I think that this is the wrong approach. Better to monitor it, prove that it happened, and remove offenders from the IXP's altogether. The IXP contracts should include just such a provision. I don't disagree. Do you have tools for reliably detecting unacceptable default route usage? Until those tools are available and until IXP's agree to police their interconnects, the MAC filtering approach is the only real solution available. Your fellow IXP members are deserving of your trust, until they show that they aren't, and the paternalistic "let's remove the temptation" approach is just offensive. This is a nice thought, entirely approprate for a more cooperative Internet of an earlier age. Unfortunately, today it would seem that "better fences make better neighbors" is a far more practical philosophy. --Vince