Ntp Associations; Ntp Broadcast Associations - Cisco Nexus 7000 Series Configuration Manual

NTP Associations

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
• 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
available on the Internet. If the network is isolated from the Internet, Cisco NX-OS allows you to configure
the time as though it were synchronized through NTP, even though it was not.
Note You can create NTP peer relationships to designate the time-serving hosts that you want your network
device to consider synchronizing with and to keep accurate time if a server failure occurs.
The time kept on a device is a critical resource, so we strongly recommend that you use the security features
of NTP to avoid the accidental or malicious setting of incorrect time. Two mechanisms are available: an access
list-based restriction scheme and an encrypted authentication mechanism.
NTP Associations
An NTP association can be one of the following:
• A peer association—The device can either synchronize to another device or allow another device to
• A server association—The device synchronizes to a server.
You need to configure only one end of an association. The other device can automatically establish the
association.

NTP Broadcast Associations

In a broadcast-based NTP association, an NTP server sends NTP broadcast packets throughout a network.
Broadcast clients listen for the NTP broadcast packets sent by the server and do not engage in any polling.
NTP broadcast servers allow you to synchronize a large number of clients without creating a lot of NTP traffic
because unsolicited messages are sent to a designated IPv4 local broadcast address, and ordinarily no request
is expected from the clients.
Cisco Nexus 7000 Series NX-OS System Management Configuration Guide
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clock or a GPS time source).
synchronize to it.
Configuring NTP