rmation. A simplified view of an MPLS network is shown in Figure 13.4 and compared with an overlay approach. An MPLS network consists of switches; however, unlike the switches of MPLS label switch router (LSR) Fig. 13.4 MPLS architecture is shown in (a) compared to the overlay approach shown in (b) as presented in Figure 13.2. Chapter 13 Multiprotocol Label Switching an overlay network, they can be integrated with the IP layer and hence are termed Label Switch Routers (LSRs). Note that the common diagrammatic form of an LSR combines the symbols for a switch and router as shown in Figrue 13.4(a). The figure demonstrates that by making the LSRs aware of the IP layer, the quantity of peering at the IP layer is greatly reduced. This is an immediate benefit for IP routing protocols that have improved perfor- mance with a lower number of peers. However, a more important aspect is that MPLS clearly separates the control plane from the forwarding (switching) function. The fact that IP routers closely couple the control of IP paths and the forwarding function is one of the problems with IP, as will be shown later with VPNs and traffic engineering. Now, with MPLS, it is possible to make use of the IP control plane where it is appropriate but to allow the forward- ing function to be controlled separately where needed. Labels in MPLS The main feature that allows MPLS to function is that the traffic to be transported (normally but not always IP) is encapsulated in a labeled packet. An MPLS label is some form of fixed entity that is associated with each packet and defines how the packet should be switched within an LSR. Furthermore, the labels only have local significance between two neighbor- ing LSRs and are swapped as the packet is forwarded through the LSR. It would be nice to be able to draw a definitive label as used by MPLS. However, if we consider MPLS to be truly "multiprotocol," we see that the label can be a number of different formats depending on the layer-2 technology used for connectivity. Layer-2 technologies, 