ntp.plus.net resolution from Safeguard DNS different to standard DNS

[#1]

I've noticed that since about 11pm Sunday, the name "ntp.plus.net" is mostly not being resolved by the Safeguard DNS servers, but seems okay with the standard DNS servers.

The standard ones are okay:
$ dig @212.159.6.9 +short ntp.plus.net.
212.159.13.49
212.159.6.10
212.159.6.9
212.159.13.50
and those 4 IPs do respond to NTP

However with the Safeguard DNS, it often does not resolve to an IP:
$ dig @213.120.234.42 +short ntp.plus.net.
ntp.vision.bt.com.plus.net.

but occasionally it does

$ dig @213.120.234.42 +short ntp.plus.net.
ntp.vision.bt.com.
ntp.gslb.vision.bt.com.
213.123.16.85

and when it does, that IP does respond to NTP.
(It does not respond to pings, which is fair enough for an NTP server, but that is also different behaviour compared with the original 4 IPs which do respond to pings).

Anyone else seeing this? Is the Plusnet NTP service moving to BT in the long term?

[#2 · Accepted solution]

Good spot.

Looking into this now...

Edit: looks to be related to a planned change last night that didn't go quite as planned. Seems there's some residual gremlins in the works, so the team are looking into it.

[#3]

Thanks @bobpullen it appears to have gone back to normal about 2pm Monday.

$ dig @213.120.234.42 +short ntp.plus.net.
212.159.6.10
212.159.13.49
212.159.6.9
212.159.13.50

$ dig @213.120.234.38 +short ntp.plus.net.
212.159.13.50
212.159.6.9
212.159.6.10
212.159.13.49

[#4]

From about 11pm Friday it has changed again but it is consistent, the standard DNS and Safeguard DNS servers all give the same similar answers now:

$ dig @212.159.6.9 +short ntp.plus.net.
ntp.vision.bt.com.
ntp.gslb.vision.bt.com.
213.123.22.85

$ dig @213.120.234.42 +short ntp.plus.net.
ntp.vision.bt.com.
ntp.gslb.vision.bt.com.
213.123.17.85

Do you know if this a permanent change and the BT time servers will be providing the NTP service from now on, or a temporary one whilst you do maintenance for example? The original servers still provide NTP:

$ ntpdate -q 212.159.13.49 212.159.6.10 212.159.6.9 212.159.13.50
server 212.159.13.49, stratum 2, offset -0.000841, delay 0.03871
server 212.159.6.10, stratum 2, offset -0.000423, delay 0.03891
server 212.159.6.9, stratum 2, offset -0.000322, delay 0.03868
server 212.159.13.50, stratum 2, offset -0.000027, delay 0.03877
8 Oct 09:44:09 ntpdate[17736]: adjust time server 212.159.6.9 offset -0.000322 sec

Thanks

[#5]

@seebee permanent change I believe.