AS-Path Prepend
draft-mcbride-grow-as-path-prepend-01

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Last updated 2020-07-26
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Network Working Group                                         M. McBride
Internet-Draft                                                 Futurewei
Intended status: Best Current Practice                         D. Madory
Expires: January 27, 2021                                         Oracle
                                                             J. Tantsura
                                                                  Apstra
                                                           July 26, 2020

                            AS-Path Prepend
                 draft-mcbride-grow-as-path-prepend-01

Abstract

   AS_Path prepending provides a tool to manipulate the BGP AS_Path
   attribute through prepending multiple entries of an AS.  AS_Path
   prepend is used to deprioritize a route or alternate path.  By
   prepending the local ASN multiple times, ASes can make advertised AS
   paths appear artificially longer.  Excessive AS_Path prepending has
   caused routing issues in the internet.  This document provides
   guidance,to the internet community, with how best to utilize AS_Path
   prepend in order to avoid negatively affecting the internet.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 27, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of

McBride, et al.         Expires January 27, 2021                [Page 1]
Internet-Draft               AS-Path Prepend                   July 2020

   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Problems  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Excessive Prepending  . . . . . . . . . . . . . . . . . .   3
     2.2.  Prepending during a routing leak  . . . . . . . . . . . .   3
     2.3.  Route Competition . . . . . . . . . . . . . . . . . . . .   4
     2.4.  Prepending to All . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Memory  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.6.  Errant announcement . . . . . . . . . . . . . . . . . . .   6
   3.  Best Practices  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Border Gateway Protocol (BGP) [RFC4271] specifies the AS_Path
   attribute which enumerates the ASs that must be traversed to reach
   the networks listed in the BGP UPDATE message.  If the UPDATE message
   is propagated over an external link, then the local AS number is
   prepended to the AS_PATH attribute, and the NEXT_HOP attribute is
   updated with an IP address of the router that should be used as a
   next hop to the network.  If the UPDATE message is propagated over an
   internal link, then the AS_PATH attribute and the NEXT_HOP attribute
   are passed unmodified.

   A common practice among operators is to prepend multiple entries of
   an AS (known as AS_Path prepend) in order to deprioritize a route or
   a path.  This has worked well in practice but the practice is
   increasing, with both IPv4 and IPv6, and there are inherit risks to
   the global internet especially with excessive AS_Path prepending.
   Prepending is frequently employed in an excessive manner such that it
   renders routes vulnerable to disruption or misdirection.  AS_Path
   prepending is discussed in Use of BGP Large Communities [RFC8195] and
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