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Determination of the Physical Path Through the Network: The Label Switched Path (LSP)

The complete path through a label switched network is called the label switched path (LSP). It is determined in one of two ways. With the first method, traditional routing protocols (such as OSPF or BGP) are used to discover IP addresses. This information, the next node to an address, is correlated with a label, yielding the LSP. With the second method, the LSP can be set up (configured manually) according to the idea of constraint-based routing (CR). This approach may use a routing protocol to assist in setting up the LSP, but the LSP is "constrained" by other factors, such as the need to provide a certain QOS level. Indeed, delaysensitive traffic is the prime candidate for constrained routing.

The end-to-end LSP is called an LSP tunnel, which is a concatenation of each LSP segment between each node, as shown in Figure 1–8. The characteristics of the LSP tunnel, such as bandwidth allocation, are determined by negotiations between the nodes, but after the negotiation, the ingress node (the beginning of the LSP), shown as node B in Figure 1–8, defines the traffic flow by its choice of the label (L). As the traffic is sent through the tunnel, the idea is not to examine the contents of any other headers but to examine just the label header. Therefore, the remainder of the traffic is "tunneled" through the LSP without examination or alteration. At the end of the LSP tunnel, the egress node (node D in this example) removes the label and passes the IP traffic to an IP node.

Figure 1-8Figure 1–8 LSP tunnels.

As explained in Chapters 7, 8, and 9, LSP tunnels enable the implementation of traffic engineering (TE) policies related to network performance optimization. For example, LSP tunnels can be automatically or manually routed away from network failures, congestion, and bottlenecks. Also, multiple parallel LSP tunnels can be established between two nodes, and traffic between the two nodes can be mapped onto the LSP tunnels according to local policy.

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