4000 Router Series MPLS Configuration Guide

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, and 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.

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.

An LSR assigns a label for a 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.