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

Nuno Pereira nuno.pereira at g9telecom.pt
Wed Jul 1 16:18:32 UTC 2015



Following last night's leap second, we had some issues with our NTP servers,
especially in a clients with 4 servers configured, but not in clients with 1
source configured.


We have 2 types of configuration (beside the one in the NTP server):


Config 1 (clients with access to the external network):

*	2 NTP servers in the LAN, configured with "iburst prefer";
*	2 external NTP servers, configured with "iburst".


Config 2 (clients without access to the external network):

*	1 NTP server in the LAN, configured with "iburst prefer" or "iburst"
(in this case to "prefer" or not is the same").



The 2 external servers configured had problems with the leap second, having
one second offset after it happen, while the LAN servers got no issues (they
had a leap file, and reported leap_armed within the 24 hours before the

This lead to something like this being reported by "ntpq -p" (don't have


     remote           refid      st t when poll reach   delay   offset  jitter


xlan_server_1      2 u 1013 1024  377    1.019   -0.483   0.687

xlan_server_2      2 u  922 1024  377    1.042   -0.499   0.665

xext_server_1    2 u  384 1024  377    3.360    1002.688

xext_server_2    2 u  388 1024  377    3.360    1001.582


I mean, all 4 were considered false tickers.



In the meanwhile, in the clients where I had no access to the external
network, having only 1 server to sync to (lan_server_1), things worked with no



>From what I've read in this list and in the docs, the best configuration is to
have 4 servers, and that's what's brought by default in the CentOS and Debian
servers, but this issue brought again the even number of servers issue that
can arise with just 2.


How can 4 be worst than 1?

Do I have to go to a 5 servers configuration, in order to avoid this? Or go
for 4 servers in the LAN?

I'm having difficulties to convince my colleagues that we must configure 4
servers (they think that exaggerated), with them thinking that the best is to
have just one, and now I got this issue.



Thank you.


Nuno Pereira



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