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RFC 4451 - BGP MULTI_EXIT_DISC (MED) Considerations


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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.

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

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
      9.1.2.2).  The MULTI_EXIT_DISC attribute received from a
      neighboring AS MUST NOT be propagated to other neighboring ASes.

      A BGP speaker MUST implement a mechanism (based on local
      configuration) that allows the MULTI_EXIT_DISC attribute to be
      removed from a route.  If a BGP speaker is configured to remove
      the MULTI_EXIT_DISC attribute from a route, then this removal MUST
      be done prior to determining the degree of preference of the route
      and prior to performing route selection (Decision Process phases 1
      and 2).

      An implementation MAY also (based on local configuration) alter
      the value of the MULTI_EXIT_DISC attribute received over EBGP.  If
      a BGP speaker is configured to alter the value of the

      MULTI_EXIT_DISC attribute received over EBGP, then altering the
      value MUST be done prior to determining the degree of preference
      of the route and prior to performing route selection (Decision
      Process phases 1 and 2).  See Section 9.1.2.2 for necessary
      restrictions on this.

   Section 9.1.2.2 (c) of [BGP4] defines the following route selection
   criteria regarding MEDs:

      c) Remove from consideration routes with less-preferred
         MULTI_EXIT_DISC attributes.  MULTI_EXIT_DISC is only comparable
         between routes learned from the same neighboring AS (the
         neighboring AS is determined from the AS_PATH attribute).
         Routes that do not have the MULTI_EXIT_DISC attribute are
         considered to have the lowest possible MULTI_EXIT_DISC value.

         This is also described in the following procedure:

       for m = all routes still under consideration
           for n = all routes still under consideration
               if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
                   remove route m from consideration

         In the pseudo-code above, MED(n) is a function that returns the
         value of route n's MULTI_EXIT_DISC attribute.  If route n has
         no MULTI_EXIT_DISC attribute, the function returns the lowest
         possible MULTI_EXIT_DISC value (i.e., 0).

         Similarly, neighborAS(n) is a function that returns the
         neighbor AS from which the route was received.  If the route is
         learned via IBGP, and the other IBGP speaker didn't originate
         the route, it is the neighbor AS from which the other IBGP
         speaker learned the route.  If the route is learned via IBGP,
         and the other IBGP speaker either (a) originated the route, or
         (b) created the route by aggregation and the AS_PATH attribute
         of the aggregate route is either empty or begins with an
         AS_SET, it is the local AS.

         If a MULTI_EXIT_DISC attribute is removed before re-advertising
         a route into IBGP, then comparison based on the received EBGP
         MULTI_EXIT_DISC attribute MAY still be performed.  If an
         implementation chooses to remove MULTI_EXIT_DISC, then the
         optional comparison on MULTI_EXIT_DISC, if performed, MUST be
         performed only among EBGP-learned routes.  The best EBGP-
         learned route may then be compared with IBGP-learned routes
         after the removal of the MULTI_EXIT_DISC attribute.  If
         MULTI_EXIT_DISC is removed from a subset of EBGP-learned
         routes, and the selected "best" EBGP-learned route will not

         have MULTI_EXIT_DISC removed, then the MULTI_EXIT_DISC must be
         used in the comparison with IBGP-learned routes.  For IBGP-
         learned routes, the MULTI_EXIT_DISC MUST be used in route
         comparisons that reach this step in the Decision Process.
         Including the MULTI_EXIT_DISC of an EBGP-learned route in the
         comparison with an IBGP-learned route, then removing the
         MULTI_EXIT_DISC attribute, and advertising the route has been
         proven to cause route loops.

2.2.  MEDs and Potatoes

   Let's consider a situation where traffic flows between a pair of
   hosts, each connected to a different transit network, which is in
   itself interconnected at two or more locations.  Each transit network
   has the choice of either sending traffic to the closest peering to
   the adjacent transit network or passing traffic to the
   interconnection location that advertises the least-cost path to the
   destination host.

