Level3 based network issues are often very difficult to diagnose.
This is due to Level3 operating their BGP network via MPLS (Multi-Protocol Label Switching), a wonderful and useful technology (but difficult to diagnose when your network spans the entire world!)
Take this snippet of a traceroute for example:
2 ge-6-18.car1.Manchester1.Level3.net (195.50.119.73) 0.868 ms 0.981 ms 0.770 ms 3 ae-4-90.edge1.Washington4.Level3.net (4.69.149.207) 80.715 ms 80.546 ms 80.671 ms
For a regular network, this would indicate a direct link between Manchester UK and Washington US. However, this link does not exist.
The Manchester and Washington routers are talking directly between each other, but the underlying path is hidden from view (the actual path travels from Manchester to London before travelling to either New York, Newark or New Jersey in the US and onwards).
Think of MPLS as a network of tunnels, a tunnel exists between Manchester and Washington. The path taken between the two may automatically re-route behind the scenes. It’s entirely possible that it could route via Japan and back (although highly unlikely due to policies) and still only appear as a single hop.
The underlying network will often be OSPF based (an interior gateway routing protocol), calculating the “cost” between two destinations based on a variety of factors.
This makes it very resilient, but very difficult to diagnose. You now have to diagnose two separate network paths, nested together.