Cisco Cisco IOS Software Release 12.0(23)S
MPLS Label Distribution Protocol (LDP)
Feature Overview
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Cisco IOS Release 12.0(23)S
This document describes the use of the Multiprotocol Label Switching (MPLS) Label Distribution
Protocol (LDP), which enables peer label switch routers (LSRs) in an MPLS network to exchange label
binding information for supporting hop-by-hop forwarding along normally routed paths. The document
includes the following sections:
Protocol (LDP), which enables peer label switch routers (LSRs) in an MPLS network to exchange label
binding information for supporting hop-by-hop forwarding along normally routed paths. The document
includes the following sections:
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Feature Overview
The Cisco MPLS LDP, as standardized by the Internet Engineering Task Force (IETF) and as enabled by
Cisco IOS software, allows the construction of highly scalable and flexible IP Virtual Private Networks
(VPNs) that support multiple levels of services.
Cisco IOS software, allows the construction of highly scalable and flexible IP Virtual Private Networks
(VPNs) that support multiple levels of services.
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.
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 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.
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.
From an historical and functional standpoint, LDP is a superset of the Cisco prestandard Tag Distribution
Protocol (TDP), which also supports MPLS forwarding along normally routed paths. For those features
that LDP and TDP share in common, the pattern of protocol exchanges between network routing
platforms is identical. The differences between LDP and TDP for those features supported by both
protocols are largely embedded in their respective implementation details, such as the encoding of
protocol messages.
Protocol (TDP), which also supports MPLS forwarding along normally routed paths. For those features
that LDP and TDP share in common, the pattern of protocol exchanges between network routing
platforms is identical. The differences between LDP and TDP for those features supported by both
protocols are largely embedded in their respective implementation details, such as the encoding of
protocol messages.
This release of LDP, which supports both the LDP and TDP protocols, provides the means for
transitioning an existing network from a TDP environment to an LDP environment. Thus, you can run
LDP and TDP simultaneously on any router platform. The routing protocol that you select can be
configured on a per-interface basis for directly connected neighbors and on a per-session basis for
nondirectly connected (targeted) neighbors. In addition, an LSP across an MPLS network can be
supported by LDP on some hops and by TDP on other hops.
transitioning an existing network from a TDP environment to an LDP environment. Thus, you can run
LDP and TDP simultaneously on any router platform. The routing protocol that you select can be
configured on a per-interface basis for directly connected neighbors and on a per-session basis for
nondirectly connected (targeted) neighbors. In addition, an LSP across an MPLS network can be
supported by LDP on some hops and by TDP on other hops.