[ntp:questions] Number of Stratum 1 & Stratum 2 Peers

David Lord snews at lordynet.org
Thu Dec 4 11:33:24 UTC 2014

Brian Utterback wrote:
> On 12/2/2014 4:00 AM, Rob wrote:
>> The whole "have 3 servers to select a majority" thing is absolutely not
>> required when your servers are accurately synchronized themselves and
>> your requirements are "only" within-a-second.  It is true that when you
>> have two servers the clients cannot know which one is right, but it is
>> trivial to keep servers within a millisecond of eachother with GPS and
>> within 10 milliseconds using only network peering.  To that is two
>> orders of magnitude better than you require.
> Be careful with this generalization. While it may be "trivial", it isn't 
> "automatic". I deal with customers all the time that have configured 
> exactly two servers on their clients and then are surprised later when 
> all of the clients become unsynchronized and start free drifting. I 
> always recommend against it. I still think that it takes four to 
> guarantee a majority but I don't have proof of that. Someday I will 
> spend some time to either prove or disprove it, but alas, time is 
> something I don't generally have extra to spend. But you are better off 
> with one than two from an operational standpoint.

The ntp html docs on selection state that four are needed to
guarantee a majority and give an example of this case.


> Brian Utterback

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