Dell PowerConnect 5548 User Guide - Page 172

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 information 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" on page 173. 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. • Stratum 1 - A server that is directly linked to a Stratum 0 time source is used. • Stratum 2 - The time source is distanced from the Stratum 1 server over a network path, for example, a Stratum 2 server receives the time over a network link, via NTP, from a Stratum 1 server. 172 Dell PowerConnect 55xx Systems User Guide