Border Gateway Protocol (BGP) Persistent Route Oscillation Condition
RFC 3345

Network Working Group                                       D. McPherson
Request for Comments: 3345                                           TCB
Category: Informational                                          V. Gill
                                                   AOL Time Warner, Inc.
                                                               D. Walton
                                                               A. Retana
                                                     Cisco Systems, Inc.
                                                             August 2002

  Border Gateway Protocol (BGP) Persistent Route Oscillation Condition

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   In particular configurations, the BGP scaling mechanisms defined in
   "BGP Route Reflection - An Alternative to Full Mesh IBGP" and
   "Autonomous System Confederations for BGP" will introduce persistent
   BGP route oscillation.  This document discusses the two types of
   persistent route oscillation that have been identified, describes
   when these conditions will occur, and provides some network design
   guidelines to avoid introducing such occurrences.

1. Introduction

   The Border Gateway Protocol (BGP) is an inter-Autonomous System
   routing protocol.  The primary function of a BGP speaking system is
   to exchange network reachability information with other BGP systems.

   In particular configurations, the BGP [1] scaling mechanisms defined
   in "BGP Route Reflection - An Alternative to Full Mesh IBGP" [2] and
   "Autonomous System Confederations for BGP" [3] will introduce
   persistent BGP route oscillation.

   The problem is inherent in the way BGP works: locally defined routing
   policies may conflict globally, and certain types of conflicts can
   cause persistent oscillation of the protocol.  Given current
   practices, we happen to see the problem manifest itself in the
   context of MED + route reflectors or confederations.

McPherson, et al.            Informational                      [Page 1]
RFC 3345       BGP Persistent Route Oscillation Condition    August 2002

   The current specification of BGP-4 [4] states that the
   MULTI_EXIT_DISC is only comparable between routes learned from the
   same neighboring AS.  This limitation is consistent with the
   description of the attribute: "The MULTI_EXIT_DISC attribute may be
   used on external (inter-AS) links to discriminate among multiple exit
   or entry points to the same neighboring AS." [1,4]

   In a full mesh iBGP network, all the internal routers have complete
   visibility of the available exit points into a neighboring AS.  The
   comparison of the MULTI_EXIT_DISC for only some paths is not a
   problem.

   Because of the scalability implications of a full mesh iBGP network,
   two alternatives have been standardized: route reflectors [2] and AS
   confederations [3].  Both alternatives describe methods by which
   route distribution may be achieved without a full iBGP mesh in an AS.

   The route reflector alternative defines the ability to re-advertise
   (reflect) iBGP-learned routes to other iBGP peers once the best path
   is selected [2].  AS Confederations specify the operation of a
   collection of autonomous systems under a common administration as a
   single entity (i.e. from the outside, the internal topology and the
   existence of separate autonomous systems are not visible).  In both
   cases, the reduction of the iBGP full mesh results in the fact that
   not all the BGP speakers in the AS have complete visibility of the
   available exit points into a neighboring AS.  In fact, the visibility
   may be partial and inconsistent depending on the location (and
   function) of the router in the AS.

   In certain topologies involving either route reflectors or
   confederations (detailed description later in this document), the
   partial visibility of the available exit points into a neighboring AS
   may result in an inconsistent best path selection decision as the
   routers don't have all the relevant information.  If the
   inconsistencies span more than one peering router, they may result in
   a persistent route oscillation.  The best path selection rules
   applied in this document are consistent with the current
   specification [4].

   The persistent route oscillation behavior is deterministic and can be
   avoided by employing some rudimentary BGP network design principles
   until protocol enhancements resolve the problem.

   In the following sections a taxonomy of the types of oscillations is