Implementation Principle Of Ntp - 3Com Switch 4500 26-Port Configuration Manual

294 C 18: NTP C
HAPTER ONFIGURATION
Implementation
Principle of NTP
The local clock of an Switch 4500 cannot operate as a reference clock. It can
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serve as a NTP server only after synchronized.
Figure 81 shows the implementation principle of NTP.
Ethernet switch A (LS_A) is connected to Ethernet switch B (LS_B) through
Ethernet ports. Both have their own system clocks, and they need to synchronize
the clocks of each other through NTP. To help you to understand the
implementation principle, we suppose that:
Before the system clocks of LS_A and LS_B are synchronized, the clock of LS_A
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is set to 10:00:00 am, and the clock of LS_B is set to 11:00:00 am.
LS_B serves as the NTP server, that is, the clock of LS_A will be synchronized to
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that of LS_B.
It takes one second to transfer an NTP packet from LS_A to LS_B or from LS_A
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to LS_B.
Figure 81 Implementation principle of NTP
NTP packet
NTP Packet NTP Packet NTP Packet NTP Packet NTP Packet NTP Packet NTP Packet
1. 1. 1. 1. 1. 1. 1. 1.
LS_A LS_A LS_A LS_A LS_A LS_A LS_A LS_A
2. 2. 2. 2. 2. 2. 2. 2.
LS_A LS_A LS_A LS_A LS_A LS_A LS_A LS_A
3. 3. 3. 3. 3. 3. 3. 3.
LS_A LS_A LS_A LS_A LS_A LS_A LS_A LS_A
NTP Packet received at 10:00:03 NTP Packet received at 10:00:03 NTP Packet received at 10:00:03 am NTP Packet received at 10:00:03 am NTP Packet received at 10:00:03 NTP Packet received at 10:00:03 am NTP packet received at 10:00:03 am NTP Packet received at 10:00:03
4. 4. 4. 4. 4. 4. 4. 4.
LS_A LS_A LS_A LS_A LS_A LS_A LS_A LS_A
The procedure of synchronizing the system clock is as follows:
LS_A sends an NTP packet to LS_B, with a timestamp 10:00:00 am (T
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identifying when it is sent.
When the packet arrives at LS_B, LS_B inserts its own timestamp 11:00:01 am
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(T ) into the packet.
2
When the NTP packet leaves LS_B, LS_B inserts its own timestamp 11:00:02 am
■
(T ) into the packet.
3
10:00:00 am
10:00:00 am 10:00:00 am 10:00:00 am
10:00:00am 10:00:00am 10:00:00am
Network Network Network Network Network Network Network Network
NTP packet 10:00:00 am
NTP Packet 10:00:00 am NTP Packet 1 0:00:00am NTP Packet 1 0:00:00 am NTP Packet 1 0:00:00am NTP Packet 1 0:00:00am NTP Packet 1 0:00:00am NTP Packet 1 0:00:00 am
Network Network Network Network Network Network Network Network
NTP packet 10:00:00 am11:00:01 am11:00:02 am
NTP Packet NTP Packet NTP Packet NTP Packet NTP Packet NTP Packet NTP Packet 10:00:00 am11:00:01 am11:00:02 am 10:00:00 am 11:00:01 am 11:00:02 am 10:00:00am 11:00:01am 11:00:02am 10:00:00am 11:00:01am 11:00:02am 10:00:00 am 11:00:01 am 11:00:02 am 10:00:00am 11:00:01am 11:00:02am 10:00:00am 11:00:01am 11:00:02am
Network Network Network Network Network Network Network Network
Network Network Network Network Network Network Network Network
LS_B LS_B LS_B LS_B LS_B LS_B
LS_B LS_B
11:00:01 am
11:00:01 am 11:00:01am 11:00:01 am 11:00:01am 11:00:01 am 11:00:01am 11:00:01am
LS_B LS_B LS_B LS_B LS_B LS_B LS_B LS_B
LS_B LS_B LS_B LS_B LS_B LS_B LS_B LS_B
LS_B LS_B LS_B LS_B LS_B LS_B LS_B LS_B
)
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