[ntp:questions] Two servers, one client : one problem

unruh unruh at wormhole.physics.ubc.ca
Wed May 4 05:13:04 UTC 2011

On 2011-05-04, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> On 5/3/2011 12:58 PM, Yann I. wrote:
>> Hi !
>> I have some problems about the configuration of ntp servers and client.
>> You'll find the configurations used for the test and some outputs (of "ntpq
>> -p" for example).
>> First of all, the both servers are standalone and are not synchronized with
>> an external ntp server. Later, they'll be connected to external ntp server.
>> Later...
>> The servers and client don't use iptable.
> NTP is hierarchical!  It expects to get time from an external source 
> such as NTP servers on the internet.  The hierarchy starts with an 
> "atomic clock"  such as those operated by NIST or equivalent.  These are 
> referred to as "Stratum 1".  Servers that get time from a stratum 1 
> server are referred to as "Stratum 2".  Servers that get time from a 
> stratum 2 server are referred to as "Stratum 3".
> It looks as if you have created, or attempted to create a "circle jerk".
> Another problem is that you have only two servers configured.  If they 
> disagree, which one will you believe?  Four servers are required for the 
> minimum "robust" configuration.

Three are required. Four is an additional "belts and braces". Three
allow two to outvote one rogue clock. Four allow you to outvote two
disparate rogue clocks but fail on two rogue clocks which agree with
each other. Ie, a minimum is three.


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