Basic Specification for IP Fast Reroute: Loop-Free Alternates
RFC 5286

Network Working Group                                      A. Atlas, Ed.
Request for Comments: 5286                                            BT
Category: Standards Track                                  A. Zinin, Ed.
                                                          Alcatel-Lucent
                                                          September 2008

     Basic Specification for IP Fast Reroute: Loop-Free Alternates

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes the use of loop-free alternates to provide
   local protection for unicast traffic in pure IP and MPLS/LDP networks
   in the event of a single failure, whether link, node, or shared risk
   link group (SRLG).  The goal of this technology is to reduce the
   packet loss that happens while routers converge after a topology
   change due to a failure.  Rapid failure repair is achieved through
   use of precalculated backup next-hops that are loop-free and safe to
   use until the distributed network convergence process completes.
   This simple approach does not require any support from other routers.
   The extent to which this goal can be met by this specification is
   dependent on the topology of the network.

Atlas, et al.               Standards Track                     [Page 1]
RFC 5286         IP Fast Reroute: Loop-Free Alternates    September 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Failure Scenarios  . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Requirement Language . . . . . . . . . . . . . . . . . . .  8
   2.  Applicability of Described Mechanisms  . . . . . . . . . . . .  8
   3.  Alternate Next-Hop Calculation . . . . . . . . . . . . . . . .  9
     3.1.  Basic Loop-Free Condition  . . . . . . . . . . . . . . . . 10
     3.2.  Node-Protecting Alternate Next-Hops  . . . . . . . . . . . 10
     3.3.  Broadcast and Non-Broadcast Multi-Access (NBMA) Links  . . 11
     3.4.  ECMP and Alternates  . . . . . . . . . . . . . . . . . . . 12
     3.5.  Interactions with IS-IS Overload, RFC 3137, and Costed
           Out Links  . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.5.1.  Interactions with IS-IS Link Attributes  . . . . . . . 14
     3.6.  Selection Procedure  . . . . . . . . . . . . . . . . . . . 14
     3.7.  LFA Types and Trade-Offs . . . . . . . . . . . . . . . . . 18
     3.8.  A Simplification: Per-Next-Hop LFAs  . . . . . . . . . . . 19
   4.  Using an Alternate . . . . . . . . . . . . . . . . . . . . . . 20
     4.1.  Terminating Use of Alternate . . . . . . . . . . . . . . . 20
   5.  Requirements on LDP Mode . . . . . . . . . . . . . . . . . . . 22
   6.  Routing Aspects  . . . . . . . . . . . . . . . . . . . . . . . 22
     6.1.  Multi-Homed Prefixes . . . . . . . . . . . . . . . . . . . 22
     6.2.  IS-IS  . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     6.3.  OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
       6.3.1.  OSPF External Routing  . . . . . . . . . . . . . . . . 24
       6.3.2.  OSPF Multi-Topology  . . . . . . . . . . . . . . . . . 25
     6.4.  BGP Next-Hop Synchronization . . . . . . . . . . . . . . . 25
     6.5.  Multicast Considerations . . . . . . . . . . . . . . . . . 25
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 26
   Appendix A.  OSPF Example Where LFA Based on Local Area
                Topology Is Insufficient  . . . . . . . . . . . . . . 27

Atlas, et al.               Standards Track                     [Page 2]
RFC 5286         IP Fast Reroute: Loop-Free Alternates    September 2008

1.  Introduction

   Applications for interactive multimedia services such as Voice over
   IP (VoIP) and pseudowires can be very sensitive to traffic loss, such
   as occurs when a link or router in the network fails.  A router's
   convergence time is generally on the order of hundreds of
   milliseconds; the application traffic may be sensitive to losses
   greater than tens of milliseconds.

   As discussed in [FRAMEWORK], minimizing traffic loss requires a
   mechanism for the router adjacent to a failure to rapidly invoke a