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MPLS defines a fundamental separation between the grouping of packets that are to be forwarded in the same manner (the forwarding equivalence classes, or FECs), and the labels used to mark the packets. This is purely to enhance the flexibility of the approach. At any one node, all packets within the same FEC could be mapped onto the same locally significant label (given that they have the same requirements). However, there are instances where one may wish to engineer the network in such a way that several different labels are used (e.g., when wishing to explicitly differentiate between streams). The assignment of a particular packet to an FEC is done once, at the entry point to the network. MPLS-capable routers (label-switched routers, LSRs) then use only the label and CoS field in order to make packet forwarding and classification decisions. Label merging is possible where multiple incoming labels are to receive the same FEC.

MPLS packets are able to carry a number of labels, organized in a last-in first-out stack (Fig.10). This can be useful in a number of instances, such as where two levels of routing are taking place across transit routing domains. Regardless of the existence of the hierarchy, in all instances the forwarding of a packet is based on the label at the top of the stack. In order for a packet to travel through a tunnel, the node at the transmitting side of the tunnel pushes a label relating to the tunnel onto the stack, and sends the packet to the next hop in the tunnel.





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