A Framework for Loop-Free Convergence
RFC 5715

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Internet Engineering Task Force (IETF)                          M. Shand
Request for Comments: 5715                                     S. Bryant
Category: Informational                                    Cisco Systems
ISSN: 2070-1721                                             January 2010

                 A Framework for Loop-Free Convergence

Abstract

   A micro-loop is a packet forwarding loop that may occur transiently
   among two or more routers in a hop-by-hop packet forwarding paradigm.

   This framework provides a summary of the causes and consequences of
   micro-loops and enables the reader to form a judgement on whether
   micro-looping is an issue that needs to be addressed in specific
   networks.  It also provides a survey of the currently proposed
   mechanisms that may be used to prevent or to suppress the formation
   of micro-loops when an IP or MPLS network undergoes topology change
   due to failure, repair, or management action.  When sufficiently fast
   convergence is not available and the topology is susceptible to
   micro-loops, use of one or more of these mechanisms may be desirable.

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/rfc5715.

Shand & Bryant                Informational                     [Page 1]
RFC 5715          A Framework for Loop-Free Convergence     January 2010

Copyright Notice

   Copyright (c) 2010 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
   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 ....................................................3
   2. The Nature of Micro-Loops .......................................4
   3. Applicability ...................................................5
   4. Micro-Loop Control Strategies ...................................6
   5. Loop Mitigation .................................................8
      5.1. Fast Convergence ...........................................8
      5.2. PLSN .......................................................8
   6. Micro-Loop Prevention ..........................................10
      6.1. Incremental Cost Advertisement ............................10
      6.2. Nearside Tunneling ........................................12
      6.3. Farside Tunnels ...........................................13
      6.4. Distributed Tunnels .......................................14
      6.5. Packet Marking ............................................14
      6.6. MPLS New Labels ...........................................15
      6.7. Ordered FIB Update ........................................16
      6.8. Synchronised FIB Update ...................................18
   7. Using PLSN in Conjunction with Other Methods ...................18
   8. Loop Suppression ...............................................19
   9. Compatibility Issues ...........................................20
   10. Comparison of Loop-Free Convergence Methods ...................20
   11. Security Considerations .......................................21
   12. Acknowledgments ...............................................21
   13. Informative References ........................................21

Shand & Bryant                Informational                     [Page 2]
RFC 5715          A Framework for Loop-Free Convergence     January 2010

1.  Introduction

   When there is a change to the network topology (due to the failure or
   restoration of a link or router, or as a result of management
   action), the routers need to converge on a common view of the new
   topology and the paths to be used for forwarding traffic to each
   destination.  During this process, referred to as a routing
   transition, packet delivery between certain source/destination pairs
   may be disrupted.  This occurs due to the time it takes for the