LDP IGP Synchronization for Broadcast Networks
RFC 6138

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Internet Engineering Task Force (IETF)                      S. Kini, Ed.
Request for Comments: 6138                                    W. Lu, Ed.
Updates: 5443                                                   Ericsson
Category: Informational                                    February 2011
ISSN: 2070-1721

             LDP IGP Synchronization for Broadcast Networks

Abstract

   RFC 5443 describes a mechanism to achieve LDP IGP synchronization to
   prevent black-holing traffic (e.g., VPN) when an Interior Gateway
   Protocol (IGP) is operational on a link but Label Distribution
   Protocol (LDP) is not.  If this mechanism is applied to broadcast
   links that have more than one LDP peer, the metric increase procedure
   can only be applied to the link as a whole but not to an individual
   peer.  When a new LDP peer comes up on a broadcast network, this can
   result in loss of traffic through other established peers on that
   network.  This document describes a mechanism to address that use-
   case without dropping traffic.  The mechanism does not introduce any
   protocol message changes.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6138.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

Kini & Lu                     Informational                     [Page 1]
RFC 6138           LDP IGP Sync for Broadcast Networks     February 2011

   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. Problem Statement ...............................................2
   4. Solution ........................................................4
   5. Scope ...........................................................5
   6. Applicability ...................................................5
   7. Security Considerations .........................................6
   8. Conclusions .....................................................6
   9. References ......................................................7
      9.1. Normative References .......................................7
      9.2. Informative References .....................................7
   Acknowledgments ....................................................7
   Appendix A. Computation of "Cut-Edge" ..............................8
   Appendix B. Sync without Support at One End ........................8

1.  Introduction

   In RFC 5443 [LDP-IGP-SYNC], when [LDP] is not fully operational on a
   link, the IGP advertises the link with maximum cost to avoid any
   transit traffic on the link if possible.  When LDP becomes
   operational, i.e., all the label bindings have been exchanged, the
   link is advertised with its correct cost.  This tries to ensure that
   the LDP Label Switch Path (LSP) is available all along the IGP
   shortest path.  The mechanisms in [LDP-IGP-SYNC] have limitations
   when applied to a broadcast link.  These are described in Section 3.
   A solution is defined in Section 4.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Problem Statement

   On broadcast networks, a router's Link State Advertisement (LSA)
   contains a single cost to the broadcast network rather than a
   separate cost to each peer on the broadcast network.  The operation
   of the mechanism in [LDP-IGP-SYNC] is analyzed using the sample
   topology in Figure 1, where routers A, B, C, and E are attached to a

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RFC 6138           LDP IGP Sync for Broadcast Networks     February 2011