The SNTP protocol

SNTP provides the following operating modes:

  • Broadcast mode
    The switch acquires time updates by accepting the time value from the first SNTP time broadcast detected. (In this case, the SNTP server must be configured to broadcast time updates to the network broadcast address; see the documentation provided with your SNTP server application.) Once the switch detects a particular server, it ignores time broadcasts from other SNTP servers unless the configurable Poll Interval expires three consecutive times without an update received from the first-detected server. If the Poll Interval (configurable up to 720 seconds) expires three times without the switch detecting a time update from the original server, the switch accepts a broadcast time update from the next server it detects.Directs the switch to acquire its time synchronization from data broadcast by any SNTP server to the network broadcast address. The switch uses the first server detected and ignores any others. However, if the Poll Interval (configurable up to 720 seconds) expires three times without the switch detecting a time update from the original server, the switch accepts a broadcast time update from the next server it detects.
    NOTE:

    To use Broadcast mode, the switch and the SNTP server must be in the same subnet.

  • Unicast mode
    Directs the switch to poll a specific server periodically for SNTP time synchronization.The default value between each polling request is 720 seconds, but can be configured.At least one manually configured server IP address is required.
    NOTE:

    At least one key-id must be configured as trusted, and it must be associated with one of the SNTP servers. To edit or remove the associated key-id information or SNTP server information, SNTP authentication must be disabled.

    The switch periodically requests a time update, for the purposes of time synchronization, from the configured SNTP server. (You can configure one server using the menu interface, or up to three servers using the CLI sntp server command.) This option provides increased security over the Broadcast mode by specifying which time server to use instead of using the first one detected through a broadcast. The default value between each polling request is 720 seconds, but can be configured. At least one manually configured server IP address is required.

    When running SNTP unicast time polling as the time synchronization method, the switch requests a time update from the server you configured, with either the server address parameter in the menu interface, or the primary server in a list of up to three SNTP servers configured using the CLI. If the switch does not receive a response from the primary server after three consecutive polling intervals, the switch tries the next server (if any) in the list. If the switch tries all servers in the list without success, it sends an error message to the Event Log and reschedules to try the address list again after the configured Poll Interval time has expired.

    If there are already three SNTP server addresses configured on the switch, and you want to use the CLI to replace one of the existing addresses with a new one, you must delete the unwanted address before you configure the new one.