I have a couple of Ubuntu servers that time and time again only receive peers with a stratum of 16. If I restart the ntp daemon a couple of times, eventually I will get one or two usable peers (with stratum 1 or 2).
I fail to find out why so often I get unusable peers. The servers are basic installs of Ubuntu 14.04 with a default ntp.conf and static IP (so no ntp config from DHCP).
What can be the cause of this problem and how can I solve this issue?