Signaling Protocol Overview

When transit traffic is routed through an IP network, MPLS is often used to engineer its passage. Although the exact path through the transit network is of little importance to either the sender or the receiver of the traffic, network administrators often want to route traffic more efficiently between certain source and destination address pairs. By adding a short label with specific routing instructions to each packet, MPLS switches packets from router to router through the network rather than forwarding packets based on next-hop lookups. The resulting routes are called label-switched paths (LSPs). LSPs control the passage of traffic through the network and speed traffic forwarding.

You can create LSPs manually, or through the use of signaling protocols. J Series devices support two signaling protocols—the Label Distribution Protocol (LDP) and the Resource Reservation Protocol (RSVP).

LDP Signaling Protocol

The Label Distribution Protocol (LDP) is a signaling protocol that runs on a device configured for MPLS support. The LDP configuration is added to the existing interior gateway protocol (IGP) configuration and included in the MPLS configuration. To configure a network to use LDP for LSP establishment, you first enable MPLS on all transit interfaces in the MPLS network and then enable LDP sessions on the interfaces.

The successful configuration of both MPLS and LDP initiates the exchange of TCP packets across the LDP interfaces. The packets establish TCP-based LDP sessions for the exchange of MPLS information within the network. Enabling both MPLS and LDP on the appropriate interfaces is sufficient to establish LSPs.

RSVP Signaling Protocol

The Resource Reservation Protocol (RSVP) is a more flexible and powerful way to engineer traffic through a transit network. Like LDP, RSVP establishes LSPs within an MPLS network when you enable both MPLS and RSVP on the appropriate interfaces. However, whereas LDP is restricted to using the configured IGP's shortest path as the transit path through the network, RSVP uses a combination of the Constrained Shortest Path First (CSPF) algorithm and Explicit Route Objects (EROs) to determine how traffic is routed through the network.

Basic RSVP sessions are established in exactly the same way that LDP sessions are established. By configuring both MPLS and RSVP on the appropriate transit interfaces, you enable the exchange of RSVP packets and the establishment of LSPs. However, RSVP also lets you configure link authentication, explicit LSP paths, and link coloring. For more information about these topics, see the JUNOS MPLS Applications Configuration Guide.