LDP operation

LDP operates in the following phases:

LDP discovers peers in the following ways:

• Basic Discovery—Sends Link Hello messages to multicast address 224.0.0.2 that identifies all routers on the subnet, so all directly connected LSRs can discover the LSR and establish a hello adjacency.

• Extended Discovery—Sends LDP Targeted Hello messages to a specific IP address, so the destination LSR can discover the LSR and establish a hello adjacency. This mechanism is mainly used in MPLS L2VPN and VPLS. For more information, see "Configuring MPLS L2VPN," and "Configuring VPLS."

LDP can establish two hello adjacencies with a directly connected neighbor through both discovery mechanisms. It 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.

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.

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 an 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.