Level3 using uRPF on their access routers

Hi all,

Does anybody know if Level3 is performing Unicast Reverse Path Forwarding on
it's interfaces with it's customers?

Did anybody have problems with this fact (if so)?

We have a multihoming customer, and we found out that packets sent to Level3
sourced with IP blocks winning over other provider (the return path not
being prefered over Level3) are being filtered on Level3 network entrance
(pretty much uRPF behavior), but they told me and they swear they are not
using uRPF.

Thanks in advanced,