[This message was written with AI assistance because I am not an expert in this area, however a substantial amount of manual analysis lies behind it.]

We're seeing persistent high latency and apparent trombone routing on traffic between an end-user site in Cambridge, UK and a destination hosted on TDC's network (AS3292) in Denmark. We'd appreciate any insight from operators familiar with this topology, particularly regarding GTT (AS3257) peering with TDC.

Context / timeline:
The end-user's building-managed ISP changed their upstream international connectivity provider. Prior to this change, routing to the affected destination appeared to transit Colt directly into TDC in Europe with no issues. Since the change, we have observed the latency problem described below. We have not been able to confirm with certainty which provider the ISP switched to, but the timing strongly correlates with the onset of the issue.

Traceroute from Cogent's own network (London → Denmark) shows traffic routed Liverpool → Montreal (Cogent backbone) → GTT in New York → back across the Atlantic to TDC:

1  *
2  be2422.ccr82.lon05.atlas.cogentco.com (154.54.57.61)  2.062 ms
3  port-channel3464.ccr91.lhr01.atlas.cogentco.com (154.54.75.150)  1.017 ms
4  be2133.ccr22.lpl01.atlas.cogentco.com (154.54.63.237)  6.998 ms
5  be3043.ccr22.ymq01.atlas.cogentco.com (154.54.44.166)  76.101 ms
6  be2089.rcr21.ymq02.atlas.cogentco.com (154.54.45.114)  76.498 ms
7  gtt.ymq02.atlas.cogentco.com (154.54.10.178)  76.219 ms
8  ae4.cr14-nyc3.ip4.gtt.net (141.136.105.9)  74.967 ms
9  ip4.gtt.net (208.116.142.170)  75.552 ms
10 lo0-0.lynqe10.dk.ip.tdc.net (93.178.170.33)  175.604 ms

Traceroute from NTT's looking glass (London router) shows a similar pattern, this time via Ashburn/Washington DC before handing to GTT:

1  ae-13.r27.asbnva02.us.bb.gin.ntt.net (129.250.2.111)  78.084 ms
2  ae-17.a05.asbnva02.us.bb.gin.ntt.net (129.250.5.7)  77.432 ms
3  ae-0.gtt.asbnva02.us.bb.gin.ntt.net (129.250.8.22)  78.836 ms
4  ae3.cr3-was1.ip4.gtt.net (213.200.112.218)  76.768 ms
5  ip4.gtt.net (208.97.204.10)  77.534 ms
6  lo0-0.lynqe10.dk.ip.tdc.net (93.178.170.33)  181.747 ms

By contrast, a traceroute via Colt (London) reaches the same destination in three hops entirely within Europe, at ~22ms:

1  212.36.131.20 [MPLS: Label 102112 Exp 0]  2 msec
2  ae23-0.ldn4nqp1.uk.ip.tdc.net (195.66.224.64)  2 msec
3  lo0-0.lynqe10.dk.ip.tdc.net (93.178.170.33)  22 msec

Observation:
Two independent networks (Cogent and NTT) both hand off to GTT (AS3257) to reach this TDC (AS3292) prefix, and in both cases GTT appears to carry the traffic via North America before returning it to Denmark — adding roughly 150ms round-trip versus the direct European path Colt provides. This suggests GTT's interconnection with TDC for this prefix may currently be limited to (or preferred via) North American exchange points, despite TDC clearly supporting direct European peering (as demonstrated by Colt).

Would anyone with visibility into GTT's or TDC's peering topology be able to confirm whether a European interconnect exists or is planned for this route? Any pointers to the right contact for a peering/routing query at either network would also be appreciated.