L2TPv3 has been designed to transport layer-2 frames of various types over a layer-3 backbone. Some aspects of the configuration are similar to that of EoMPLS – just without the need to deploy MPLS!
Packets are tunneled across the layer-3 network completely transparently, such that the two devices appear to be connected together. In the case of ethernet tunneling, there is no learning of MAC addresses – what goes in one side, comes out the other. That includes CDP, Spanning Tree and so on.