The Accumulated IGP Metric Attribute for BGP
RFC 7311

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Internet Engineering Task Force (IETF)                      P. Mohapatra
Request for Comments: 7311                              Sproute Networks
Category: Standards Track                                    R. Fernando
ISSN: 2070-1721                                                 E. Rosen
                                                     Cisco Systems, Inc.
                                                               J. Uttaro
                                                                    AT&T
                                                             August 2014

              The Accumulated IGP Metric Attribute for BGP

Abstract

   Routing protocols that have been designed to run within a single
   administrative domain (IGPs) generally do so by assigning a metric to
   each link and then choosing, as the installed path between two nodes,
   the path for which the total distance (sum of the metric of each link
   along the path) is minimized.  BGP, designed to provide routing over
   a large number of independent administrative domains (autonomous
   systems), does not make its path-selection decisions through the use
   of a metric.  It is generally recognized that any attempt to do so
   would incur significant scalability problems as well as inter-
   administration coordination problems.  However, there are deployments
   in which a single administration runs several contiguous BGP
   networks.  In such cases, it can be desirable, within that single
   administrative domain, for BGP to select paths based on a metric,
   just as an IGP would do.  The purpose of this document is to provide
   a specification for doing so.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc7311.

Mohapatra, et al.            Standards Track                    [Page 1]
RFC 7311              AIGP Metric Attribute for BGP          August 2014

Copyright Notice

   Copyright (c) 2014 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. Specification of Requirements ...................................4
   3. AIGP Attribute ..................................................4
      3.1. Applicability Restrictions and Cautions ....................6
      3.2. Handling a Malformed AIGP Attribute ........................6
      3.3. Restrictions on Sending/Receiving ..........................6
      3.4. Creating and Modifying the AIGP Attribute ..................7
           3.4.1. Originating the AIGP Attribute ......................7
           3.4.2. Modifications by the Originator .....................8
           3.4.3. Modifications by a Non-Originator ...................8
   4. Decision Process ...............................................10
      4.1. When a Route Has an AIGP Attribute ........................11
      4.2. When the Route to the Next Hop Has an AIGP Attribute ......11
   5. Deployment Considerations ......................................12
   6. IANA Considerations ............................................13
   7. Security Considerations ........................................13
   8. Acknowledgments ................................................13
   9. References .....................................................14
      9.1. Normative Reference .......................................14
      9.2. Informative References ....................................14

Mohapatra, et al.            Standards Track                    [Page 2]
RFC 7311              AIGP Metric Attribute for BGP          August 2014

1.  Introduction

   There are many routing protocols that have been designed to run
   within a single administrative domain.  These are known collectively
   as "Interior Gateway Protocols" (IGPs).  Typically, each link is
   assigned a particular "metric" value.  The path between two nodes can