The reason that the MPLS network is “multiprotocol” is due to the fact that it is compatible with the Asynchronous Transport Mode (ATM), Internet Protocol (IP), and frame relay network protocols.

With regards to the Open Systems Interconnection (OSI), MPLS allows most packets to be forwarded at the layer 2 (switching) level instead of at the layer 3 (routing) level, although MPLS rests outside the seven-layer box of OSI as forwarding mechanisms are separated from their underlying data-link services.

More on MPLS Vpns.