When I had AT&T VDSL they had weak peering. Some traffic that should have gone across town would routinely get routed several states away before coming back to town, because they hadn't peered with the local exchanges. Some traffic was ok.
When I switched to an ISP with local peering, ping times between my home and my server downtown dropped to a few milliseconds. It's like being on my LAN.
If this is still an issue for AT&T, traceroute+reverse DNS of each router may indicate it.
This poor peering isn't directly a throughput problem, but really increases the chance of being routed through a congested link, which you could sometimes guestimate from ping times and peeringdb.
When I switched to an ISP with local peering, ping times between my home and my server downtown dropped to a few milliseconds. It's like being on my LAN.
If this is still an issue for AT&T, traceroute+reverse DNS of each router may indicate it.
This poor peering isn't directly a throughput problem, but really increases the chance of being routed through a congested link, which you could sometimes guestimate from ping times and peeringdb.