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Label Switching

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Label switching is technique for overcoming the inefficiency of traditional layer 3 hop-by-hop routing. Labels are assigned to packets that allows network devices to forward packets in layer 2 at high speed. The label points to an entry in a forwarding table that specifies where the packet should be forwarded. This label switching technique is much faster than the traditional routing method where each packet is examined before a forwarding decision is made. According to RFC 2475 (An Architecture for Differentiated Services, December 1998):

Examples of the label switching (or virtual circuit) model include Frame Relay, ATM, and MPLS. In this model path forwarding state and traffic management or QoS state is established for traffic streams on each hop along a network path. Traffic aggregates of varying granularity are associated with a label switched path at an ingress node, and packets/cells within each label switched path are marked with a forwarding label that is used to lookup the next-hop node, the per-hop forwarding behavior, and the replacement label at each hop. This model permits finer granularity resource allocation to traffic streams, since label values are not globally significant but are only significant on a single link; therefore resources can be reserved for the aggregate of packets/cells received on a link with a particular label, and the label switching semantics govern the next-hop selection, allowing a traffic stream to follow a specially engineered path through the network.

A related topic is "Multilayer Switching," which discusses silicon-based wire-speed routing devices that examine not only layer 3 packet information, but also layer 4 (transport) and layer 7 (application) information.

This topic continues in "The Encyclopedia of Networking and Telecommunications" with a discussion of the following:

  • Label switching techniques
  • Topology-driven label assignment
  • Signaling/request/control-driven label assignment
  • Traffic-driven label assignment
  • Traffic engineering across explicit routes
  • LSRs (label switching routers)
  • GSMP (General Switch Management Protocol)

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