[juancb]

To clarify my previous post, asymmetric routing is strictly an L3 behavior, and ECMP routing can also be an L3 behavior where a router chooses one of many equal-cost next hops based purely on data in the IP headers. The exact behavior of course depends on the ECMP load-balancing algorithm in use, whether it's per packet, per destination, or using a hash. And furthermore whether it's strictly IP or if it looks deeper into the packet and uses L3+L4 headers in its decision making.

Both asymmetric routing and ECMP routing are visible from L3. In the latter case, the routing decision can utilize some L4 data, so some L4 frobbing to get useful data points in practice is necessary for useful real-world diagnosis.

I agree with others that the OSI model is a good metaphor and a framework for reasoning about networking, but it is far from perfect, and the reality for those designing and operating network protocols and devices is messy.

MPLS is admittedly invisible and there isn't a thing you can do about it in the same way that you can't expect traceroute to give you a view of the switch ports it went through on a LAN. Of course it is useful to understand and keep in mind the fact that there may be, sometimes huge, gaps in your traceroutes. A sudden huge jump in RTT from one hop to the next can be confusing when trying to understand and troubleshoot a network issue.