Explanation sought for weird broadband problem - resolved, I hope.

[#1]

Somebody I went to see today had a weird broadband problem I have never met before.  Our local exchange has just been upgraded to 21CN and ever since this man was put on ADSL2+ he has had problems with his internet connection.  I found some sites would load and run normally, such as http://www.bbc.co.uk/ whilst others like http://www.google.co.uk would not run at all.   I could run a speed test and achieve a respectable result at http://www.thinkbroadband.com/speedtest.html but http://www.broadbandspeedchecker.co.uk/ would not load at all.  It looked as if I could not ping the sites I could not load, but I only checked this quickly.  And the sites that would or would not work did not remain constant, particularly, I think, after restarting the router.
The problem was the same on his computer and my laptop.  I reset his router (Home Hub v 3) and tried it in the test socket to no avail.  The router stats were (superficially) very good indeed.  I tried an alternate DNS server and that did not seem to help.  The router could be faulty but this was his second one, supplied because the problems arose with the original Home Hub.  And at that point I ran out of time and ideas.
The only thing I know of that might cause this type of problem is a MTU set badly off - but I reset the Hub (end of a paper-clip down the hole) which should have restored the correct value.  So the only thing left that might be faulty would be the new ADSL2+ line card in the telephone exchange - which fits with the timing of when problems arose.  But what sort of a fault could produce this sort of phenomenology?  

   

[#2]

That is an unusual issue, the only thing I could recommend IMHO is to try another router that's isn't a home hub.

[#3]

Did you use nslookup to confirm whether the names resolved?

[#4]

No, Jelv, I ran out of time and I use commands like nslookup so infrequently that I tend to forget about them unless I have some reference material in front of me.
On the subject of trying another router, I have noticed that my router (i.e.using my Plusnet credentials) will connect to my Plusnet account on any broadband line in the area (all on the same telephone exchange).  Does that open up possibilities for additional testing?   

[#5]

I'd be very surprised if you can connect on any line, but you will be able to connect on any other Plusnet line. So if your friend is on BT your login should not work on his line.

[#6]

Quote from: jelv

I'd be very surprised if you can connect on any line

Last week I went to see someone who uses TalkTalk - which is supposed to be LUU in my area.  Plugged in my own router and it had established a connection to Plusnet before I had a chance to change the settings.  Also done something similar on Orange.  So although the stats aren't enough to truly say it is any line, it certainly is not confined to other Plusnet lines.

[#7]

May be a case that some lines don't require log in credentials. So even though you had Plusnet credentials when you tried the TalkTalk line were you issue with a Plusnet IP address?

[#8]

Quote from: itsme

were you issued with a Plusnet IP address?

Good point; I did not check that. 

[#9]

Its just a thought, but after you reset router, could the pcs have been using cached dns entrys, and the dns cache needs clearing... Its possible the homehubs routing tables had become corrupt. You could try browsing to plusnets or googles actual ip to rule out a dns issue. Other than that, i would agree with adam to try another router. It is an odd one though

[#10]

Sounds to me like the sites that performed well were in ISP cache but that you have a DNS problem, unplug whole thing for an hour and then try a different router.
Do a factory reset on the current router and put in credentials only.
Check whether your login is on plusdsl or plusdsl2  on your login - when I had ADSL2 issues we found that Plusdsl2 had issues at Plusnet it was not the total problem but it did affect it.
For NSLOOKUP you can type NSLOOKUP /? or just NSLOOKUP and then type ? then Enter key for even more detail.
DNS is in the PC too so try changing the lan card settings.
One sure fire way to stop bad resolving of internal networks is to change the subnet to a totally different one and change PC's to automatically get IP and DNS, if these are static you may have issues.
If running Win7 then the definition of the network as Home, Work or Public can affect it
TraceRT will help too because it can show any weird paths,  also some trojans disable Google or put in an alternative IP in your Hosts file, hopefully you are not running windows as an Admin, otherwise you are wide open.

[#11]

Thanks for the various suggestions but let me remind you that the problem occurred on two computers simultaneously; one of which I brought with me.
Is there a way to view a computer's DNS cache so you can note down some cached entries to see if they are wrong?   

[#12]

From a Command Prompt window type ipconfig /displaydns

[#13]

A BT engineer came and reportedly fixed the problem.  He reportedly said that the output power on the 'line card' (my terminology) in the telephone exchange was set too low.  I can't really see how this relates to the observed phenomenology.