Definition of MPLS

The MPLS labels are advertised between routers so that they can build a label-to-label mapping. These labels are attached to the IP packets, enabling the routers to forward the traffic by looking at the label and not the destination IP address. The packets are forwarded by label switching instead of by IP switching.

The label switching technique is not new. Frame Relay and ATM use it to move frames or cells throughout a network. In Frame Relay, the frame can be any length, whereas in ATM, a fixed-length cell consists of a header of 5 bytes and a payload of 48 bytes. The header of the ATM cell and the Frame Relay frame refer to the virtual circuit that the cell or frame resides on. The similarity between Frame Relay and ATM is that at each hop throughout the network, the “label” value in the header is changed. This is different from the forwarding of IP packets. When a router forwards an IP packet, it does not change a value that pertains to the destination of the packet; that is, it does not change the destination IP address of the packet. The fact that the MPLS labels are used to forward the packets and no longer the destination IP address have led to the popularity of MPLS. These benefits—such as the better integration of IP over ATM and the popular MPLS virtual private network (VPN) application—are explained in the “Benefits of MPLS” section of this chapter.