Is it the line protocol that is flapping, or the interface is flapping as well? If it is just the line protocol, it could be an encapsulation thing, os something else on layer 2. If the interface is flapping as well, it most likely layer 1 i.e. physical.
Was the loopback done from both ends? You can ask the provider to loop their ends and you look at the result. Your interface should show looped. If it does not, or it is still flapping, the problem is most likely at your end, probably a bad interface.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...