3Com Switch 4200G Configuration Guide

292 CHAPTER 35: NTP CONFIGURATION
Working Principle of NTP The working principle of NTP is shown in Figure 89.
In Figure 89, 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 89 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.
■ Before this NTP packet leaves LS_B, LS_B inserts its own timestamp once again,
which identifies 11:00:02am (noted as T
3
).
■ When receiving the response packet, LS_A inserts a new timestamp, which
identifies 10:00:03am (noted as T
4
), into it.
Netw ork
Netw ork
NTP Packet 10:00:00am
Netw ork
Netw ork
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NT P Pac ket
NT P Pac ket
Netw ork
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 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NT P Pac ket
NT P Pac ket
Netw ork
Netw ork
NTP Packet 10:00:00am
Netw ork
Netw ork
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NT P Pac ket
NT P Pac ket
Netw ork
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 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Pac ket
NTP Packet
Netw ork
Netw ork
NTP Packet 10:00:00am
Netw ork
Netw ork
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NT P Pac ket
NT P Pac ket
Netw ork
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 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NT P Pac ket
NT P Pac ket
Netw ork
Netw ork
NTP Packet 10:00:00am
Netw ork
Netw ork
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NT P Pac ket
NT P Pac ket
Netw ork
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 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Pac ket
NTP Packet