The NTP is designed to time-synchronize a network of
devices. NTP runs over User Datagram Protocol (UDP), which runs over IP. NTP is
documented in RFC 1305.
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 then distributes this time across the network. NTP is extremely
efficient; no more than one packet per minute is necessary to synchronize two
devices to within a millisecond of one another.
NTP uses the concept
describe how many NTP hops away a device 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 through NTP from a stratum 1 time
server, and so on. A device running NTP automatically chooses as its time
source the device with the lowest stratum number with which it communicates
through NTP. This strategy effectively builds a self-organizing tree of NTP
synchronizing to a device whose time might not be accurate by never
synchronizing to a device that is not synchronized. NTP also compares the time
reported by several devices and does not synchronize to a device whose time is
significantly different than the others, even if its stratum is lower.
between devices running NTP (known as associations) are usually statically
configured; each device is given the IP address of all devices with which it
should form associations. Accurate timekeeping is possible by exchanging NTP
messages between each pair of devices with an association. However, in a LAN
environment, NTP can be configured to use IP broadcast messages instead. This
alternative reduces configuration complexity because each device can simply be
configured to send or receive broadcast messages. However, in that case,
information flow is one-way only.
The time kept on a
device is a critical resource; you should use the security features of NTP to
avoid the accidental or malicious setting of an incorrect time. Two mechanisms
are available: an access list-based restriction scheme and an encrypted
of NTP does not support stratum 1 service; it is not possible to connect to a
radio or atomic clock. We recommend that the time service for your network be
derived from the public NTP servers available on the IP Internet.
The Figure shows a
typical network example using NTP.
A is the NTP master, with the
B, C, and D configured in NTP server mode, in server association with
E is configured as an NTP peer to the upstream and downstream
Figure 1. Typical NTP
If the network is
isolated from the Internet, Cisco’s implementation of NTP allows a device to
act as if it is synchronized through NTP, when in fact it has learned the time
by using other means. Other devices then synchronize to that device through
When multiple sources
of time are available, NTP is always considered to be more authoritative. NTP
time overrides the time set by any other method.
include NTP software for their host systems, and a publicly available version
for systems running UNIX and its various derivatives is also available. This
software allows host systems to be time-synchronized as well.