Reference Guide

80 Configuring System Information
DELL CONFIDENTIAL – PRELIMINARY 3/6/14 - FOR PROOF ONLY
If Unicast polling is not enabled or if no servers are defined on the device,
the device accepts time information from any SNTP server of the type that
is enabled, which responds.
•Anycast
Polling for Anycast information is used when the SNTP server’s IP address
is not defined or it cannot be reached. If this method is enabled, time
information can be received from any SNTP server on the network. The
device time and date are synchronized when it proactively requests
synchronization information.
Anycast polling to get time information is preferable to Broadcast polling,
because it is more secure.
Time levels T3 and T4 are used to determine from which server time
information is accepted.
•
Broadcast
Broadcast info
rmation is used if receiving Broadcast packets has been
enabled, and one of the following situations occurs:
– The SNTP server IP address has not been defined.
– Several time-information packets are received and the Broadcast time
is best according to the algorithm defined in Algorithm for Selecting
Designated SNTP Server.
Broadcast is the least secure method of receiving time, because it is both
unsecured and the time information was not specifically requested by the
device. Anycast is also unsecured, but time-information packets are only
accepted if they were requested.
Stratums
Each SNTP server is characterized by stratums, which define the accuracy of
its clock. The stratum is the distance, in terms of NTP hops, from the most
authoritative time server. The lower the stratum (where zero is the lowest),
the more accurate the clock. The switch accepts time from stratum 1 and
above.
The following provides examples of clocks from various stratums:
• Stratum 0 — A real time clock is used as the time source, for example, a
GPS system.