I first reported this to inoc@gt-t.net, but they refused to look into it, as I'm not a customer of theirs. (An attitude which makes me very happy I'm not, incidentally.). So I'll dump the info here in case someone a more senior techie in GTT is reading this list and would like to take a closer look. Traffic from GTT's Stockholm PoP (in my case delivered to them there by AS3549 to 77.67.82.145 a.k.a xe-5-2-0.stk30.ip4.tinet.net) destined to/past their Frankfurt PoP gets either routed to 141.136.107.218 (ae5.fra61.ip4.tinet.net) or 89.149.185.105 (xe-5-0-1.fra61.ip4.tinet.net). It appears they're using ECMP between these two links, as I can "choose" by slightly modifying the source and/or destination address when I'm tracing (even though the addresses remain within the same DFZ prefix). When the path to 141.136.107.218 is chosen, the RTT increases by ~100ms and there is significant packet loss. See below traceroutes, which are done at the same time from the same source. The return path is known to be good. If I change my outbound path to the destinations I'm tracing to below in AS12741 to go via AS3356->AS174, the problem immediately disappears. So for now I'm strong depreffing all as-paths matching .*3257.* and I'm happy, but I'm thinking someone else might be interested in looking into this... Good path (xe-5-2-0.stk30. -> xe-5-0-1.fra61.ip4.tinet.net): ------------------------------------------------------------ $ sudo mtr -nrc 100 5.226.119.193 pompel.hmg9.vgnett.no Snt: 100 Loss% Last Avg Best Wrst StDev 87.238.51.121 0.0% 0.9 1.2 0.8 4.1 0.5 87.238.62.80 0.0% 1.3 1.4 0.8 5.4 0.8 87.238.62.84 0.0% 0.3 1.3 0.3 50.2 5.4 64.210.69.85 0.0% 0.5 6.4 0.5 200.8 26.5 67.16.136.6 0.0% 14.1 13.8 7.3 43.8 3.4 67.17.74.46 0.0% 7.6 7.6 7.5 12.9 0.6 77.67.82.145 0.0% 7.5 11.9 7.5 70.0 12.1 89.149.185.105 0.0% 30.0 30.8 30.0 71.8 4.8 77.67.73.78 0.0% 47.8 48.7 46.7 50.7 1.2 83.238.250.44 0.0% 54.8 55.6 53.9 57.2 1.0 83.238.251.217 0.0% 55.2 56.0 54.6 60.3 1.1 81.210.5.70 0.0% 54.1 54.3 53.9 59.4 0.6 5.226.119.193 0.0% 59.0 58.4 57.0 67.9 1.7 Bad path (xe-5-2-0.stk30. -> ae5.fra61.ip4.tinet.net): ------------------------------------------------------ $ sudo mtr -nrc 100 5.226.119.170 pompel.hmg9.vgnett.no Snt: 100 Loss% Last Avg Best Wrst StDev 87.238.51.121 0.0% 1.0 1.0 0.8 4.0 0.3 87.238.62.80 0.0% 1.7 1.5 0.8 7.5 1.0 87.238.62.84 0.0% 0.3 2.2 0.3 66.7 8.4 64.210.69.85 0.0% 0.5 4.6 0.5 153.4 17.9 67.16.146.93 0.0% 7.3 7.4 7.3 21.5 1.4 67.17.74.42 0.0% 16.9 15.9 7.5 17.0 1.1 77.67.82.145 0.0% 7.5 9.8 7.5 66.9 9.6 141.136.107.218 7.0% 153.6 152.7 128.2 182.4 5.3 77.67.75.38 12.0% 156.6 154.9 140.2 163.4 4.0 83.238.251.98 8.0% 157.7 157.1 143.6 176.5 5.8 83.238.250.5 6.0% 163.4 162.5 150.0 175.3 4.9 77.252.5.118 3.0% 159.0 162.2 150.6 226.0 14.5 5.226.126.223 7.0% 157.0 161.0 149.6 223.8 12.6 5.226.119.170 5.0% 153.5 161.2 147.7 226.5 11.4 Tore