Discussion

You can also have all the routers synchronize time (chime) with each other. This is
only practical for relatively small networks of, say, less than 50 routers. (For larger
networks, you may want to have all the routers in a region or POP chime with each
other.) You still want to have some stratum 1 clocks someplace in the mix. This
arrangement allows the entire network to keep reasonable time even if some or all of
the stratum 1 clocks go down for a while.
Note that you can configure both server and peer statements on the same router.
NTP automatic time synchronization works only if the times on the two systems are
very close. Very close means between 128 milliseconds and 128 seconds apart. Time
differences less than 128 milliseconds are dealt with by slowly slewing the time
(speeding up or slowing down the clock), which means that the time is always monotonically
increasing. Between 128 milliseconds and 128 seconds, the time is stepped,
which means that it may go backward. Above this, the time is not changed at all. If
the time is more than 1,000 seconds off, NTP records a system log message:
Mar 16 16:41:41 5htp-fxp0 xntpd[28243]: time error 4217 over 1000 seconds; set
clock manually
If the time is this far off, you need to reset the clock manually:
aviva@router1# set date ntp
This command uses the NTP servers that you have configured. You do not have to
reboot the router for the new time to take effect.
On an operational note, one system will not synchronize to another that is not itself
synchronized. Because the synchronization process is recursive, there must be an
authoritative time source as the stratum 1 clock or all systems will be free running.
An NTP purist will ensure that there are at least three or, better yet, four, independent
sources of time available (traceable to three different stratum 1 servers) because
this allows “false tickers” to be detected and discarded. At least two independent
sources are needed for robustness, because if that one stratum 1 server goes down,
the entire synchronization tree will basically come apart and all systems will free run
in the mean time. Normally this isn’t too serious, but if the stratum 1 server never
comes back, there will be no time synchronization whatsoever.
6.5 Authenticating NTP
Problem
You want to make sure that the router gets time updates only from known and
trusted NTP servers.
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.
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