LSP Setup
Before forwarding packets, MPLS must allocate labels to packets and establish an LSP. LSPs can be either static or dynamic.
Establishing Static LSPs
You can manually allocate labels to set up static LSPs. A static LSP is valid for only the local node, and nodes on the LSP are unaware of the entire LSP.
A static LSP is set up without any label distribution protocols or exchange of control packets. Static LSPs have low costs and are recommended for small-scale networks with simple and stable topologies. Static LSPs cannot adapt to network topology changes and must be configured by an administrator.
Establishing Dynamic LSPs
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 Interior Gateway Protocol (IGP) and Border Gateway Protocol (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 the S2720, S5700, and S6700 V200R019C10 Configuration Guide - IP Unicast Routing.
Procedure for Establishing Dynamic LSPs
MPLS labels are distributed from downstream LSRs to upstream LSRs. As shown in Figure 1, 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 table and establish an LSP.
