LSP establishment and label distribution

This section describes how MPLS sets up LSPs and distribute labels.

Establishing LSPs is to bind FECs to labels on each LSR involved and notify its adjacent LSRs of the bindings, so as to establish the LFIB on each LSR. LSPs can be manually established through configuration, or dynamically established through label distribution protocols.

• Establishing a static LSP through manual configuration:

  Static LSPs do not dynamically change in response to network topology changes, and are suitable for small-scale, stable, and simple networks. To establish a static LSP, assign a label to the FEC on each LSR along the packet forwarding path.

• Establishing an LSP through a label distribution protocol:

  Label distribution protocols are MPLS signaling protocols. They can classify FECs, distribute labels, and establish and maintain LSPs. Label distribution protocols include protocols designed specifically for label distribution, such as the Label Distribution Protocol (LDP), and protocols extended to support label distribution, such as BGP and RSVP-TE.

This document discusses LDP only. For more information about LDP, see "LDP."

In this document, the term "label distribution protocols" refers to all protocols for label distribution. The term "LDP" refers to the RFC 5036 LDP.

A dynamic LSP is established in the following procedure:

A downstream LSR classifies FECs according to destination addresses. It assigns a label to a FEC, and distributes the FEC-label binding to its upstream LSR, which then establishes an LFIB entry for the FEC according to the binding information. After all LSRs along the packet forwarding path establish a LFIB entry for the FEC, an LSP is established for packets of this FEC.

If equal-cost routes exist on the LSRs, MPLS establishes equal-cost LSPs based on these routes, and shares loads among the equal-cost LSPs.

An LSR informs its upstream LSRs of labels assigned to FECs through label advertisement. The label advertisement modes include downstream unsolicited (DU) and downstream on demand (DoD). The label distribution control modes include independent and ordered.

Label management specifies the mode for processing a received label binding that is not useful at the moment. The processing mode, called "label retention mode," can be either liberal or conservative.

As shown in Figure 16, the label advertisement modes include the DU mode and the DoD mode.

• In DU mode, an LSR assigns a label to a FEC and then distributes the FEC-label binding to its upstream LSR without solicitation. The switch supports only the DU mode.

• In DoD mode, an LSR assigns a label to a FEC and distributes the FEC-label binding to its upstream LSR only when it receives a label request from the upstream LSR.

To establish an LSP, an upstream LSR and its downstream LSR must use the same label advertisement mode.

Label distribution control modes include the independent mode and the ordered mode.

• In independent mode, an LSR can distribute label bindings upstream at any time. This means that an LSR may have distributed a label binding for a FEC to its upstream LSR before it receives a binding for that FEC from its downstream LSR. As shown in Figure 17, in independent label distribution control mode, if the label advertisement mode is DU, an LSR assigns labels to its upstream even if it has not obtained labels from its downstream. If the label advertisement mode is DoD, the LSR distributes a label to its upstream as long as it receives a label request from the upstream.

• In ordered mode, an LSR distributes its label binding for a FEC upstream only when it receives a label binding for the FEC from its downstream or it is the egress of the FEC. In Figure 16, label distribution control is in ordered mode. If the label advertisement mode is DU, an LSR distributes a label upstream only when it receives a label binding for the FEC from its downstream. If the label advertisement mode is DoD, after an LSR (Transit in this example) receives a label request from its upstream (Ingress), the LSR (Transit) sends a label request to its downstream (Egress). Then, after the LSR (Transit) receives the label binding from its downstream (Egress), it distributes a label binding to the upstream (Ingress).

Label retention modes include the liberal mode and the conservative mode.

• In liberal mode, an LSR keeps any received label binding regardless of whether the binding is from the next hop for the FEC or not. This mode allows for quicker adaptation to route changes but wastes label resources because LSRs keep extra labels. The switch supports only the liberal mode.

• In conservative mode, an LSR keeps only label bindings that are from the next hops for the FECs. This allows LSRs to maintain fewer labels but makes LSRs slower in adapting to route changes.