3Com Switch 8800 Configuration Guide
Chapter 49 NTP Configuration
49-2
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Before synchronizing the system clocks on Ethernet Switch A and B, the clock on
Ethernet Switch A is set to 10:00:00am, and that on B is set to 11:00:00am.
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Ethernet Switch B serves as an NTP time server. That is, Ethernet Switch A
synchronizes the local clock with the clock of B.
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It takes 1 second to transmit a data packet from either A or B to the opposite end.
The system clocks are synchronized as follows:
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Ethernet Switch A sends an NTP packet to Ethernet Switch B. The packet carries
the timestamp 10:00:00am (T
1
) that tells when it left Ethernet Switch A.
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When the NTP packet arrives at Ethernet Switch B, Ethernet Switch B adds a local
timestamp 11:00:01am (T
2
) to it.
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When the NTP packet leaves Ethernet Switch B, Ethernet Switch B adds another
local timestamp 11:00:02am (T
3
) to it.
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When Ethernet Switch A receives the acknowledgement packet, it adds a new
timestamp 10:00:03am (T
4
) to it.
Now Ethernet Switch A collects enough information to calculate the following two
important parameters:
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The delay for a round trip of an NTP packet traveling between the Switch A and B:
Delay= (T
4
-T
1
) - (T
3
-T
2
).
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Offset of Ethernet Switch A clock relative to Ethernet Switch B clock: offset=
( (T
2
-T
1
) + (T
4
-T
3
) ) /2.
In this way, Ethernet Switch A uses the above information to set the local clock and
synchronize it with the clock on Ethernet Switch B.
The operating principle of NTP is briefly introduced above. For details, refer to
RFC1305.
49.2 NTP Configuration
NTP is used for time synchronization throughout a network. The following sections
describe the NTP configuration tasks.
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Configuring NTP Operating Mode
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Configuring NTP ID Authentication
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Setting NTP Authentication Key
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Setting Specified Key as Reliable
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Designating an Interface to Transmit NTP Messages
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Setting NTP Master Clock
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Setting Authority to Access a Local Ethernet Switch
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Setting Maximum Local Sessions
49.2.1 Configuring NTP Operating Mode
You can set the NTP operating mode of an Ethernet Switch according to its location in
the network and the network structure. For example, you can set a remote server as the
