[ntp:questions] prefer keyword and server failover

Danny Mayer mayer at ntp.isc.org
Mon Apr 28 02:31:36 UTC 2008

Andrew Stone wrote:
> Hi all,
> I would like to configure my ntp client to use 2 servers only. I want
> it to receive updates from only one of them, but then use the other when
that one fails. What is the best way to do this? Will setting the
primary server to use the 'prefer' keyword work?

You totally misunderstand the way that NTP works. Having multiple 
servers is not meant as a load-balancing or failover mechanism, it meant 
to be used to find the best and most reliable NTP server over any period 
  and to switch as necessary when the currently chosen server has become 
worse (whether accuracy, precision, delay or other reasons) than other 
servers in the list. Failover is purely automatic and you are unlikely 
to ever notice. Two servers is the worst option since it will be unable 
to decide which one is better (usually). Three is the minimum 
recommended and four is preferred. You do *not* normally need to worry 
about additional NTP packets being sent to the servers, they barely 
notice the extra load, unless it's one the heavily loaded public servers 
such as the ones run by NIST.


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