamp
11:00:01am (T
When the NTP packet leaves Switch B, Switch B adds another local timestamp
11:00:02am (T
When Switch A receives the acknowledgement packet, it adds a new timestamp
10:00:03am (T
Now, Switch A collects enough information to calculate the following two important
parameters:
The delay for a round trip of an NTP packet traveling between Switch A and B:
Delay= (T
Offset of Switch A clock relative to Switch B clock: offset= ( (T
10:00:00 AM
message
Network
NTP message
Network
NTP message 10:00:00 AM
Network
NTP Packet received at 10: 00:03
Network
) that tells when it left Switch A.
1
) to it.
2
) to it.
3
) to it.
4
-T ) - (T -T ).
4 1 3 2
LS _B
10:00:00 AM : 11:00:01 AM
LS_B
11:00:01 AM 11:00:02 AM
LS_B
1-2
Chapter 1 NTP Configuration
LS_B
-T ) + (T -T
2 1 4 3
) ) /2.