The Network Time
Protocol (NTP) synchronizes timekeeping among a set of distributed time servers
and clients. This synchronization allows you to correlate events when you
receive system logs and other time-specific events from multiple network
NTP uses the User
Datagram Protocol (UDP) as its transport protocol. All NTP communication uses
the Universal Time Coordinated (UTC) standard. 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. 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 each other.
NTP uses a stratum to
describe how many NTP hops away that a network device is from an authoritative
time source. A stratum 1 time server has an authoritative time source (such as
an atomic clock) directly attached to the server. A stratum 2 NTP server
receives its time through NTP from a stratum 1 NTP server, which in turn
connects to the authoritative time source.
synchronizing to a network device that may keep accurate time. NTP never
synchronizes to a system that is not synchronized itself. NTP compares the time
reported by several network devices and does not synchronize to a network
device that has a time that is significantly different than the others, even if
its stratum is lower.
Cisco NX-OS cannot act
as a stratum 1 server. You cannot connect to a radio or atomic clock. We
recommend that the time service that you use for your network is derived from
the public NTP servers available on the Internet.
If the network is
isolated from the Internet, Cisco NX-OS allows you to configure a network
device so that the device acts as though it is synchronized through NTP, when
it has determined the time by using other means. Other network devices can then
synchronize to that network device through NTP.
NTP supports IPv4 addresses.
NTP allows you to
create a peer relationship between two networking devices. A peer can provide
time on its own or connect to an NTP server. If both the local device and the
remote peer point to different NTP servers, your NTP service is more reliable.
The local device maintains the right time even if its NTP server fails by using
the time from the peer.
The following figure
shows a network with two NTP stratum 2 servers and two switches.
Figure 1. NTP Peer and
In this configuration,
switch 1 and switch 2 are NTP peers. switch 1 uses stratum-2 server 1, while
switch 2 uses stratum-2 server 2. If stratum-2 server-1 fails, switch 1
maintains the correct time through its peer association with switch 2.