To forward customer traffic across the core network backbone, SPBM uses IEEE 802.1ah Provider Backbone Bridging (PBB) MAC-in-MAC encapsulation, which hides the customer MAC (C-MAC) addresses in a backbone MAC (B-MAC) address pair. MAC-in-MAC encapsulation defines a B-MAC source address (BMAC-SA) and a B-MAC destination address (BMAC-DA) to identify the backbone source and destination addresses.
The originating node creates a MAC header that is used for delivery from end to end. As the MAC header stays the same across the network, there is no need to swap a label or do a route lookup at each node, allowing the frame to follow the most efficient forwarding path end to end.
Encapsulating customer MAC addresses in B-MAC addresses improves network scalability (no end-user C-MAC learning is required in the core) and also significantly improves network robustness (loops in access networks do not impact forwarding results in the backbone infrastructure.)