{"id":4092,"date":"2025-11-13T00:00:00","date_gmt":"2025-11-13T00:00:00","guid":{"rendered":"https:\/\/lp.szlogic.cn\/glossary\/ldp-label-distribution-protocol-in-mpls-network\/"},"modified":"2026-06-18T08:11:00","modified_gmt":"2026-06-18T08:11:00","slug":"ldp-label-distribution-protocol-in-mpls-network","status":"publish","type":"post","link":"https:\/\/lp.szlogic.cn\/ru\/glossary\/ldp-label-distribution-protocol-in-mpls-network","title":{"rendered":"What Is LDP? The Foundation of MPLS Label Switching"},"content":{"rendered":"
\"What<\/figure>\n\n\n\n

\ud83d\udd39 What Is LDP?<\/h2>\n\n\n\n

LDP (Label Distribution Protocol)<\/strong> is a control protocol used in MPLS (Multiprotocol Label Switching)<\/strong><\/a> networks to automatically assign and exchange labels between routers.
Instead of relying solely on traditional IP routing, MPLS uses short, fixed-length labels to forward packets efficiently through pre-established paths known as Label Switched Paths (LSPs)<\/strong><\/a>.<\/p>\n\n\n\n

Through LDP, routers \u2014 specifically Label Switch Routers (LSRs)<\/strong> and Label Edge Routers (LERs)<\/strong> \u2014 can dynamically communicate label mapping information, forming a virtual circuit-like path across the MPLS backbone.<\/p>\n\n\n\n

\ud83d\udd39 How LDP Works<\/h2>\n\n\n\n

1. Discovery and Session Setup<\/h3>\n\n\n\n

Routers discover neighboring LSRs by exchanging LDP Hello messages<\/strong>.
Once a connection is established (typically using TCP port 646), they form an LDP session<\/strong> to exchange label information.<\/p>\n\n\n\n

2. Label Mapping and Advertisement<\/h3>\n\n\n\n

Each LSR associates a local label with an IP prefix from its routing table \u2014 for example:<\/p>\n\n\n\n

10.10.0.0\/16 \u2192 Label 102\n<\/code><\/pre>\n\n\n\n
These label mappings are advertised to neighbors, allowing them to build a consistent Label Forwarding Information Base (LFIB)<\/strong>.<\/p>\n\n\n\n
3. Label Switched Path Creation<\/h3>\n\n\n\n
When each router receives label information from its peers, it builds the label stack needed for packet forwarding.
This process forms a complete Label Switched Path (LSP)<\/strong><\/a> from ingress LER to egress LER, enabling packets to travel through the network without repeated IP lookups.<\/p>\n\n\n\n
\"MPLS<\/figure>\n\n\n\n
\ud83d\udd39 LDP vs RSVP-TE: Understanding the Difference<\/h2>\n\n\n\n
\n\n<\/colgroup>
Feature<\/p><\/th>
LDP<\/strong><\/p><\/th>
RSVP-TE<\/strong><\/p><\/th><\/tr>
Path Control<\/strong><\/p><\/td>
Based on the IGP shortest path<\/p><\/td>
Explicitly defined by the administrator<\/p><\/td><\/tr>
Traffic Engineering<\/strong><\/p><\/td>
No<\/p><\/td>
Yes<\/p><\/td><\/tr>
QoS \/ Bandwidth Reservation<\/strong><\/p><\/td>
Limited<\/p><\/td>
Supported<\/p><\/td><\/tr>
Complexity<\/strong><\/p><\/td>
Simpler<\/p><\/td>
More complex<\/p><\/td><\/tr>
Use Case<\/strong><\/p><\/td>
General MPLS forwarding<\/p><\/td>
Backbone traffic optimization<\/p><\/td><\/tr><\/tbody>\n<\/table>\n<\/figure>\n\n\n\n
While LDP<\/strong> provides a simple and scalable way to establish label paths, RSVP-TE (Resource Reservation Protocol – Traffic Engineering)<\/strong> allows more advanced control, such as bandwidth reservation and QoS enforcement in large carrier networks.<\/p>\n\n\n\n
\ud83d\udd39 Types of LDP<\/h2>\n\n\n\n
\u25b7 Basic LDP (Hop-by-Hop LDP)<\/h3>\n\n\n\n
Labels are assigned based on IGP routing information (OSPF, IS-IS).
Each hop independently determines the next label, forming a best-effort LSP.<\/p>\n\n\n\n
\u25b7 Targeted LDP (tLDP)<\/h3>\n\n\n\n
Used to establish LSPs between non-directly connected routers.
It\u2019s essential for MPLS Layer 2\/3 VPNs<\/strong>, where label exchange happens between remote edge routers.<\/p>\n\n\n\n
\ud83d\udd39 Advantages of Using LDP<\/h2>\n\n\n\n