12.1.6 Notes on using NTP

• NTP assumes that the equipment keeps the correct time within the error. Therefore, if the time difference between related devices is unexpectedly large, the time is not synchronized.

• If the time difference between the Switch and the synchronization destination NTP servers is 128 milliseconds or more, you need to check for 900 seconds prior to synchronization.

• In the following cases, the time is synchronized even if the time difference with the host server is 1000 seconds (about 17 minutes) or more.

  • When the Switch starts up

  • NTP configuration setting

  • When an operation-command restart ntp or set clock is executed

• When the device is restarted, an error of about 1 second occurs. For this reason, approximately 10 minutes are required for the host NTP to resynchronize and provide time to the lower-level NTP clients after a restart due to a power failure or an operation-command reload.

• NTP client of the Switch does not synchronize with the host NTP servers whose dispersion exceeds 1 second. However, if prefer parameter is specified in the configuration command ntp server, the system ignores the excessive dispersion and synchronizes it. Specify prefer parameter if you want to synchronize dispersion to the host NTP servers that exceed 1 second.

• If the IP address of a switch is configured as that of its loopback interface, use the IP address of the loopback interface as the source IP address for sending NTP packets. Therefore, if you set the Switch as the synchronization source or destination, specify the IP address of the loopback interface as the IP address of the Switch. When adding, changing, or deleting a IP addressfor a loopback interface, use the operation-command restart ntp to reinitialize NTP program.

• At the moment of leap second insertion by NTP, "23:59:60" is displayed as "23:59:59". The status of item 3 in the following table.

  Table 12-6: UTC of leap second insertion and the time of the Switch

  No.	UTC	Switch
  1	12/31 23:59:58	12/31 23:59:58
  2	12/31 23:59:59	12/31 23:59:59
  3	12/31 23:59:60	12/31 23:59:59
  4	01/01 00:00:00	01/01 00:00:00
  5	01/01 00:00:01	01/01 00:00:01