One of the reasons why I said " The following isnt strictly true but it hopefully gives an idea of what Im getting at" it was a kind of extreme example
No-one can ever say what routing you are going to take over the BT cloud. - But it certainly is possible to come off at different PoPs depending upon where the onward transit is.
For example a couple of tracerts Ive just done now
1) coming off at the Ilford PoPC:\Documents and Settings\kitz>tracert bbc.co.uk
Tracing route to bbc.co.uk [212.58.224.131] over a maximum of 30 hops:
1 1 ms 1 ms <1 ms 192.168.7.100
2 9 ms 9 ms 8 ms 217.47.204.58
3 8 ms 8 ms 8 ms 217.47.251.161
4 10 ms 8 ms 8 ms 217.41.173.13
5 14 ms 8 ms 8 ms 217.41.173.150
6 8 ms 8 ms 8 ms 217.41.173.46
7 8 ms 8 ms 8 ms 217.47.251.114
8 8 ms 8 ms * londonc-access2-s9.ukcore.bt.net [194.72.2.249]
9 14 ms 14 ms 13 ms core1-pos0-1-5-0.ilford.ukcore.bt.net [62.6.201.117]
10 13 ms 13 ms 13 ms core1-pos9-0-0.telehouse.ukcore.bt.net [62.6.201.118]
11 15 ms 13 ms 15 ms 194.74.65.6
12 13 ms 13 ms 13 ms 212.58.238.129
13 13 ms 13 ms 13 ms rdirwww-vip.thdo.bbc.co.uk [212.58.224.131] example 2 - coming off at ealing PoPC:\Documents and Settings\kitz>tracert www.kitz.co.uk
Tracing route to kitz.co.uk [195.62.28.175] over a maximum of 30 hops:
1 1 ms 1 ms 1 ms 192.168.7.100
2 9 ms 9 ms 8 ms 217.47.204.58
3 8 ms 8 ms 8 ms 217.47.251.161
4 9 ms 8 ms 8 ms 217.41.173.13
5 8 ms 9 ms 13 ms 217.41.173.150
6 8 ms 8 ms 8 ms 212.140.206.46
7 9 ms 9 ms 7 ms 217.47.158.83
8 17 ms 8 ms 8 ms core1-pos3-1.manchester.ukcore.bt.net [194.72.2.41]
9 14 ms 14 ms 14 ms core1-pos0-7-0-12.ealing.ukcore.bt.net [62.6.200.109]
10 14 ms 13 ms 14 ms 62.6.200.110
11 13 ms 12 ms 13 ms t2c1-ge14-0-0.uk-eal.eu.bt.net [166.49.168.25]
12 13 ms 13 ms 13 ms t2c2-p1-0.uk-lon1.eu.bt.net [166.49.208.210]
13 14 ms 13 ms 13 ms t2a1-ge7-0-0.uk-lon1.eu.bt.net [166.49.135.110]
14 14 ms 14 ms 19 ms ge-5-0-4.lon11.ip.tiscali.net [213.200.78.5]
15 14 ms 14 ms 14 ms uk-webhosting-gw.ip.tiscali.net [213.200.78.114]
16 15 ms 15 ms 15 ms thermalseven.footholds.net [195.62.28.175] [/i]
Example 3 - coming off at Birmingham PoPC:\Documents and Settings\kitz>tracert www.metoffice.gov.uk
Tracing route to www.metoffice.gov.uk [151.170.240.7] over a maximum of 30 hops:
1 <1 ms 1 ms <1 ms 192.168.7.100
2 15 ms 13 ms 15 ms 217.47.204.58
3 13 ms 13 ms 14 ms 217.47.251.161
4 11 ms 10 ms 10 ms 217.41.173.13
5 15 ms 11 ms 10 ms 217.41.173.150
6 10 ms 11 ms 11 ms 217.41.173.54
7 14 ms 13 ms 11 ms 217.41.165.242
8 10 ms 10 ms 10 ms core2-pos12-3.manchester.ukcore.bt.net [194.72.2.157]
9 13 ms 13 ms 13 ms core2-pos5-1.birmingham.ukcore.bt.net [194.74.16.186]
10 10 ms 10 ms 12 ms interconnect1-gig2-0.birmingham.fixed.bt.net [62.6.196.10]
11 24 ms 22 ms 22 ms ftip002779089-ms-fo.interconnect1.birmingham.fixed-nte.bt.net [194.73.17.146]
12 23 ms 24 ms 26 ms www.meto.gov.uk [151.170.240.7]
Trace complete. Bletchly PoP
C:\Documents and Settings\kitz>tracert bt.com
