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Label Distribution Protocol
5.1 Label Distribution Protocol
Label Distribution Protocol (LDP) is used to distribute labels in non-traffic-engineered
applications. LDP allows routers to establish LSPs through a network by mapping
network-layer routing information directly to data link LSPs.
An LSP is defined by the set of labels from the ingress LER to the egress LER. 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
ingress LER 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 procedures 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 by each LSR, where each LSR splices
incoming labels for the FEC to the outgoing label assigned to the next hop for the
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 (DU). For LDP on the 7705 SAR,
Downstream Unsolicited (DU) mode is implemented.
This section contains the following topics:
368
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LDP and MPLS
•
LDP Architecture
•
LDP Subsystem Interrelationships
•
Execution Flow
•
Label Exchange
•
LDP Filters
•
LDP FEC Statistics
•
Multi-area and Multi-instance Extensions to LDP
•
ECMP Support for LDP
•
Graceful Restart Helper
•
Graceful Handling of Resource Exhaustion
•
LDP Support for Unnumbered Interfaces
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MPLS Guide
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