2 · LDP & Label Distribution Protocol Mechanics¶

Label Distribution Protocol (LDP, RFC 5036) is the standard control plane protocol used to build Label-Switched Paths (LSPs) across an MPLS network by binding local labels to IP prefixes learned from the underlay IGP (OSPF or IS-IS).

LDP Session Establishment (UDP Discovery + TCP Session)¶

LDP uses a two-phase initialization process combining UDP discovery and TCP reliable session state:

sequenceDiagram
    autonumber
    participant R1 as Router 1 (1.1.1.1)
    participant R2 as Router 2 (2.2.2.2)

    R1->>R2: UDP Port 646 Hello Multicast (224.0.0.2)<br/>Transport Address: 1.1.1.1
    R2->>R1: UDP Port 646 Hello Multicast (224.0.0.2)<br/>Transport Address: 2.2.2.2

    Note over R1,R2: Router with HIGHER Transport IP (2.2.2.2) initiates TCP connection
    R2->>R1: TCP SYN (Port 646)
    R1-->>R2: TCP SYN-ACK
    R2->>R1: TCP ACK

    R2->>R1: LDP Initialization Message
    R1-->>R2: LDP Initialization Message + Keepalive
    R2-->>R1: LDP Keepalive (State: OPERATIONAL)

Discovery Phase (UDP Port 646): Routers send periodic LDP Link Hellos to multicast address 224.0.0.2 on all LDP-enabled interfaces.
Session Phase (TCP Port 646): The router with the higher numerical LDP Transport IP initiates a 3-way TCP handshake to port 646.
PDU Exchange: Routers exchange LDP Initialization and Keepalive PDUs until reaching state OPERATIONAL.

Label Distribution Modes: DU vs DoD, Independent vs Ordered¶

Operating Mode	Options	Description	Standard Usage
Label Distribution	Downstream Unsolicited (DU)	Routers automatically advertise label bindings to all LDP peers without waiting for a request.	Default on Cisco & Arista
	Downstream on Demand (DoD)	Routers only advertise a label binding when explicitly requested by an upstream peer.	ATM / legacy circuits
Label Control	Ordered Control	A router only advertises a label for a prefix if it is the egress router OR has received a label from its downstream next-hop.	Default & Standard
	Independent Control	Routers create and advertise label bindings for any route in their RIB immediately, regardless of downstream state.	Legacy behavior
Label Retention	Liberal Retention	Keeps all received label bindings in the LIB even if they are not from the current IGP next-hop.	Default (Fast Reroute)
	Conservative Retention	Discards received label bindings if they are not from the current IGP next-hop.	Memory constrained

LDP-IGP Synchronization¶

When a link recovers, OSPF/IS-IS converges faster than LDP session establishment. If traffic is forwarded over the link before LDP has distributed labels, packets are dropped.

LDP-IGP Sync Solution: OSPF/IS-IS advertises maximum metric (65535) on the recovering link until the LDP session reaches OPERATIONAL state, keeping transit traffic on backup paths until labels are fully programmed!