Alcatel-Lucent 6850-48 ネットワークガイド

Configuring MPLS

MPLS Overview

OmniSwitch AOS Release 6 Network Configuration Guide

page 10-13

packet to its final destination using information in the forwarding table. Each LSP can have only one 
egress router. The ingress and egress routers in an LSP cannot be the same router.

A router in the network can act as an ingress, egress, or transit router for one or more LSPs, depending on 
the network design.

There are two types of Label Switched Paths (LSPs):

• Static LSPs. A static LSP specifies a static path. All routers that the LSP traverses must be configured 

manually with labels. No signaling is required.

• Signaled LSP. LSPs are set up using a signaling protocol, such as the Label Distribution Protocol 

(LDP). The signaling protocol allows labels to be assigned from an ingress router to the egress router. 
Signaling is triggered by the ingress routers. Configuration is required only on the ingress router and is 
not required on intermediate routers. Signaling also facilitates path selection.

A signaled LSP is confined to one IGP area for LSPs using constrained-path. They cannot cross an auton-
omous system (AS) boundary.

Static LSPs can cross AS boundaries. The intermediate hops are manually configured so the LSP has no 
dependence on the IGP topology or a local forwarding table.

The Label Distribution Protocol (LDP) is a protocol used to distribute labels in non-traffic-engineered 
applications. LDP allows routers to establish Label Switched Paths (LSPs) through a network by mapping 
network-layer routing information directly to data link layer-switched paths.

An LSP is defined by the set of labels from the ingress Label Switching Router (LSR) to the egress LSR. 
LDP associates a Forwarding Equivalence Class (FEC) with each LSP it creates. A FEC is a collection of 
common actions associated with a class of packets. When an LSR assigns a label to a FEC, it must let 
other LSRs in the path know about the label. LDP helps to establish the LSP by providing a set of proce-
dures that LSRs can use to distribute labels.

The FEC associated with an LSP specifies which packets are mapped to that LSP. LSPs are extended 
through a network as each LSR splices incoming labels for a FEC to the outgoing label assigned to the 
next hop for the given FEC.

LDP allows an LSR to request a label from a downstream LSR so it can bind the label to a specific FEC. 
The downstream LSR responds to the request from the upstream LSR by sending the requested label.

LSRs can distribute a FEC label binding in response to an explicit request from another LSR. This is 
known as Downstream On Demand (DOD) label distribution. LSRs can also distribute label bindings to 
LSRs that have not explicitly requested them. This is called Downstream Unsolicited (DUS).

LDP performs label distribution only in MPLS environments. The LDP operation begins with a Hello 
discovery process to find LDP peers in the network. LDP peers are two LSRs that use LDP to exchange 
label-FEC mapping information. An LDP session is created between LDP peers. A single LDP session 
allows each peer to learn the other's label mappings (LDP is bi-directional) and to exchange label binding 
information.