[ntp:questions] Regarding NTP configuration ntp.conf

[Richard B. Gilbert]

Kevin Oberman wrote:
>> Date: Thu, 17 Dec 2009 14:24:56 +0800
>> From: <arpit.gupta at Emerson.com>
>> Sender: questions-bounces+oberman=es.net at lists.ntp.org
>>
>> Hi,
>>   I wish to know that If I am providing 2 server names in ntp.conf
>> without "prefer" option then with which server my system will sync the
>> time. 
>> I googled it I came to know that the server which has less "jitter" it
>> will sync with. Other thing is both the servers are at same strand
>> value. 
>> Can anyone please explain it. I will be thankful to you .
> 
> I think you probably meant stratum instead of "strand". In any case, the
> algorithm is a bit more complex than "jitter", but it is the dominant
> contributor. I'm not entirely sure what you are asking, though. It sound
> like you  found the answer.
> 
> I will say that having two servers is probably the worst case. You
> really want three, five, or seven. Those allow for "good" servers to
> out-vote a bad server. If you only have two, there is no indication as
> to what the "best" time is. The jitter may be low, but the time may be
> off by a lot. If you have three servers, if one goes bad, ntp will ignore
> it and always pick from the servers that have about the sime time.

Three is not a good number of servers.  If any one of the three fails 
either by not responding or by serving incorrect time you are reduced to 
two servers; back to the worst case.  The magic numbers are four, five, 
or seven, protecting you from the failure of one, two, or three, 
respectively, of the configured servers.