LSP Setup

Label Distribution Protocols for Dynamic LSPs

Dynamic LSPs are established using label distribution protocols. As the control protocol or signaling protocol for MPLS, a label distribution protocol defines FECs, distributes labels, and establishes and maintains LSPs.

MPLS can use the following protocols for label distribution:

• LDP

  The Label Distribution Protocol (LDP) is designed for distributing labels. It sets up an LSP hop by hop according to IGP and BGP routing information.

  For details about LDP implementation, see Understanding MPLS LDP in the MPLS LDP Configuration.

• RSVP-TE

  Resource Reservation Protocol Traffic Engineering (RSVP-TE) is an extension of RSVP and is used to set up a constraint-based routed LSP (CR-LSP). In contrast to LDP LSPs, RSVP-TE tunnels are characterized by bandwidth reservation requests, bandwidth constraints, link "colors" (designating administrative groups), and explicit paths.

  For details about RSVP-TE implementation, see Understanding MPLS TE in the MPLS TE Configuration.

• MP-BGP

  MP-BGP is an extension to BGP and allocates labels to MPLS VPN routes and inter-AS VPN routes.

  For details about MP-BGP implementation, see BGP Configuration in CloudEngine 12800 and 12800E Series Switches Configuration Guide - IP Routing.

Procedure for Establishing Dynamic LSPs

MPLS labels are distributed from downstream LSRs to upstream LSRs. As shown in Figure 1-5, a downstream LSR identifies FECs based on the IP routing table, allocates a label to each FEC, and records the mapping between labels and FECs. The downstream LSR then encapsulates the mapping into a message and sends the message to the upstream LSR. As this process proceeds on all the LSRs, the LSRs create a label forwarding information base (LFIB) and establish an LSP.

Figure 1-5 Establishing a dynamic LSP