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MPLS Concept


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The MPLS working group is addressing the issues of the scalability of routing, the provision of more flexible routing services, increased performance, and more simplified integration of layer 3 routing and circuit-switching technologies, with the overall goal of providing a standard label-swapping architecture.

MPLS introduces a new forwarding concept for IP networks. The idea is similar to that in asynchronous transfer mode (ATM) and frame relay networks. A path is first established using a signaling protocol; then a label in the packet header, rather than the IP destination address, is used for making forwarding decisions in the network. In this way, MPLS introduces the notion of connection-oriented forwarding in an IP network. MPLS thus offers a new solution for directing the traffic along the computed paths-a significant requirement for traffic engineering, establishing a path and sending traffic along that path. This provides the network engineer with a level of functionality equivalent to what virtual circuits provide in ATM networks. In the absence of MPLS, providing even the simplest traffic engineering functions (e.g., explicit routing) in an IP network is very cumbersome.

The following is a very brief introduction to MPLS. Two signaling protocols may be used for path setup in MPLS:

         the Label Distribution Protocol (LDP) and

         extensions to RSVP.

The path set up by the signaling protocol is called a label switched path (LSP). Routers that support MPLS are called label switched routers (LSRs). An LSP typically originates at an edge LSR, traverses one or more core LSRs and then terminates at another edge LSR. The ingress edge LSR maps the incoming traffic onto LSPs using the notion of a forwarding equivalence class (FEC). An FEC is described by a set of attributes such as the destination IP address prefix. All packets that match a given FEC will be sent on the LSP corresponding to that FEC. This is done by prepending the appropriate label to the IP packet. The core LSRs forward labeled packets using only information contained in the label; the rest of the IP header is not consulted. When an LSR receives a packet it looks up the entry in its label information base (LIB), and determines the output interface and new outgoing label for the packet. Finally, the egress edge LSR will remove the label from the packet and forward it as a regular IP packet. Naturally, this description omits many of the subtle details, but they are beyond the scope of this section. The MPLS signaling protocols used for traffic engineering are described in the sequel.

Figure 9 - A simplified LSR forwarding engine





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