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Configuring LDP
Overview
The Label Distribution Protocol (LDP) dynamically distributes FEC-label mapping information
between LSRs to establish LSPs.
Terminology
LDP session
Two LSRs establish a TCP-based LDP session to exchange FEC-label mappings.
LDP peer
Two LSRs that use LDP to exchange FEC-label mappings are LSR peers.
Label spaces and LDP identifiers
Label spaces include the following types:
•
Per-interface label space
—Each interface uses a single, independent label space. Different
interfaces can use the same label values.
•
Per-platform label space
—Each LSR uses a single label space. The device only supports the
per-platform label space.
A six-byte LDP Identifier (LDP ID) identifies a label space on an LSR. It is in the format of <LSR
ID>:<
label space number
>, where:
•
The LSR ID takes four bytes to identity the LSR.
•
The label space number takes two bytes to identify a label space within the LSR.
A label space number of 0 indicates that the label space is a per-platform label space. A label space
number other than 0 indicates a per-interface label space.
LDP uses the same LDP ID format on IPv4 and IPv6 networks. An LDP ID must be globally unique.
FECs and FEC-label mappings
MPLS groups packets with the same characteristics (such as the same destination or service class)
into a class, called an FEC. The packets of the same FEC are handled in the same way on an MPLS
network.
LDP can classify FECs by destination IP address and by PW. This document describes FEC
classification by destination IP address. For information about FEC classification by PW, see
"Configuring MPLS L2VPN" and "Configuring VPLS."
An LSR assigns a label for an FEC and advertises the FEC-label mapping, or FEC-label binding, to
its peers in a Label Mapping message.
LDP messages
LDP mainly uses the following types of messages:
•
Discovery
messages
—Declare and maintain the presence of LSRs, such as Hello messages.
•
Session
messages
—Establish, maintain, and terminate sessions between LDP peers, such
as Initialization messages used for parameter negotiation and Keepalive messages used to
maintain sessions.
