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This chapter describes how the Network Time Protocol (NTP) synchronizes a host's clock with another time source and provides procedures for specifying the following:
For information about starting and stopping the NTP service, controlling access to it, and setting basic and communication options, see the chapter titled "Configuring Services with the Service Configuration Manager".
NTP synchronizes the system clock of your host with the system clock of another computer or time source. By using this protocol, you can make sure that all hosts on your network are set to the same time, and that your network is running the correct time for your area.
To use NTP, you must have a connection to the Internet. Otherwise, you can use a local system as an NTP server.
For a list of public NTP servers and information about using them, you can get the file /pub/ntp/doc/clock.txt using anonymous FTP from louie.udel.edu at the University of Delaware.
NTP works by polling another server or time source to retrieve the correct time. It works in one of two modes:
To specify the host(s) with which to synchronize in client mode:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Server label.
Step 5 Click on the Edit... button next to the Server field.
Step 6 Enter the IP address of one or more remote hosts with which you want to synchronize.
Step 7 Click on OK.
Step 8 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
To specify the host(s) with which to synchronize in symmetric mode:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Peer label.
Step 5 Click on the Edit... button next to the Peer field.
Step 6 Enter the IP address of one or more remote hosts you want to synchronize with.
Step 7 Click on OK.
Step 8 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
If the NTP service loses contact with a remote host in either client or symmetric mode, it continues to poll to reconnect with the remote host. If you do not want the system to poll the disconnected host, specify the remote host as passive.
A remote host that is specified as passive is not polled if the connection to that host is lost. If the remote host becomes available, it re-establishes communication with the NTP server.
To specify a host as passive:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Passive label.
Step 5 Click on the Edit... button next to the Passive field.
Step 6 Enter the IP address of one or more remote hosts you want to specify for passive mode.
Step 7 Click on OK.
Step 8 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
This section describes options for controlling how NTP changes your system clock.
If NTP needs to adjust your system clock, it "slews" the clock by running it 10% faster or 10% slower to keep it synchronized. If your system clock is off by more than the specified amount of time, NTP does not slew the system clock, but rather changes it directly.
To specify the maximum amount that your system clock can be off for NTP to slew the system clock:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Set-threshold label.
Step 5 Enter the maximum number of seconds NTP is allowed to slew the system clock in the Set-threshold field. The default value is .5 seconds.
Step 6 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
NTP includes a parameter that serves as a sanity check and limits how far NTP can change your system clock. To specify the maximum change NTP can make:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Waytoobig label.
Step 5 Enter the maximum number of seconds NTP can change your system clock in the Waytoobig field. The default value is 4000 seconds.
Step 6 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
You can specify the precision of your system clock by providing a number that describes the resolution of your system clock as a power of two. Choose the nearest power of two that is greater than the actual clock resolution. For example, for a clock with a resolution of 100 Hz, the precision would be 7 (27 is 128). The default value selected for your hardware should be correct. Do not change this parameter unless you are directed to do so by a Cisco customer support engineer.
To specify the precision of your system clock:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Precision label.
Step 5 Enter the precision of your system clock in the Precision field.
Step 6 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
You can configure the NTP service to send a message to the default Syslog log file (MultiNet/log/info.log in the installation directory) every time it adjusts the logical clock. This is useful if you want to gather statistical information to analyze the logical clock behavior. The logical clock is the software clock maintained by the NTP protocol.
To configure the NTP service to send messages to the system log:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the LogClock checkbox.
Step 5 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
 | Caution This file may grow very large in a short period of time. It is recommended that you log logical clock changes for debugging purposes only. |
NTP stores the drift compensation register in a file that is loaded at initialization and used to store updated drift values. To specify the path name of the drift file:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Driftfile label.
Step 5 Enter the path and file name of the drift file in the Driftfile field.
Step 6 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
By enabling the Trusting option, you can specify that NTP allow peers not specified in your NTP configuration to change your system clock. By default, trusting is disabled. To enable trusting:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the Trusting label.
Step 5 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
You can specify the debug level at which the NTP service operates. Increasing the debug level provides progressively more information in NTP messages, starting with 10 and increasing in increments of 10 up to 100. The default level is -1, which disables all message output, including operational messages.
To specify the debug level for the NTP server:
Step 1 Start the SCM.
Step 2 Choose NTP from the Available Services box.
Step 3 Click on the Config tab.
Step 4 Click on the checkbox next to the DebugLevel label.
Step 5 Enter the debug level in the DebugLevel field.
Step 6 Choose Save Configuration from the File menu.
Changes take effect after you stop and restart the NTP service.
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