3Com 10014298 Switch User Manual


 
NTP 279
Ethernet Switch B serves as an NTP time server and Ethernet Switch A
synchronizes the local clock with the clock of B.
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:
Ethernet Switch A sends an NTP packet to Ethernet Switch B. The packet
carries the timestamp 10:00:00am (T1) that tells when it left Ethernet Switch A.
When the NTP packet arrives at Ethernet Switch B, Ethernet Switch B adds a
local timestamp 11:00:01am (T2) to it.
When the NTP packet leaves Ethernet Switch B, Ethernet Switch B adds
another local timestamp 11:00:02am (T3) to it.
When Ethernet Switch A receives the acknowledgement packet, it adds a new
timestamp 10:00:03am (T4) to it.
Next, Ethernet Switch A collects enough information to calculate the following
two important parameters:
The delay for a round trip of an NTP packet traveling between the Switch A and
B: Delay= (T4-T1) - (T3-T2).
Offset of Ethernet Switch A clock relative to Ethernet Switch B clock: offset= (
(T2-T1) + (T3-T4) ) /2.
Ethernet Switch A uses this information to set the local clock and to synchronize it
with the clock on Ethernet Switch B.
Configuring NTP is described in the following sections:
Configuring NTP
NTP Configuration Examples
Configuring NTP NTP configuration includes the tasks described in the following sections:
Configuring NTP Operating Mode
Configuring NTP ID Authentication
Setting the NTP Authentication Key
Setting the Specified Key to Be Reliable
Designating an Interface to Transmit the NTP Message
Setting the NTP Master Clock
Enabling or Disabling an Interface to Receive an NTP Message
Setting the Authority to Access a Local Switch
Setting Maximum Local Sessions
Displaying and Debugging NTP
Configuring NTP Operating Mode
The Switch 7750 can serve as an NTP client but not as an NTP server.