MPLS and the OSI Reference Model
The OSI reference model consists of seven layers. Refer to Figure 2-4 for the OSI reference model. Figure 2-4 OSI Reference Model
Application Presentation Session Transport Network Data Link Physical
The bottom layer is Layer 1, or the physical layer, and the top layer is Layer 7, or the application layer. Whereas the physical layer concerns the cabling, mechanical, and electrical characteristics, Layer 2, the data link layer, is concerned with the formatting of the frames. Examples of the data link layer are Ethernet, PPP, HDLC, and Frame Relay. The significance of the data link layer is only on one link between two machines, but not beyond. This means that the data link layer header is always replaced by the machine at the other end of the link. Layer 3, the network layer, is concerned with the formatting of packets end to end. It has significance beyond the data link. The most well-known example of a protocol operating at Layer 3 is IP.
Where does MPLS fit in? MPLS is not a Layer 2 protocol because the Layer 2 encapsulation is still present with labeled packets. MPLS also is not really a Layer 3 protocol because the Layer 3 protocol is still present, too. Therefore, MPLS does not fit in the OSI layering too well. Perhaps the easiest thing to do is to view MPLS as the 2.5 layer and be done with it.
Continue reading here: Label Switch Router
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