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Introduction to MPLS LDP
MPLS LDP provides the means for 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 for the purpose of exchanging label binding information.
MPLS LDP enables one LSR to inform another LSR of the label bindings it has made. Once a pair of
routers communicate the LDP parameters, they establish a label-switched path (LSP). MPLS LDP enables
LSRs to distribute labels along normally routed paths to support MPLS forwarding. This method of label
distribution is also called hop-by-hop forwarding. With IP forwarding, when a packet arrives at a router the
router looks at the destination address in the IP header, performs a route lookup, and forwards the packet to
the next hop. With MPLS forwarding, when a packet arrives at a router the router looks at the incoming
label, looks up the label in a table, and then forwards the packet to the next hop. MPLS LDP is useful for
applications that require hop-by-hop forwarding, such as MPLS VPNs.
MPLS LDP Functional Overview
Cisco MPLS LDP provides the building blocks for MPLS-enabled applications, such as MPS Virtual
Private Networks (VPNs).
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 (LSPs), forward label traffic across
an MPLS backbone to particular destinations. These capabilities enable service providers to implement
MPLS-based IP VPNs and IP+ATM services across multivendor MPLS networks.
LDP and TDP Support
LDP supercedes Tag Distribution Protocol (TDP). See the table below for information about LDP and TDP
support in Cisco IOS releases.
Use caution when upgrading the image on a router that uses TDP. Ensure that the TDP sessions are
established when the new image is loaded. You can accomplish this by issuing the global configuration
command mpls label protocol tdp. Issue this command and save it to the startup configuration before
loading the new image. Alternatively, you can enter the command and save the running configuration
immediately after loading the new image.
Table 1
LDP and TDP Support
Train and Release LDP/TDP Support
12.0S Train
• TDP is enabled by default.
• Cisco IOS Release 12.0(29)S and earlier releases: TDP is
supported for LDP features.
• Cisco IOS Release 12.0(30)S and later releases: TDP is
not support for LDP features.
Introduction to MPLS LDP
Information About MPLS LDP
MPLS LDP Configuration Guide, Cisco IOS Release 12.4
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