LSP Establishment

Procedure

MPLS assigns packets to a FEC, distributes labels that identify the FEC, and establishes an LSP. Packets travel along the LSP.

On the network shown in Figure 1, packets destined for 3.3.3.3 are assigned to a FEC. Downstream LSRs assign labels for the FEC to upstream LSRs and use a label advertisement protocol to inform its upstream LSR of the mapping between the labels and the FEC. Each upstream LSR adds the mapping to a label forwarding table. An LSP is established using the label mapping information.

Figure 1 Procedure for establishing an LSP

LSPs can be either static or dynamic. Static LSPs are established manually. Dynamic LSPs are established using a routing protocol and a label distribution protocol.

Establishing Dynamic LSPs

Dynamic LSPs are set up automatically by one of the following label distribution protocols:

• Label Distribution Protocol (LDP)

  LDP is specially defined to distribute labels. When LDP sets up an LSP in hop-by-hop mode, LDP identifies a next hop based on the routing forwarding table on each LSR. Information contained in the routing forwarding table is collected by Interior Gateway Protocol (IGP) and BGP, from which LDP is independent.

  In addition to LDP, BGP and RSVP can also be extended to distribute MPLS labels.

• Resource Reservation Protocol-Traffic Engineering (RSVP-TE)

  The RSVP-TE signaling protocol is an extension to RSVP. RSVP is designed for the integrated service model and is used to reserve the resources of nodes along a path. RSVP works on the transport layer and does not transmit application data. This is because RSVP is a network control protocol, similar to the Internet Control Message Protocol (ICMP).

  RSVP-TE establishes constraint-based routed LSPs (CR-LSPs).

  Unlike LDP LSPs, CR-LSPs support the following parameters:

  • Bandwidth reservation requests

  • Bandwidth constraint

  • Link colors

  • Explicit paths

• Multiprotocol Extensions for Border Gateway Protocol (MP-BGP)

  MP-BGP is an extension to BGP. MP-BGP defines community attributes. MP-BGP supports label distribution for packets transmitted over MPLS virtual private network (VPN) routes and labeled inter-AS VPN routes.