16
•
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 in the following ways:
•
Basic Discovery
—Sends Link Hello messages to multicast address 224.0.0.2 that identifies all
routers on the subnet. 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. The
destination LSR can discover the LSR and establish a hello adjacency. This mechanism is
typically used in MPLS L2VPN and VPLS. For more information, see "
."
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.
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 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.
