MPLS - Using RSVP for Traffic Engineering

MPLS uses various technologies and protocols to be able to perform its functions. Label Distribution Protocol (LDP) is used to automatically generate and exchange labels, while RSVP is used in the context of MPLS traffic engineering (MPLS-TE). Both of these protocols can be used simultaneously in some scenarios.

RSVP alone can generate labels. However, in an MPLS-TE configuration using OSPF for example, both LDP and RSVP can be used, but when they’re used together, they serve different purposes. Here’s a detailed explanation of why both are often used together:

  1. Dual Use Cases:
    • LDP for Standard Traffic: LDP can be used for general-purpose traffic where strict path requirements and resource reservations are not needed. This ensures that basic MPLS forwarding is always available, leveraging the simplicity of LDP.
    • RSVP for TE Traffic: RSVP is used for specific traffic engineering needs, where traffic demands explicit paths, bandwidth reservations, or other constraints. This allows for better control and optimization of network resources for critical traffic flows.
  2. Separation of Concerns:
    • Using both protocols allows for a clear separation between standard MPLS forwarding (handled by LDP) and traffic-engineered paths (handled by RSVP). This separation simplifies network management and ensures that each protocol is used for its intended purpose.
  3. Compatibility and Redundancy:
    • Having both LDP and RSVP configured provides a fallback mechanism. If RSVP fails to establish a TE tunnel, the traffic can still fall back to an LDP-signaled LSP, ensuring continuity of service.
  4. Granular Control:
    • By configuring both, network operators can have granular control over which traffic should use traffic-engineered paths and which should use default LDP paths. This is particularly useful in complex networks where different traffic types have different service requirements.

Using both LDP and RSVP in MPLS-TE configurations provides flexibility, redundancy, and optimized use of network resources. LDP handles standard label distribution for best-effort traffic, while RSVP handles the more complex requirements of traffic-engineered paths. This dual approach ensures that the network can support a wide range of service requirements and maintain high performance and reliability.

https://networklessons.com/mpls/mpls-traffic-engineering-te-ospf-configuration

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