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

Brian Utterback brian.utterback at oracle.com
Wed Dec 3 13:58:23 UTC 2014

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.

Brian Utterback

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