Chapter 39 Configuring Ntp; Understanding How Ntp Works - Cisco WS-C2950SX-48-SI Configuration Manual

Configuring NTP
This chapter describes how to configure the Network Time Protocol (NTP) on the Catalyst enterprise
LAN switches.
Note For complete syntax and usage information for the commands used in this chapter, refer to the
Catalyst 4500 Series, Catalyst 2948G, and Catalyst 2980G Switches Command Reference.
This chapter consists of these sections:
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Understanding How NTP Works

NTP synchronizes timekeeping among a set of distributed time servers and clients. With this
synchronization, you can correlate events to the time that system logs were created and the time that
other time-specific events occur. An NTP server must be accessible by the client switch.
NTP uses the User Datagram Protocol (UDP) as its transport protocol. All NTP communication uses
Coordinated Universal Time (UTC), which is the same as Greenwich Mean Time. An NTP network
usually gets its time from an authoritative time source, such as a radio clock or an atomic clock that is
attached to a time server. NTP distributes this time across the network. NTP is extremely efficient; no
more than one packet per minute is necessary to synchronize two machines to within a millisecond of
one another.
NTP uses a stratum to describe how many NTP hops away a machine is from an authoritative time
source. A stratum 1 time server has a radio or atomic clock that is directly attached, a stratum 2 time
server receives its time from a stratum 1 time server, and so on. A machine running NTP automatically
chooses as its time source the machine with the lowest stratum number that it is configured to
communicate with through NTP. This strategy effectively builds a self-organizing tree of NTP speakers.
NTP has two ways to avoid synchronizing to a machine whose time might be ambiguous:
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Understanding How NTP Works, page 39-1
Default NTP Configuration, page 39-2
Configuring NTP on the Switch, page 39-2
NTP never synchronizes to a machine that is not synchronized itself.
NTP compares the time that is reported by several machines and does not synchronize to a machine
whose time is significantly different from the others, even if its stratum is lower.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
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