Tracing route to bt.com [217.32.165.148] over a maximum of 30 hops:
1 1 ms 1 ms 1 ms 192.168.7.100
2 10 ms 8 ms 8 ms 217.47.204.58
3 8 ms 9 ms 8 ms 217.47.204.161
4 8 ms 9 ms 10 ms 217.41.173.9
5 10 ms 8 ms 9 ms 217.41.173.66
6 11 ms 9 ms 8 ms 217.41.173.150
7 9 ms 8 ms 8 ms 212.140.206.38
8 8 ms 8 ms 8 ms 217.47.67.83
9 8 ms 8 ms 8 ms core1-pos7-1.manchester.ukcore.bt.net [194.72.2.62]
10 11 ms 11 ms 11 ms core1-pos13-1.bletchley.ukcore.bt.net [194.74.16.142]
11 11 ms 11 ms 11 ms vhsaccess1-pos7-0.bletchley.fixed.bt.net [62.6.197.134]
12 12 ms 11 ms 11 ms ftip002587721-p.vhsaccess1.bletchley.fixed-nte.bt.net [217.32.244.70]
Glasgow Pop
C:\Documents and Settings\kitz>tracert bt.net
Tracing route to bt.net [193.113.211.125] over a maximum of 30 hops:
1 1 ms 1 ms 1 ms 192.168.7.100
2 13 ms 8 ms 8 ms 217.47.204.58
3 8 ms 8 ms 9 ms 217.47.251.161
4 9 ms 10 ms 10 ms 217.41.173.13
5 9 ms 10 ms 8 ms 217.41.173.150
6 9 ms 9 ms 8 ms 217.41.173.62
7 8 ms 8 ms 8 ms 217.47.109.114
8 9 ms 9 ms 9 ms londonc-access3-s28.ukcore.bt.net [194.72.2.189]
9 13 ms 13 ms 13 ms core2-pos9-0.glasgow.ukcore.bt.net [194.74.16.178]
10 15 ms 14 ms 14 ms vhsaccess1-gig2-0.glasgow.fixed.bt.net [217.32.170.158]
11 13 ms 12 ms 13 ms ftip002676159.vhsaccess1.glasgow.fixed-nte.bt.net [213.121.156.38]
However, I must admit that Ealing, Ilford, and Kingston do seem to be the most common that Ive seen.
AND like you I too dont seem to be able to "come off" at Manchester - why I dont know - What should route me direct to Manchester seems to go down to Ealing and back up again.
BUT we do know that at least on the "normal" BT system, it does allow for Manchester RAS to Manchester PoPs. ie theres the Zen customers on the Manchester RAS going direct to a Manchester server without having to go via london.
I seem to recall quite a few instances in the past where Ive seen Scotland users go through the Edinburgh RAS and then it also lists the Manchester/Birmingham PoP as a hop.
As regards to someone on RIN on the same RAS - well thats just a weird one - and as you know that doesnt even go anywhere near Colossus (core) - but then again I suppose BT dont expect anyone to be able to do too much damage with a direct connection when the upstream is going to be limited anyhow to 448/832 kbps
C:\Documents and Settings\kitz>tracert [to mr_chris - IP address removed}
Tracing route to host86-156-xxx-xxx.range86-156.btcentralplus.com [86.156.xxx.xxx] over a maximum of 30 hops:
1 1 ms <1 ms <1 ms 192.168.7.100
2 9 ms 8 ms 9 ms 217.47.204.58
3 8 ms 8 ms 8 ms 217.47.204.161
4 8 ms 10 ms 9 ms 217.41.173.9
5 49 ms 15 ms 9 ms 217.41.173.66
6 11 ms 11 ms 10 ms 217.41.173.150
7 9 ms 9 ms 11 ms 172.16.243.37
8 8 ms 8 ms * 212.140.206.14
9 9 ms 8 ms 8 ms 217.47.158.23
10 21 ms 17 ms 17 ms host86-156-xxx-xx.range86-156.btcentralplus.com [86.156.xxx.xxx]
Trace complete.At the end of the day - this is BT and what they do and how (and why) is still sometimes a bit of a mystery