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MPLS in the SDN Era by Krzysztof Grzegorz Szarkowicz, Antonio Sanchez Monge

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Chapter 3. Layer 3 Unicast MPLS Services

So far, this book has illustrated the primary role of MPLS: building tunnels (LSPs) across the service provider (SP) or the data center core in order to transport packets. In previous examples, the ingress PE performs a route lookup on each IPv4 packet before placing it on an LSP. This route lookup process takes into account Layer 3 (L3) fields contained in the IPv4 header; and the user packets are unicast—destined to a single host. Putting it all together, IPv4 Internet Transit over MPLS is an L3 Unicast MPLS Service. It is, historically speaking, the first MPLS service and in terms of volume it also remains the most widely used.

IPv4 Internet over MPLS is an unlabeled service, in the sense that packets typically have no MPLS label when they arrive to the egress (service) PE. While in the LSP, packets only carry transport labels, but no service labels. What is a service label? It is simply an MPLS label that identifies a service. At first glance, it is impossible to distinguish transport labels from service labels: they look the same. The difference lies in the way Label Switch Routers (LSRs) and Label Edge Routers (LERs) interpret them, as determined by the signaling process: when an LSR/LER advertises a label, it is also mapping it to something with a precise meaning.

If an MPLS service is labeled, the ingress PE typically pushes a stack of two MPLS headers—as long as the egress PE is more than one hop away. The outer and the inner headers ...

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