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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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► The packet does not contain a label. In this scenario, the LSP egress receives<br />

a st<strong>and</strong>ard network layer packet. The node uses the local <strong>IP</strong> routing table to<br />

make a forwarding decision.<br />

A.2.2 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. The LSP for<br />

the traffic flow shown in Figure A-2 on page 933 is .<br />

In an MPLS network, LSPs can be established in one of two ways:<br />

► Independent LSP control: Each LSR makes an independent decision to bind<br />

a label to an FEC. It then distributes the label to its peer nodes. This is similar<br />

to conventional <strong>IP</strong> routing; each node makes an independent decision as to<br />

how to forward a packet.<br />

► Ordered LSP control: An LSR binds a label to a particular FEC only if it is the<br />

egress LSR for that FEC, or if it has already received a label binding for that<br />

FEC from its next hop for that FEC. In an environment implementing traffic<br />

engineering policies, ordered LSP control is used to ensure that traffic in a<br />

particular FEC follows a specific path.<br />

Section A.2.5, “Label distribution protocols” on page 938 describes the<br />

procedures used to exchange label information in an MPLS environment.<br />

A.2.3 Label stack <strong>and</strong> label hierarchies<br />

A label stack (FILO) is used in tunneling 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 />

The set of traffic sent through the tunnel constitutes an FEC. Each LSR in the<br />

tunnel must assign a label to that FEC.<br />

To send a packet through the tunnel, the tunnel ingress node pushes a label<br />

understood by the tunnel egress node onto the label stack. The tunnel ingress<br />

node then pushes a label understood by the next hop node <strong>and</strong> forwards the<br />

data packet through the tunnel.<br />

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

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