Enabling Slew Mode for NTP

Products

• VMware vCenter Server

Issue/Introduction

This article provides the steps to enable slew mode for NTP. Slew mode may be required if the vCenter Server is affected by the leap second change.

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to correspond to the mean solar time.

Systems configured as Network Time Protocol (NTP) clients will receive a NTP packet with a leap second flag on the day of the adjustment.

Resolution

Switch the ntpd daemon to slew mode 24 hours prior to the leap second, continue in slew mode for 48 hours afterwards and then revert to normal operation.

To switch the ntp daemon to slew mode:

1. Open a console or SSH session to the virtual appliance.
2. Open the /etc/sysconfig/ntp configuration file using a text editor.
3. Find the NTPD_OPTIONS setting below:
   
   NTPD_OPTIONS="-g -u ntp:ntp"
    
4. Change the configuration parameter to add the -x switch. If the setting is not present, add the line in the end of the file:
   
   NTPD_OPTIONS="-x -g -u ntp:ntp"
    
5. Save and close the file using esc -> :wq!
6. Restart the ntpd service by running this command:
   
   service ntpd restart

When the leap second window is over, the NTP mode can be safely changed back again.

To switch the ntp daemon back to regular operation after the leap second:

1. Open a console or SSH session to the virtual appliance.
2. Open the /etc/sysconfig/ntp configuration file using a text editor.
3. Find the NTPD_OPTIONS setting. It looks similar to:
   
   NTPD_OPTIONS="-x -g -u ntp:ntp"
    
4. Change the configuration parameter to remove the -x switch or remove the line completely.
   
   NTPD_OPTIONS="-g -u ntp:ntp"
    
5. Save and close the file.
6. Restart the ntpd service by running this command:
   
   service ntpd restart

Additional Information