CHAPTER 4
Configuring NTP
This chapter contains the following sections:
• Information About NTP, on page 21
• NTP as a Time Server, on page 22
• Distributing NTP Using CFS, on page 22
• Clock Manager, on page 22
• Virtualization Support, on page 22
• Licensing Requirements for NTP, on page 23
• Guidelines and Limitations for NTP, on page 23
• Default Settings, on page 24
• Configuring NTP, on page 24
• Related Documents for NTP, on page 34
• Feature History for NTP, on page 34
Information About NTP
The Network Time Protocol (NTP) synchronizes the time of day among a set of distributed time servers and
clients so that you can correlate events when you receive system logs and other time-specific events from
multiple network devices. NTP uses the User Datagram Protocol (UDP) as its transport protocol. All NTP
communications use Coordinated Universal Time (UTC).
An NTP server usually receives its time from an authoritative time source, such as a radio clock or an atomic
clock attached to a time server, and then 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 each
other.
NTP uses a stratum to describe the distance between a network device and an authoritative time source:
• A stratum 1 time server is directly attached to an authoritative time source (such as a radio or atomic
clock or a GPS time source).
• A stratum 2 NTP server receives its time through NTP from a stratum 1 time server.
Before synchronizing, NTP compares the time reported by several network devices and does not synchronize
with one that is significantly different, even if it is a stratum 1. Because Cisco NX-OS cannot connect to a
radio or atomic clock and act as a stratum 1 server, we recommend that you use the public NTP servers
Cisco Nexus 3548 Switch NX-OS System Management Configuration Guide, Release 7.x
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