MPLS Label Distribution Protocols

Label distribution protocols create and maintain the label-to-FEC bindings along an LSP from MPLS domain ingress to MPLS domain egress. A label distribution protocol is a set of procedures by which one LSR informs a peer LSR of the meaning of the labels used to forward traffic between them. It enables each peer to learn about the other peer’s label mappings. The label distribution protocol provides the information MPLS uses to create the forwarding tables in each LSR in the MPLS domain.

Note: Label distribution protocols are sometimes referred to as signaling protocols. However, label distribution is a more accurate description of their function and is preferred in this text.

The following protocols are currently used for label distribution:

• BGP—Border Gateway Protocol
• LDP—Label Distribution Protocol
• RSVP-TE—Resource Reservation Protocol with traffic-engineering extensions that enable label binding and explicit route capability

  Note: To reduce confusion, this text uses the lowercase term, label distribution protocol, to refer to the generic class of protocols. The acronym, LDP, refers only to the particular protocol named Label Distribution Protocol.

BGP and LDP have no traffic-engineering capability and support only best-effort LSPs. LDP supports topology-driven MPLS networks in best-effort, hop-by-hop implementations. RSVP-TE is used primarily for MPLS applications that require traffic engineering (TE) or quality of service (QoS) capabilities, but they also support best-effort LSPs.

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