Spot on. It gets messier when a failure in a T1 carrier between your CDN and a customer is causing issues. Neither of you (nor possibly even the CDN) has the power to fix it. Well, other than bypass the CDN and hope the traffic passes through different peers.
On the up side, MTR is indeed a useful tool for finding the issue.
On the up side, MTR is indeed a useful tool for finding the issue.