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

Chris Albertson albertson.chris at gmail.com
Thu Apr 21 02:07:35 UTC 2011

It's a matter of opinion now,

I think if the servers are under your control and supervision two is
enough to provide pretty good availability and three might not have as
much bang per buck.   But if you don't control the servers then you
don't know if any of them are "correct" so I'd want enough so that
ntps can apply its' clock section algorithm  to identify a bad one.
But if they are your servers you can keep track of them and I think
you are only worried about availabilty

How many GPS receivers to buy.  I'm conservative so I'd want two and
not the same or even related brands.  But you likely want only one
antenna and a splitter.   You will not get the time from two antenna
locations to match at the nanosecond level and antenna installations
can be the most expensive part if you need to install a roof mast and
lead coax cable and have lightening protection and grounding per
electric code. It can cost more than the computer if you have to pay a
pro to do it.

On Wed, Apr 20, 2011 at 6:14 PM, Ben Rockwood <benr at cuddletech.com> 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
> ==============================================================================
> *    3 u  756 1024  377    0.494    0.596
> 0.352
> +    3 u  953 1024  377    0.135    0.144
> 0.241
> Thank you.
> benr.
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Chris Albertson
Redondo Beach, California

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