[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
> 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
> *10.0.91.10 22.214.171.124 3 u 756 1024 377 0.494 0.596
> +10.0.91.82 126.96.36.199 3 u 953 1024 377 0.135 0.144
> Thank you.
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
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