[ntp:questions] Using two NTP Server: Bad?

Richard B. Gilbert rgilbert88 at comcast.net
Thu Apr 21 13:32:21 UTC 2011


On 4/20/2011 9:14 PM, Ben Rockwood wrote:
> I've read in the past that clients should always have an odd-number of
> NTP servers; 1 server or 3 servers but not 2.  If I recall the reason
> was that clients could become "confused" and needs a tie breaker.
>
> Question 1: I want to challenge this old assumption.  Is there truth to
> it?
>
> In many environments there is a desire to have 2 NTP servers for
> redundancy purposes, but not 3 due to limited resources.
>
> Question 2: Furthermore, if you have 2 local NTP servers is it
> preferable to have them sync off of different sources to avoid a client
> syncing servers that are using the same reference clock?  ie: Is this bad?:
>
> $ ntpq -pn
>       remote           refid      st t when poll reach   delay   offset
> jitter
> ==============================================================================
> *10.0.91.10      207.171.7.152    3 u  756 1024  377    0.494    0.596
> 0.352
> +10.0.91.82      207.171.7.152    3 u  953 1024  377    0.135    0.144
> 0.241
>
>
>
> Thank you.
>
> benr.

One server: if it fails you have nothing!
Two servers: If the two differ, which one do you believe?
Three servers: degenerates too easily to the two server case.
Four servers: Allows the failure of one server.
Five servers: Allows the failure of two.
Seven servers: Allows the failure of three.

Where "failure" is defined as either no response, or responding with an 
incorrect time.





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