[ntp:questions] Remaining synced on an unsynchronised peer?

michael.buetow at comsoft.de michael.buetow at comsoft.de
Tue Dec 1 15:26:20 UTC 2009

Ok, some definite new results:

Firstly, I was wrong about this being a regression between
ntp-4.1.2-4.EL3.1 and ntp-4.2.2p1-9.el5 .
The regression is due to an additional change we made, and that is to
use the "prefer" keyword on one of the external servers.

When "prefer" is used, the peer who lost touch with the outside world
will remain synchronised on its partner which lost touch slightly
later (even if this is due to timing in poll cycles).

When "prefer" is not used and the external sources have become
completely unreachable (0), their symmetric peers will no longer
synchronise on them.

It seems to me that if "prefer" was used, even a "freewheeling" ntpd
that's lost all upstream reachability will still serve the time.
I can see that pretty well in our systems because in addition to host1
and host2, there are 8 other hosts, synchronised to host1/host2 with a
"prefer" on host1.
These "slaves" all see host1 and host2 as candidates, despite the fact
that when the external LANs are cut, one of them is definitely utterly

Ouch, I didn't know "prefer" would do this...

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