BGP Wedgies
RFC 4264

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Network Working Group                                         T. Griffin
Request for Comments: 4264                       University of Cambridge
Category: Informational                                        G. Huston
                                                                   APNIC
                                                           November 2005

                              BGP Wedgies

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 (2005).

Abstract

   It has commonly been assumed that the Border Gateway Protocol (BGP)
   is a tool for distributing reachability information in a manner that
   creates forwarding paths in a deterministic manner.  In this memo we
   will describe a class of BGP configurations for which there is more
   than one potential outcome, and where forwarding states other than
   the intended state are equally stable.  Also, the stable state where
   BGP converges may be selected by BGP in a non-deterministic manner.
   These stable, but unintended, BGP states are termed here "BGP
   Wedgies".

Table of Contents

   1. Introduction ....................................................2
   2. Describing BGP Routing Policy ...................................2
   3. BGP Wedgies .....................................................3
   4. Multi-Party BGP Wedgies .........................................6
   5. BGP and Determinism .............................................7
   6. Security Considerations .........................................8
   7. References ......................................................9
      7.1. Normative References .......................................9
      7.2. Informative References .....................................9

Griffin & Huston             Informational                      [Page 1]
RFC 4264                      BGP Wedgies                  November 2005

1.  Introduction

   It has commonly been assumed that the Border Gateway Protocol (BGP)
   [RFC1771] is a tool for distributing reachability information in a
   manner that creates forwarding paths in a deterministic manner.  This
   is a 'problem statement' memo that describes a class of BGP
   configurations for which there is more than one stable forwarding
   state.  In this class of configurations there exist multiple stable
   forwarding states.  One of these stable forwarding states is the
   intended state, with other stable forwarding states being unintended.
   The BGP convergence process of selection of a stable forwarding state
   may operate in a non-deterministic manner in such cases.

   These stable, but unintended, BGP states are termed here "BGP
   Wedgies".

2.  Describing BGP Routing Policy

   BGP routing policies generally reflect each network administrator's
   objective to optimize their position with respect to their network's
   cost, performance, and reliability.

   With respect to cost optimization, the local network's default
   routing policy often reflects a local preference to prefer routes
   learned from a customer to routes learned from some form of peering
   exchange.  In the same vein, the local network is often configured to
   prefer routes learned from a peer or a customer over those learned
   from a directly connected upstream transit provider.  These
   preferences may be expressed via a local preference configuration
   setting, where the local preference overrides the AS path length
   metric of the base BGP operation.

   In terms of engineering reliability in the inter-domain routing
   environment it is commonly the case that a service provider may enter
   into arrangements with two or more upstream transit providers,
   passing routes to all upstream providers, and receiving traffic from
   all sources.  If the path to one upstream fails, the traffic will
   switch to other links.  Once the path is recovered, the traffic
   should switch back.

   In such situations of multiple upstream providers it is also common
   to place a relative preference on the providers, so that one
   connection is regarded as a preferred, or "primary" connection, and
   other connections are regarded as less preferred, or "backup"
   connections.  The intent is typically that the backup connections
   will be used for traffic only for the duration of a failure in the
   primary connection.

Griffin & Huston             Informational                      [Page 2]
RFC 4264                      BGP Wedgies                  November 2005

   It is possible to express this primary / backup policy using local AS
   path prepending, where the AS path is artificially lengthened towards
   the backup providers, using additional instances of the local AS.
   This is not a deterministic selection algorithm, as the selected
   primary provider may in turn be using AS path prepending to its