Reference Guide

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System Time and Date
System time and date settings and the network time protocol (NTP) are supported on the S4820T platform.
You can set system times and dates and maintained through the NTP. They are also set through the Dell Networking
operating system (FTOS) command line interfaces (CLIs) and hardware settings.
Network Time Protocol
The network time protocol (NTP) synchronizes timekeeping among a set of distributed time servers and clients.
The protocol also coordinates time distribution in a large, diverse network with various interfaces. In NTP, servers
maintain the time and NTP clients synchronize with a time-serving host. NTP clients choose from among several NTP
servers to determine which offers the best available source of time and the most reliable transmission of information.
NTP is a fault-tolerant protocol that automatically selects the best of several available time sources to synchronize to.
You can combine multiple candidates to minimize the accumulated error. Temporarily or permanently insane time
sources are detected and avoided.
Dell Networking recommends configuring NTP for the most accurate time. In FTOS, you can configure other time
sources (the hardware clock and the software clock).
NTP is designed to produce three products: clock offset, roundtrip delay, and dispersion, all of which are relative to a
selected reference clock.
• Clock offset — represents the amount to adjust the local clock to bring it into correspondence with the
reference clock.
• Roundtrip delay — provides the capability to launch a message to arrive at the reference clock at a specified
time.
• Dispersion — represents the maximum error of the local clock relative to the reference clock.
Because most host time servers synchronize via another peer time server, there are two components in each of these
three products, those determined by the peer relative to the primary reference source of standard time and those
measured by the host relative to the peer.
In order to facilitate error control and management of the subnet itself, each of these components is maintained
separately in the protocol. They provide not only precision measurements of offset and delay, but also definitive
maximum error bounds, so that the user interface can determine not only the time, but the quality of the time as well.
In what may be the most common client/server model, a client sends an NTP message to one or more servers and
processes the replies as received. The server interchanges addresses and ports, overwrites certain fields in the
message, recalculates the checksum and returns the message immediately. Information included in the NTP message
allows the client to determine the server time regarding local time and adjust the local clock accordingly. In addition, the
message includes information to calculate the expected timekeeping accuracy and reliability, as well as select the best
from possibly several servers.
Following conventions established by the telephone industry [BEL86], the accuracy of each server is defined by a
number called the stratum, with the topmost level (primary servers) assigned as one and each level downwards
(secondary servers) in the hierarchy assigned as one greater than the preceding level.
FTOS synchronizes with a time-serving host to get the correct time. You can set FTOS to poll specific NTP time-serving
hosts for the current time. From those time-serving hosts, the system chooses one NTP host with which to synchronize
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