November 2012 - Reading time: 9 minutes
Multiprotocol Label Switching (MPLS) continues to evolve as a foundational technology for modern service provider and enterprise networks. One of the most critical components in an MPLS deployment is the mechanism by which labels are distributed throughout the network. Two primary protocols — LDP (Label Distribution Protocol) and RSVP-TE (Resource Reservation Protocol - Traffic Engineering) — serve this purpose, but in vastly different ways. Understanding their core differences is key to effective network planning, especially when reliability, performance, and scalability are at stake.
Label Distribution with LDP
LDP is the most widely deployed label distribution mechanism in MPLS environments. It was designed to be relatively simple and scalable. LDP works in conjunction with the underlying IGP (Interior Gateway Protocol), such as OSPF or IS-IS, to build the label-switched path (LSP) across the network. As routers learn the IGP topology, LDP piggybacks on that information to exchange labels for destination prefixes.
Advantages of LDP include:
- Simplicity in configuration
- Automatic label assignment based on IGP
- Scalability across large core networks
However, LDP lacks granular control over path selection. It always follows the IGP's shortest path, which may not be optimal for traffic engineering or failover design.
Label Distribution with RSVP-TE
RSVP-TE, on the other hand, was designed with traffic engineering in mind. It allows explicit control over path selection and bandwidth reservation. Unlike LDP, RSVP-TE can be used to define constraints — such as avoiding a particular link or preferring a certain path. This makes it highly desirable in service provider networks where SLA compliance is crucial.
RSVP-TE establishes LSPs using signaling messages that include label bindings and resource reservation requests. These paths are precomputed, often using offline path computation or algorithms such as CSPF (Constrained Shortest Path First).
Head-to-Head Comparison
| Criteria | LDP | RSVP-TE |
|---|---|---|
| Control over Path | None (follows IGP) | Explicit (CSPF) |
| Traffic Engineering | Not Supported | Full TE support |
| Configuration Complexity | Low | High |
| Resource Reservation | No | Yes |
| Use Case | Core networks, simple topologies | SLA-bound, engineered paths |
When to Use LDP
LDP is ideal for large-scale deployments where simplicity and scalability are priorities. For example, in a core ISP network where all traffic is treated equally and routed based on destination IP, LDP minimizes complexity while offering solid performance. Many networks deploy LDP as the default mechanism and introduce RSVP-TE only where traffic engineering becomes critical.
When to Use RSVP-TE
RSVP-TE shines in scenarios that require differentiated services, latency control, or bandwidth reservation. It is commonly used in financial institutions, video transport networks, and real-time communication backbones. RSVP-TE allows service providers to build predictable, deterministic paths — an essential component for delivering premium services with guarantees.
Hybrid Deployments
Some modern networks use a combination of LDP and RSVP-TE. For instance, a core may use LDP by default, while key customer-facing services use RSVP-TE for dedicated paths. Technologies like MPLS-TE Fast Reroute (FRR) enhance resiliency by pre-signaling backup paths with RSVP-TE — something LDP cannot natively accomplish.
Conclusion
Choosing between LDP and RSVP-TE requires a solid understanding of network goals, operational overhead, and expected service levels. While LDP is simpler and fits the needs of many core environments, RSVP-TE offers control and predictability essential for traffic engineering. As MPLS evolves and integrates with SDN and Segment Routing, these traditional mechanisms will likely be replaced or complemented — but understanding the foundational elements remains critical for any network architect.