David Lord <sn...@lordynet.org> wrote: > 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.
In practice this problem does not occur when you use only your own servers that you monitor and trust, and it confuses people that want to setup NTP on their company network. They get sent away with "you need to configure and maintain 4 servers or better even more". When the servers either are synced correctly or are down, this is not required. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
