4000 Router Series MPLS Configuration Guide

• Advertisement messages—Create, alter, and remove FEC-label mappings, such as Label Mapping

messages used to advertise FEC-label mappings.

• Notification messages—Provide advisory information and notify errors, such as Notification

messages.

LDP uses UDP to transport discovery messages for efficiency, and uses TCP to transport session,

advertisement, and notification messages for reliability.

LDP operation

LDP operates in the following phases:

Discovering and maintaining LDP peers

LDP discovers peers by sending Link Hello messages to multicast address 224.0.0.2 that identifies all

routers on the subnet. In this way, all directly-connected LSRs can discover the LSR and establish a hello

adjacency.

LDP sends Hello messages at the hello interval to maintain a hello adjacency. If LDP receives no Hello

message from a hello adjacency before the hello hold timer expires, it removes the hello adjacency.

Establishing and maintaining LDP sessions

LDP establishes a session with a peer in the following steps:

1. Establishes a TCP connection with the neighbor.

2. Negotiates session parameters such as LDP version, label distribution method, and Keepalive timer,

and establishes an LDP session with the neighbor if the negotiation succeeds.

After a session is established, LDP sends LDP PDUs (an LDP PDU carries one or more LDP messages) to

maintain the session. If no information is exchanged between the LDP peers within the Keepalive interval,

LDP sends Keepalive messages at the Keepalive interval to maintain the session. If LDP receives no LDP

PDU from a neighbor before the keepalive hold timer expires, or the last hello adjacency with the

neighbor is removed, LDP terminates the session.

LDP can also send a Shutdown message to a neighbor to terminate the LDP session.

Establishing LSPs

LDP classifies FECs according to destination IP addresses in IP routing entries, creates FEC-label

mappings, and advertises the mappings to LDP peers through LDP sessions. After an LDP peer receives

a FEC-label mapping, it uses the received label and the label locally assigned to that FEC to create an

LFIB entry for that FEC. When all LSRs (from the Ingress to the Egress) establish an LFIB entry for the FEC,

an LSP is established exclusively for the FEC.