[ntp:questions] different Ntp servers...

Richard B. Gilbert rgilbert88 at comcast.net
Tue May 26 15:14:52 UTC 2009

Terje Mathisen wrote:
> Andy wrote:
>> About 10 years ago we used to have 2 seperate GPS receivers feeding
>> into two Sparc servers via their serial port. I can't remember the
>> version of NTP we ran however these were our stratum one servers, One
>> of the Sparcs developed a fault on it's serial interface which delayed
>> the input meaning the two stratum one servers started showing
>> different time so all the statum 2 servers stopped trusting both
>> stratum 1 servers as they could not determine which server, if any,
>> was telling the  truth. Ever since then we have always used at least 3
>> stratum one servers with seperate stratum zero sources to form a
>> quorum so that startum 2 can differentiate the good from the bad. I
>> believe this is set down as best practise in the ntp.org FAQ - this is
>> a practical example of why this is wise.
> The "best practice" is in fact to use at least _four_ sources, that way 
> you can lose one and still have a voting group.
> Terje

The magic numbers are 4, 5 and 7 protecting from the failure of one, 
two, or three servers respectively.  If you can't do it with seven 
servers you have more serious problems!

More information about the questions mailing list