LDP IGP Synchronization
RFC 5443

Network Working Group                                            M. Jork
Request for Comments: 5443                                       GENBAND
Category: Informational                                         A. Atlas
                                                         British Telecom
                                                                 L. Fang
                                                     Cisco Systems, Inc.
                                                              March 2009

                       LDP IGP Synchronization

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   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
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Jork, et al.                 Informational                      [Page 1]
RFC 5443                LDP IGP Synchronization               March 2009

Abstract

   In certain networks, there is dependency on the edge-to-edge Label
   Switched Paths (LSPs) setup by the Label Distribution Protocol (LDP),
   e.g., networks that are used for Multiprotocol Label Switching (MPLS)
   Virtual Private Network (VPN) applications.  For such applications,
   it is not possible to rely on Internet Protocol (IP) forwarding if
   the MPLS LSP is not operating appropriately.  Blackholing of labeled
   traffic can occur in situations where the Interior Gateway Protocol
   (IGP) is operational on a link on which LDP is not.  While the link
   could still be used for IP forwarding, it is not useful for MPLS
   forwarding, for example, MPLS VPN applications or Border Gateway
   Protocol (BGP) route-free cores.  This document describes a mechanism
   to avoid traffic loss due to this condition without introducing any
   protocol changes.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................3
   2. Proposed Solution ...............................................3
   3. Applicability ...................................................4
   4. Interaction with TE Tunnels .....................................5
   5. Security Considerations .........................................5
   6. References ......................................................6
      6.1. Normative References .......................................6
      6.2. Informative References .....................................6
   7. Acknowledgments .................................................6

1.  Introduction

   LDP [RFC5036] establishes MPLS LSPs along the shortest path to a
   destination as determined by IP forwarding.  In a common network
   design, LDP is used to provide Label Switched Paths throughout the
   complete network domain covered by an IGP such as Open Shortest Path
   First (OSPF) [RFC2328] or Intermediate System to Intermediate System
   (IS-IS) [ISO.10589.1992]; i.e., all links in the domain have IGP as
   well as LDP adjacencies.

   A variety of services a network provider may want to deploy over an
   LDP-enabled network depend on the availability of edge-to-edge label
   switched paths.  In a layer 2 (L2) or layer 3 (L3) VPN scenario, for
   example, a given Provider-Edge (PE) router relies on the availability
   of a complete MPLS forwarding path to the other PE routers for the
   VPNs it serves.  This means that all the links along the IP shortest
   path from one PE router to the other need to have operational LDP
   sessions, and the necessary label binding must have been exchanged
   over those sessions.  If only one link along the IP shortest path is

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