The Network Time Protocol (NTP) synchronizes the time 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.
An NTP server usually receives its time from a 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, as shown in Figure 6-6:
A stratum 1 time server is directly attached to an authoritative time source (such as an atomic clock).
A stratum 2 NTP server receives its time through NTP from a stratum 1 NTP 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.
Figure 6-6 NTP Stratum
Because Cisco NX-OS cannot connect to a radio or atomic clock and act as a stratum 1 server, Cisco recommends 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.
The time kept on a device is a critical resource; it is strongly recommended that you use NTP security features to avoid any accidental or malicious setting of incorrect time. Two mechanisms are available: an access list-based restriction scheme and an encrypted authentication mechanism.
