[ntp:questions] Help appreciated

Dave Hart davehart at gmail.com
Wed May 13 04:43:10 UTC 2009

On May 12, 8:41 pm, "Towli" <to... at oldboyz.dk> wrote:
> I think i may have the answer to my own question above...
> I read somewhere that a man with 2 watches never know what time it is...
> My switches sees the 2 external NTP-stratum1-servers as being closer to each
> other in time (due to their physical placement close to each other and
> relative to my core switches).
> Therefore my core switches choose to trust the to more agreeing servers as
> opposed to my internal server, which is quite alone in its view on the time
> and whose time differs from the 2, due to the closer proximity /smaller
> delay.
> Am i right in this assumption?
> I removed 1 of the external NTP servers and now my internal DCF is used as
> prefered Ntp server.

I think you answered your own question once again ;)

> Should i keep 1 of the external stratum-1-servers for backup or should i
> remove it all together? My Dcf77 sometimes looses connection to Frankfurt
> and when this happens it looses it stratum 1 value, and hence my core
> switches looses synch...

Yes, I would keep an external backup, probably even several, but
remote from each other to avoid the problem with nonoverlapping
confidence intervals you saw.  Also test the configuration with just
one local stratum 1 server even after you get your second one
operational, to be sure that losing one of those local stratum 1
servers will not cause behavior you don't want.

I would also suggest you consider using hosts running NTP version 4,
rather than switches running an adaptation of NTP version 3.  The
switches are designed and optimized to shuffle packets, not keep
time.  Precision of 2**-6 means the switches are stepping time 64
times per second, or every 15.6 milliseconds.  You shouldn't rely on
such crude machinery as the local distribution mechanism.  ntpd easily
coexists with most other software and has very light resource
requirements, even serving large numbers of clients, so it should be
possible to find a few suitable servers.  Alternatively, you can find
NTP appliances or build one yourself with a $200 Soekris or similar
embedded platform.

Dave Hart

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