Label Distribution Protocol; Ldp And Mpls - Alcatel-Lucent 7450 Manual

Label Distribution Protocol

Label Distribution Protocol

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 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 as each LSR splices incoming labels for a FEC to the outgoing label
assigned to the next hop for the given FEC. The next-hop for a FEC prefix is resolved in the
routing table. LDP can only resolve FECs for IGP and static prefixes. LDP does not support
resolving FECs of a BGP prefix.
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).
SR OS supports IPv4 and IPv6 in LDP control and data planes; as IPv6 has been added
subsequently, CLI commands have changed to support both IPv4 and IPv6. Refer to the Release
13.0.R1 SR OS Software Release Notes for more information.

LDP and MPLS

LDP performs the 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
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