3Com Switch 7750 Configuration Guide Guide

696 CHAPTER 66: NTP CONFIGURATION
with the increasing of stratum number. The clocks with the stratum of 16 are in
unsynchronized state and cannot serve as reference clocks.
Working Principle of NTP The working principle of NTP is shown in Figure 175.
In Figure 175, The Ethernet switch A (LS_A) is connected to the Ethernet switch B
(LS_B) through their Ethernet ports. Both of them have system clocks of their own,
and they need to synchronize the clocks of each other through NTP. For ease of
understanding, suppose that:
■ Before the system clocks of LS_A and LS_B are synchronized, the clock of LS_A
is set to 10:00:00am, and the clock of LS_B is set to 11:00:00am.
■ LS_B serves as the NTP time server, that is, the clock of LS_A will be
synchronized to that of LS_B.
■ It takes one second for a packet sent by one switch to reach the other.
Figure 175 Working principle of NTP
The procedures of synchronizing system clocks are as follows:
■ LS_A sends an NTP packet to LS_B, with the timestamp identifying the time
when it is sent (that is, 10:00:00am, noted as T
1
) carried.
■ When the packet arrives at LS_B, LS_B inserts its own timestamp, which
identifies 11:00:01am (noted as T
2
) into the packet.
Ne tw o rk
Netw ork
NTP Packet10:00:00am
Ne tw o rk
Ne tw o rk
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet r ec eiv ed at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Ne tw o rk
Netw ork
NTP Packet10:00:00 am
Ne tw o rk
Ne tw o rk
11:00:01 a m
10:00:00 a m 11:00:01 am 11:00:02 am
10:00:00 am
NTP Packet r ec eiv ed at 10:00:03 a m
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Ne tw o rk
Netw ork
NTP Packet10:00:00am
Ne tw o rk
Ne tw o rk
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet r ec eiv ed at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Netw o rk
Netw ork
NTP Packet 10:00:00 am
Netw ork
Netw ork
11:00:01 am
10:00:00 am 11 :00:01 am 11:00:02 am
10:00:00 a m
NTP Packet r ec eiv ed at 10:00:03 a m
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Pac ke t
NT P Pac ke t
Ne tw o rk
Netw ork
NTP Packet10:00:00am
Ne tw o rk
Ne tw o rk
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet r ec eiv ed at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Ne tw o rk
Netw ork
NTP Packet10:00:00 am
Ne tw o rk
Ne tw o rk
11:00:01 a m
10:00:00 a m 11:00:01 am 11:00:02 am
10:00:00 am
NTP Packet r ec eiv ed at 10:00:03 a m
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Ne tw o rk
Netw ork
NTP Packet10:00:00am
Ne tw o rk
Ne tw o rk
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet r ec eiv ed at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Netw o rk
Netw ork
NTP Packet 10:00:00 am
Netw ork
Netw ork
11:00:01 am
10:00:00 am 11 :00:01 am 11:00:02 am
10:00:00 a m
NTP Packet r ec eiv ed at 10:00:03 a m
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Pac ke t
NT P Pac ke t