   The former method is called "hot potato routing" (or closest-exit)
   because like a hot potato held in bare hands, whoever has it tries to
   get rid of it quickly.  Hot potato routing is accomplished by not
   passing the EBGP-learned MED into IBGP.  This minimizes transit
   traffic for the provider routing the traffic.  Far less common is
   "cold potato routing" (or best-exit) where the transit provider uses
   its own transit capacity to get the traffic to the point that
   adjacent transit provider advertised as being closest to the
   destination.  Cold potato routing is accomplished by passing the
   EBGP-learned MED into IBGP.

   If one transit provider uses hot potato routing and another uses cold
   potato, traffic between the two tends to be more symmetric.  However,
   if both providers employ cold potato routing or hot potato routing
   between their networks, it's likely that a larger amount of asymmetry
   would exist.

   Depending on the business relationships, if one provider has more
   capacity or a significantly less congested backbone network, then
   that provider may use cold potato routing.  An example of widespread
   use of cold potato routing was the NSF-funded NSFNET backbone and
   NSF-funded regional networks in the mid-1990s.

   In some cases, a provider may use hot potato routing for some
   destinations for a given peer AS and cold potato routing for others.
   An example of this is the different treatment of commercial and
   research traffic in the NSFNET in the mid-1990s.  Today, many

   commercial networks exchange MEDs with customers but not with
   bilateral peers.  However, commercial use of MEDs varies widely, from
   ubiquitous use to none at all.

   In addition, many deployments of MEDs today are likely behaving
   differently (e.g., resulting in sub-optimal routing) than the network
   operator intended, which results not in hot or cold potatoes, but
   mashed potatoes!  More information on unintended behavior resulting
   from MEDs is provided throughout this document.

3.  Implementation and Protocol Considerations

   There are a number of implementation and protocol peculiarities
   relating to MEDs that have been discovered that may affect network
   behavior.  The following sections provide information on these
   issues.

3.1.  MULTI_EXIT_DISC Is an Optional Non-Transitive Attribute

   MULTI_EXIT_DISC is a non-transitive optional attribute whose
   advertisement to both IBGP and EBGP peers is discretionary.  As a
   result, some implementations enable sending of MEDs to IBGP peers by
   default, while others do not.  This behavior may result in sub-
   optimal route selection within an AS.  In addition, some
   implementations send MEDs to EBGP peers by default, while others do
   not.  This behavior may result in sub-optimal inter-domain route
   selection.

3.2.  MED Values and Preferences

   Some implementations consider an MED value of zero less preferable
   than no MED value.  This behavior resulted in path selection
   inconsistencies within an AS.  The current version of the BGP
   specification [BGP4] removes ambiguities that existed in [RFC1771] by
   stating that if route n has no MULTI_EXIT_DISC attribute, the lowest
   possible MULTI_EXIT_DISC value (i.e., 0) should be assigned to the
   attribute.

   It is apparent that different implementations and different versions
   of the BGP specification have been all over the map with
   interpretation of missing-MED.  For example, earlier versions of the
   specification called for a missing MED to be assigned the highest
   possible MED value (i.e., 2^32-1).

   In addition, some implementations have been shown to internally
   employ a maximum possible MED value (2^32-1) as an "infinity" metric
   (i.e., the MED value is used to tag routes as unfeasible); upon
   receiving an update with an MED value of 2^32-1, they would rewrite

   the value to 2^32-2.  Subsequently, the new MED value would be
   propagated and could result in routing inconsistencies or unintended
   path selections.

   As a result of implementation inconsistencies and protocol revision
   variances, many network operators today explicitly reset (i.e., set
   to zero or some other 'fixed' value) all MED values on ingress to
   conform to their internal routing policies (i.e., to include policy
   that requires that MED values of 0 and 2^32-1 not be used in
   configurations, whether the MEDs are directly computed or
   configured), so as not to have to rely on all their routers having
   the same missing-MED behavior.

   Because implementations don't normally provide a mechanism to disable
   MED comparisons in the decision algorithm, "not using MEDs" usually
   entails explicitly setting all MEDs to some fixed value upon ingress
   to the routing domain.  By assigning a fixed MED value consistently
   to all routes across the network, MEDs are a effectively a non-issue
   in the decision algorithm.

3.3.  Comparing MEDs between Different Autonomous Systems

   The MED was intended to be used on external (inter-AS) links to
   discriminate among multiple exit or entry points to the same
   neighboring AS.  However, a large number of MED applications now
   employ MEDs for the purpose of determining route preference between
   like routes received from different autonomous systems.

