[ntp:questions] NTP client with 4 servers lost sync

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Fri Jul 3 04:13:54 UTC 2015

On 2015-07-02 13:40, Nuno Pereira wrote:
> The use of prefer was based on this idea: "I want to use only the local
> sources, unless all fail, and so I have to use an external".
>>From what I read now, I saw that prefer was a bad choice.
> I could only see the "noselect" option in order to accomplish that idea (only
> use external if all the local fail), but that also fails, as the external
> sources aren't available if the local ones fail.
> Am I right?

Yes - better just to let ntpd do its thing with all sources.
Internal sources are not available if your host or LAN router fails and
external sources are not available if your internet router fails.

>> You may want to consider adding all LAN sources to all clients, add enough
>> sources to provide an odd number, add pool servers as backup to external
>> servers,
>> and drop prefer from LAN sources to allow the combine algorithm to compute
>> stats.
> All LAN sources are just 2 (in reality just one for the moment, as they're the
> same host).
>>From my experience, the pool servers, if taken directly from pool.ntp.org, are
> very unstable and not trustable, and so I chose 2 fixed not so bad external
> servers as backup. But they failed in the leap second.

Add also explicit pool statements for pt.pool.ntp.org, maybe also es.ntp.pool.org
if none of the former are ever selected as candidates (+ tally code), possibly
also europe.pool.ntp.org, with iburst and preempt options.

> But how can you explain that a client with just one source was better than a
> client with 4 sources?

Okay as long as that single source is up and working and accurate - if it is
inaccurate, clinets would not know.

> It's not just from this leap second situation: it's from some months where we
> have some clients with just one source that are having less problems that the
> 4 sources configuration.
> 4 is not odd, I know, but in that case I have to go for a 5 sources
> configuration, as a 3 source configuration can fall into a 2 available sources
> configuration if one of them fails?
> And as we prefer to use local sources, in order to have all of our clients
> with better accuracy between them, in that case we would need to have 5
> servers of NTP! It's a little insane, in our opinion.

Three local servers, plus two good external servers, plus a few good pool servers
should allow your clients to get reliable time. Let ntpd do its own thing to get
accurate time. Collect loopstats and peerstats and compare consistency.

Take care. Thanks, Brian Inglis

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