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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Label switched path (LSP)<br />

An LSP represents a set of MPLS nodes traversed by packets belonging to a<br />

specific FEC. The set is an ordered, unidirectional list. Traffic flows from the node<br />

at the head-end of the list toward the node at the tail-end of the list.<br />

Label stack <strong>and</strong> label hierarchies<br />

A labeled packet can contain more than one label. The labels are maintained in a<br />

last-in, first-out stack. The stack implements an ordered hierarchy among the set<br />

of labels.<br />

This hierarchy is used when an MPLS node delivers a packet to a partner MPLS<br />

node, but the nodes are not consecutive routers on the hop-by-hop path for the<br />

packet. In this situation, a tunnel is created between the two MPLS nodes. The<br />

tunnel is implemented as an LSP <strong>and</strong> label switching is used to forward traffic<br />

through the tunnel.<br />

A.2 MPLS network processing<br />

A.2.1 Label swapping<br />

The primary goal of MPLS is the integration of label swapping paradigms with<br />

traditional network layer routing. This integration bring efficiencies in data<br />

forwarding as well as positioning the network for advanced QoS functions.<br />

Label swapping is the process used by an MPLS node to forward a data packet<br />

to the next hop device. This process is used regardless of whether the packet<br />

arrives labeled or unlabeled. The process is similar to the method used in ATM<br />

<strong>and</strong> frame relay networks to forward traffic through a virtual circuit.<br />

Forwarding a labeled packet<br />

An MPLS node examines the label at the top of the stack of an incoming packet.<br />

It uses the ILM to map the label to an NHLFE. The NHLFE indicates where to<br />

forward the packet <strong>and</strong> the operation to perform on the label stack. Using this<br />

information, the node encodes a new label stack <strong>and</strong> forwards the resulting<br />

packet.<br />

Forwarding an unlabeled packet<br />

An MPLS node examines the network layer header <strong>and</strong> any other pertinent<br />

information required to determine an FEC. The node uses the FTN to map the<br />

FEC to an NHLFE. Processing is now identical to a labeled packet. The NHLFE<br />

indicates where to forward the packet <strong>and</strong> the operation to perform on the label<br />

932 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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