This is probably the most important driver for MPLS, and is central to the layer 3
VPN technology that has driven much of the deployment of MPLS up to now.
?– Packets may be assigned to LSPs that follow explicit paths rather than the path
chosen by the IP routing protocols. This enables traffic engineering (a full description
of traffic engineering is beyond the scope of this chapter) and could only be accomplished
in a connectionless network by including a ???source route??? with each packet.
MPLS was designed to carry IP packets (though the multiprotocol in its name refers
to its ability to work with other network layer protocols). MPLS also uses as much as
possible of the existing IP infrastructure in its operation. Thus, MPLS uses IP addressing
and IP routing protocols, and encapsulates its signalling traffic in IP. Indeed, MPLS
is often known as IP/MPLS. MPLS forwarding operates at Layer 2; however, MPLS is
often referred to as ???Layer 2.5??? because it has some Layer 3 characteristics, and because
MPLS 423
the MPLS header (known as the MPLS label stack) generally sits between the Layer 2
header and the IP (Layer 3) header (see Figure 14.
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