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BGP MULTI_EXIT_DISC (MED) Considerations
RFC 4451

Network Working Group                                       D. McPherson
Request for Comments: 4451                          Arbor Networks, Inc.
Category: Informational                                          V. Gill
                                                                     AOL
                                                              March 2006

                BGP MULTI_EXIT_DISC (MED) Considerations

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 (2006).

Abstract

   The BGP MULTI_EXIT_DISC (MED) attribute provides a mechanism for BGP
   speakers to convey to an adjacent AS the optimal entry point into the
   local AS.  While BGP MEDs function correctly in many scenarios, a
   number of issues may arise when utilizing MEDs in dynamic or complex
   topologies.

   This document discusses implementation and deployment considerations
   regarding BGP MEDs and provides information with which implementers
   and network operators should be familiar.

McPherson & Gill             Informational                      [Page 1]
RFC 4451                 BGP MED Considerations               March 2006

Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................3
      2.1. About the MULTI_EXIT_DISC (MED) Attribute ..................3
      2.2. MEDs and Potatoes ..........................................5
   3. Implementation and Protocol Considerations ......................6
      3.1. MULTI_EXIT_DISC Is an Optional Non-Transitive Attribute ....6
      3.2. MED Values and Preferences .................................6
      3.3. Comparing MEDs between Different Autonomous Systems ........7
      3.4. MEDs, Route Reflection, and AS Confederations for BGP ......7
      3.5. Route Flap Damping and MED Churn ...........................8
      3.6. Effects of MEDs on Update Packing Efficiency ...............9
      3.7. Temporal Route Selection ...................................9
   4. Deployment Considerations ......................................10
      4.1. Comparing MEDs between Different Autonomous Systems .......10
      4.2. Effects of Aggregation on MEDs ............................11
   5. Security Considerations ........................................11
   6. Acknowledgements ...............................................11
   7. References .....................................................12
      7.1. Normative References ......................................12
      7.2. Informative References ....................................12

McPherson & Gill             Informational                      [Page 2]
RFC 4451                 BGP MED Considerations               March 2006

1.  Introduction

   The BGP MED attribute provides a mechanism for BGP speakers to convey
   to an adjacent AS the optimal entry point into the local AS.  While
   BGP MEDs function correctly in many scenarios, a number of issues may
   arise when utilizing MEDs in dynamic or complex topologies.

   While reading this document, note that the goal is to discuss both
   implementation and deployment considerations regarding BGP MEDs.  In
   addition, the intention is to provide guidance that both implementors
   and network operators should be familiar with.  In some instances,
   implementation advice varies from deployment advice.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.1.  About the MULTI_EXIT_DISC (MED) Attribute

   The BGP MULTI_EXIT_DISC (MED) attribute, formerly known as the
   INTER_AS_METRIC, is currently defined in section 5.1.4 of [BGP4], as
   follows:

      The MULTI_EXIT_DISC is an optional non-transitive attribute that
      is intended to be used on external (inter-AS) links to
      discriminate among multiple exit or entry points to the same
      neighboring AS.  The value of the MULTI_EXIT_DISC attribute is a
      four-octet unsigned number, called a metric.  All other factors
      being equal, the exit point with the lower metric SHOULD be
      preferred.  If received over External BGP (EBGP), the
      MULTI_EXIT_DISC attribute MAY be propagated over Internal BGP
      (IBGP) to other BGP speakers within the same AS (see also

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