MPLS can be thought of as a shim-layer between Layer 2 and Layer 3 of the protocol stack. MPLS provides connection services to Layer 3 functions while making use of link-layer services from Layer 2. To achieve this, MPLS defines a shim header that is inserted between the link layer header and the network layer header of transmitted frames. The format of a 32-bit MPLS shim header is illustrated in the following figure.
