Distribution Of Label Bindings; Summary Route Propagation - Cisco Catalyst 8540 MSR Configuration Manual

Chapter 16 Configuring Tag Switching and MPLS

Distribution of Label Bindings

Each label switch router (LSR) in the network makes an independent, local decision as to which label
value to use to represent an FEC. This association is known as label binding. Each LSR informs its
neighbors of the label bindings it has made. This awareness of label bindings by neighboring routers and
switches facilitates the following protocols:
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MPLS LDP provides a standard methodology for hop-by-hop, or dynamic label, distribution in an MPLS
network by assigning labels to routes that have been chosen by the underlying Interior Gateway Protocol
(IGP) routing protocols. The resulting labeled paths, called label switch paths or LSPs, forward label
traffic across an MPLS backbone to particular destinations. These capabilities enable service providers
to implement Cisco MPLS-based IP VPNs and IP+ATM services across multi-vendor MPLS networks.
LDP allows label switch routers (LSRs) to request, distribute, and release label prefix binding
information to peer routers in a network. LDP enables LSRs to discover potential peers and to establish
LDP sessions with those peers to exchange label binding information.
An LDP label binding is an association between a destination prefix and a label. The label used in a label
binding is allocated from a set of possible labels called a label space.
LDP supports two types of label spaces:
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Summary Route Propagation

Figure 16-5
discover mechanism is used to periodically transmit LDP hello messages and to signal its desire to
advertise label bindings. The LSR sends the LDP hello messages as UDP packets to the well known LDP
port (646). The hello messages carry the LDP identifier (ID) of the label space for sending LSR.
SalesLSR4 sends a hello packet with the VPI and VCI used to connect to FEC 172.68.0.0. Each LSR
then propagates that FEC replacing the VPI and VCI used to connect to its ingress interface.When a
labeled packet is being sent from an LSR to its neighbor LSR, the label value carried by the packet is the
label value that the egress LSR assigned to represent the FEC of the packet. This causes the label value
(VPI/VCI) to be swapped as the packet traverses the network.
OL-7396-01
Tag Distribution Protocol (TDP)—Used to support MPLS forwarding along normally routed paths
Resource Reservation Protocol (RSVP)—Used to support MPLS traffic engineering
Border Gateway Protocol (BGP)—Used to support MPLS virtual private networks (VPNs)
Interface-specific—An interface-specific label space uses interface resources for labels. For
example, LC-ATM interfaces use VPIs/VCIs for labels. Depending on its configuration, an LDP
platform may support zero, one, or more interface-specific label spaces.
Platform-wide—An LDP platform supports a single platform-wide label space for use by interfaces
that can share the same labels. For Cisco platforms, all interface types except LC-ATM use the
platform-wide label space.
shows the summary route propagation between four LSRs in an MPLS network. The LDP
ATM Switch Router Software Configuration Guide
MPLS Overview
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