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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Thu Apr 21 05:48:02 UTC 2011

"Ben Rockwood" <benr at cuddletech.com> wrote in message 
news:4DAF84FC.7060504 at cuddletech.com...
> 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.

If you have three servers, and one fails, which of the two remaining 
servers do clients believe?  You can configure your clients to have as 
many servers as you want, but perhaps 4 is the minimum, and I have seen 
five and seven suggested.  Of course, depending on your application, it 
may be acceptable that some of the servers are not as accurate as others 
so you could have just one or two local servers, and three Internet 
servers configured on the clients.

> 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?:
> Thank you.
> benr.

In that situation, I would agree with Chris Albertson and get two GPS 
receivers - they are now quite cheap (e.g. about US $35 including the 
antenna) so you could perhaps have two in different locations depending on 
your network and buildings.  If the servers are syncing from the Internet, 
I might have them with a configuration including, say, four pool servers, 
plus one "good" local server marked as "prefer", but a different local 
server in each case.  A server offering a good public service, such as 
Manchester and Glasgow in the UK.

Just some ideas.


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