Chapter 31 Configuring Ntp; Understanding How Ntp Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Configuring NTP
This chapter describes how to configure the Network Time Protocol (NTP) on the Catalyst 6000 family
switches.
For complete syntax and usage information for the commands used in this chapter, refer to the
Note
Catalyst 6000 Family Command Reference publication.
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. This synchronization
allows events to be correlated when system logs are created and other time-specific events occur.
An NTP server must be accessible by the client switch. NTP runs over User Datagram Protocol (UDP),
which runs over IP. NTP is documented in RFC 1305. 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 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 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 31-1
NTP Default Configuration, page 31-2
Configuring NTP, page 31-2
NTP never synchronizes to a machine that is not synchronized itself.
NTP compares the time 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 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
C H A P T E R
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