[ntp:questions] client configuration: it's sufficient 3 servers ?

Rob nomail at example.com
Fri May 24 09:27:51 UTC 2013

Riccardo Castellani <ric.castellani at alice.it> wrote:
> If I have 3 internal NTP servers, what do you think if my clients have all 
> these 3 servers in their configuration ?
> According to '5.3.2. Why should I have 
> more than one clock?' in 'http://www.ntp.org/ntpfaq/NTP-s-algo-real.htm#Q-NTP-ALGO
> I read 2 servers, it's worst case, but also 3...

It depends on what you expect and how actively you are monitoring
your network.

When you want your clients to always have correct time even if a
server fails, you need more than one server.  There are people who
claim that 2 servers is not good because they may have different
time and you will not be able to tell which one is right, but in
a correctly configured ntp server it will normally not happen that
it serves wrong time without noticing it.
(EXCEPT when you put that LOCAL clock ( in the server
config, that is one of the reasons why you should always remove that!)

So, two servers is perfectly OK when you want to increase reliability
on a local network and you have monitoring in place so that when
one server dies you will start to repair it before the other server
dies too.

Also, in many environments it does not really matter when the clients
are not synchronized for a couple of hours.  So when you have only
one server (or only one server configured in the clients), it may
still be OK when you have monitoring and someone repairs the server
or puts a temporary other server on that address within reasonable

However, when you have the 3 internal servers it is good to include
the server lines in all the clients configs.  That is the most
reliable situation overall.  I only want to indicate that it is
not mandatory in cases where you don't need 100% reliability.

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