Dell PowerConnect 3524P User Manual page 102

Time sources are established by Stratums. Stratums define the accuracy of the reference clock.
The higher the stratum (where zero is the highest), the more accurate the clock. The switch receives
time from stratum 1 and above. The following is an example of stratums:
• Stratum 0 — Indicates a real time clock is used as the time source, for example, a GPS system.
• Stratum 1 — Indicates that a server that is directly linked to a Stratum 0 time source is used.
Stratum 1 time servers provide primary network time standards.
• Stratum 2 — Indicates that 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.
Information received from SNTP servers is evaluated based on the Time level and server type.
SNTP time definitions are assessed and determined by the following time levels:
• T1 — The time at which the original request was sent by the client.
• T2 — The time at which the original request was received by the server.
• T3 — The time at which the server sent the client a reply.
• T4 — The time at which the client received the server's reply.
The device can poll the following server types for the server time: Unicast, Anycast and Broadcast.
Polling for Unicast information is used for polling a server for which the IP address is known.
SNTP servers that are configured on the device are the only ones that are polled for synchronization
information. T1-T4 are used to determine server time. This is the preferred method for synchronizing
device time as it is most secure. If this method is selected, SNTP information is accepted only from
SNTP servers defined on the device using the SNTP Servers page.
Polling for Anycast information is used when the server IP address is unknown. If this method is selected,
all SNTP servers on the network can send synchronization information . The device is synchronized
when it proactively requests synchronization information. The best response (lowest stratum) from the
first 3 SNTP servers to respond to a request for synchronization information is used to set the time value.
Time levels T3 and T4 are used to determine the server time.
Using Anycast polling to get time information for synchronizing device time is preferred to using
Broadcast polling to get time information. However, this method is less secure than unicast polling,
because SNTP packets are accepted from SNTP servers that are not configured on the device.
Broadcast information is used when the server IP address is unknown. When a Broadcast message is
sent from an SNTP server, the SNTP client listens to the message. If Broadcast polling is enabled,
any synchronization information is accepted, even if it has not been requested by the device. This is the
least secure method.
102
Configuring System Information