   A large number of implementations provide the capability to enable
   comparison of MEDs between routes received from different neighboring
   autonomous systems.  While this capability has demonstrated some
   benefit (e.g., that described in [RFC3345]), operators should be wary
   of the potential side effects of enabling such a function.  The
   deployment section below provides some examples as to why this may
   result in undesirable behavior.

3.4.  MEDs, Route Reflection, and AS Confederations for BGP

   In particular configurations, the BGP scaling mechanisms defined in
   "BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC2796]
   and "Autonomous System Confederations for BGP" [RFC3065] will
   introduce persistent BGP route oscillation [RFC3345].  The problem is
   inherent in the way BGP works: a conflict exists between information
   hiding/hierarchy and the non-hierarchical selection process imposed
   by lack of total ordering caused by the MED rules.  Given current
   practices, we see the problem manifest itself most frequently in the
   context of MED + route reflectors or confederations.

   One potential way to avoid this is by configuring inter-Member-AS or
   inter-cluster IGP metrics higher than intra-Member-AS IGP metrics
   and/or using other tie-breaking policies to avoid BGP route selection
   based on incomparable MEDs.  Of course, IGP metric constraints may be
   unreasonably onerous for some applications.

   Not comparing MEDs between multiple paths for a prefix learned from
   different adjacent autonomous systems, as discussed in section 2.3,
   or not utilizing MEDs at all, significantly decreases the probability
   of introducing potential route oscillation conditions into the
   network.

   Although perhaps "legal" as far as current specifications are
   concerned, modifying MED attributes received on any type of IBGP
   session (e.g., standard IBGP, EBGP sessions between Member-ASes of a
   BGP confederation, route reflection, etc.) is not recommended.

3.5.  Route Flap Damping and MED Churn

   MEDs are often derived dynamically from IGP metrics or additive costs
   associated with an IGP metric to a given BGP NEXT_HOP.  This
   typically provides an efficient model for ensuring that the BGP MED
   advertised to peers, used to represent the best path to a given
   destination within the network, is aligned with that of the IGP
   within a given AS.

   The consequence with dynamically derived IGP-based MEDs is that
   instability within an AS, or even on a single given link within the
   AS, can result in widespread BGP instability or BGP route
   advertisement churn that propagates across multiple domains.  In
   short, if your MED "flaps" every time your IGP metric flaps, your
   routes are likely going to be suppressed as a result of BGP Route
   Flap Damping [RFC2439].

   Employment of MEDs may compound the adverse effects of BGP flap-
   dampening behavior because it may cause routes to be re-advertised
   solely to reflect an internal topology change.

   Many implementations don't have a practical problem with IGP
   flapping; they either latch their IGP metric upon first advertisement
   or employ some internal suppression mechanism.  Some implementations
   regard BGP attribute changes as less significant than route
   withdrawals and announcements to attempt to mitigate the impact of
   this type of event.

3.6.  Effects of MEDs on Update Packing Efficiency

   Multiple unfeasible routes can be advertised in a single BGP Update
   message.  The BGP4 protocol also permits advertisement of multiple
   prefixes with a common set of path attributes to be advertised in a
   single update message.  This is commonly referred to as "update
   packing".  When possible, update packing is recommended as it
   provides a mechanism for more efficient behavior in a number of
   areas, including the following:

      o Reduction in system overhead due to generation or receipt of
        fewer Update messages.

      o Reduction in network overhead as a result of fewer packets and
        lower bandwidth consumption.

      o Less frequent processing of path attributes and searches for
        matching sets in your AS_PATH database (if you have one).
        Consistent ordering of the path attributes allows for ease of
        matching in the database as you don't have different
        representations of the same data.

   Update packing requires that all feasible routes within a single
   update message share a common attribute set, to include a common
   MULTI_EXIT_DISC value.  As such, potential wide-scale variance in MED
   values introduces another variable and may result in a marked
   decrease in update packing efficiency.

3.7.  Temporal Route Selection

   Some implementations had bugs that led to temporal behavior in
   MED-based best path selection.  These usually involved methods to
   store the oldest route and to order routes for MED, which caused
   non-deterministic behavior as to whether or not the oldest route
   would truly be selected.

