CHAPTER
34-1
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
34
Configuring NTP
This chapter describes how to configure the Network Time Protocol (NTP) on the Catalyst 6500 series
switches.
Note For complete syntax and usage information for the commands that are used in this chapter, refer to the
Catalyst 6500 Series Switch Command Reference publication.
This chapter consists of these sections:
• Understanding How NTP Works, page 34-1
• NTP Default Configuration, page 34-2
• Configuring NTP on the Switch, page 34-2
Understanding How NTP Works
NTP synchronizes the timekeeping among a set of distributed time servers and clients. This
synchronization allows the events to be correlated when the system logs are created and the 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 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:
• 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
that has its time significantly different from the others, even if its stratum is lower.
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