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Chapter 7 Adminis tering the Switch Managi ng the System Ti me and Dat e
NTP uses the concept of a stratum to de scri be h ow many NT P ho ps away a device is fro m an
author itative time sourc e. A stratu m 1 time server has a radio or at omic cloc k direct ly atta ched , 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
speakers.
NTP avoids synchro nizin g to a device whose tim e migh t not be accura te 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.
The co mmunic ations bet ween devices run ning NTP (k nown as associations) are usually statically
configured; each device is given the IP address of all devices with which it should form associations.
Accurat e t imekee ping is po ssible by exch angi ng NTP messa ges be tw ee n ea ch pa ir o f d evices wi th an
associa tion. However, i n a L AN environment, NTP ca n be c onfigured t o use IP b roadc as t m essage s
instead. This alternative reduces configuration complexity because each device can simply be
configured to send or rece ive broadc ast messa ges. However, in that case, in format ion 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
accidenta l o r malici ous se tting o f an in corr ec t ti me. Two mec ha nis ms are available: an ac cess list-based
restri ctio n scheme and an en crypte d aut hent icatio n mech anism .
Cisco’s implementation of NTP does not support stratum 1 service; it is not possible to connect to a radio
or atom ic clock. We recommend that the time service for your network b e derived from the public NTP
servers available on the IP Internet. Figure 7-3 show a typical network example using NTP.
If the network is isolated from the Internet, Cisco’s implem entat ion of NTP al lows a device to act as
though it is sy nc hron ized thro ugh N TP, w hen in fact it ha s det ermi ned t he t ime by u s ing othe r me an s.
Other devices the n synchroni ze to that device th rough NTP.
When multiple sources of time are available, NTP is always considered to be more authoritative. NTP
time overrides th e ti me s et by a ny othe r met hod .
Se veral man uf act urer s in clud e NTP software for t heir ho st syst ems, an d a p ublic ly availa ble version f or
systems running UNIX and its various derivatives is also available. This software allows host systems
to be time-synchronized as well.