3Com Switch 8800 Configuration Guide Chapter 49 NTP Configuration49-2z Before synchronizing the system clocks on Ethernet Switch A and B, the clock onEthernet Switch A is set to 10:00:00am, and that on B is set to 11:00:00am.z Ethernet Switch B serves as an NTP time server. That is, Ethernet Switch Asynchronizes the local clock with the clock of B.z 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:z Ethernet Switch A sends an NTP packet to Ethernet Switch B. The packet carriesthe timestamp 10:00:00am (T1) that tells when it left Ethernet Switch A.z When the NTP packet arrives at Ethernet Switch B, Ethernet Switch B adds a localtimestamp 11:00:01am (T 2) to it.z When the NTP packet leaves Ethernet Switch B, Ethernet Switch B adds anotherlocal timestamp 11:00:02am (T 3) to it.z When Ethernet Switch A receives the acknowledgement packet, it adds a newtimestamp 10:00:03am (T 4) to it.Now Ethernet Switch A collects enough information to calculate the following twoimportant parameters:z The delay for a round trip of an NTP packet traveling between the Switch A and B:Delay= (T 4-T1) - (T3-T2).z Offset of Ethernet Switch A clock relative to Ethernet Switch B clock: offset=( (T2-T1) + (T4-T3) ) /2.In this way, Ethernet Switch A uses the above information to set the local clock andsynchronize it with the clock on Ethernet Switch B.The operating principle of NTP is briefly introduced above. For details, refer toRFC1305.49.2 NTP ConfigurationNTP is used for time synchronization throughout a network. The following sectionsdescribe the NTP configuration tasks.z Configuring NTP Operating Modez Configuring NTP ID Authenticationz Setting NTP Authentication Keyz Setting Specified Key as Reliablez Designating an Interface to Transmit NTP Messagesz Setting NTP Master Clockz Setting Authority to Access a Local Ethernet Switchz Setting Maximum Local Sessions49.2.1 Configuring NTP Operating ModeYou can set the NTP operating mode of an Ethernet Switch according to its location inthe network and the network structure. For example, you can set a remote server as the