   The reasoning for this is that older paths are presumably more
   stable, and thus preferable.  However, temporal behavior in route
   selection results in non-deterministic behavior and, as such, is
   often undesirable.

4.  Deployment Considerations

   It has been discussed that accepting MEDs from other autonomous
   systems has the potential to cause traffic flow churns in the
   network.  Some implementations only ratchet down the MED and never
   move it back up to prevent excessive churn.

   However, if a session is reset, the MEDs being advertised have the
   potential of changing.  If a network is relying on received MEDs to
   route traffic properly, the traffic patterns have the potential for
   changing dramatically, potentially resulting in congestion on the
   network.  Essentially, accepting and routing traffic based on MEDs
   allows other people to traffic engineer your network.  This may or
   may not be acceptable to you.

   As previously discussed, many network operators choose to reset MED
   values on ingress.  In addition, many operators explicitly do not
   employ MED values of 0 or 2^32-1 in order to avoid inconsistencies
   with implementations and various revisions of the BGP specification.

4.1.  Comparing MEDs between Different Autonomous Systems

   Although the MED was meant to be used only when comparing paths
   received from different external peers in the same AS, many
   implementations provide the capability to compare MEDs between
   different autonomous systems as well.  AS operators often use
   LOCAL_PREF to select the external preferences (primary, secondary
   upstreams, peers, customers, etc.), using MED instead of LOCAL_PREF
   would possibly lead to an inconsistent distribution of best routes,
   as MED is compared only after the AS_PATH length.

   Though this may seem like a fine idea for some configurations, care
   must be taken when comparing MEDs between different autonomous
   systems.  BGP speakers often derive MED values by obtaining the IGP
   metric associated with reaching a given BGP NEXT_HOP within the local
   AS.  This allows MEDs to reasonably reflect IGP topologies when
   advertising routes to peers.  While this is fine when comparing MEDs
   between multiple paths learned from a single AS, it can result in
   potentially "weighted" decisions when comparing MEDs between
   different autonomous systems.  This is most typically the case when
   the autonomous systems use different mechanisms to derive IGP metrics
   or BGP MEDs, or when they perhaps even use different IGP protocols
   with vastly contrasting metric spaces (e.g., OSPF vs. traditional
   metric space in IS-IS).

4.2.  Effects of Aggregation on MEDs

   Another MED deployment consideration involves the impact that
   aggregation of BGP routing information has on MEDs.  Aggregates are
   often generated from multiple locations in an AS in order to
   accommodate stability, redundancy, and other network design goals.
   When MEDs are derived from IGP metrics associated with said
   aggregates, the MED value advertised to peers can result in very
   suboptimal routing.

5.  Security Considerations

   The MED was purposely designed to be a "weak" metric that would only
   be used late in the best-path decision process.  The BGP working
   group was concerned that any metric specified by a remote operator
   would only affect routing in a local AS if no other preference was
   specified.  A paramount goal of the design of the MED was to ensure
   that peers could not "shed" or "absorb" traffic for networks that
   they advertise.  As such, accepting MEDs from peers may in some sense
   increase a network's susceptibility to exploitation by peers.

6.  Acknowledgements

   Thanks to John Scudder for applying his usual keen eye and
   constructive insight.  Also, thanks to Curtis Villamizar, JR
   Mitchell, and Pekka Savola for their valuable feedback.

7.  References

7.1.  Normative References

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-
              4)", RFC 1771, March 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2796]  Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection
              - An Alternative to Full Mesh IBGP", RFC 2796, April 2000.

   [RFC3065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 3065, February 2001.

   [BGP4]     Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

7.2.  Informative References

   [RFC2439]  Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
              Flap Damping", RFC 2439, November 1998.

   [RFC3345]  McPherson, D., Gill, V., Walton, D., and A. Retana,
              "Border Gateway Protocol (BGP) Persistent Route
              Oscillation Condition", RFC 3345, August 2002.

Authors' Addresses

   Danny McPherson
   Arbor Networks

   EMail: danny@arbor.net

   Vijay Gill
   AOL

   EMail: VijayGill9@aol.com

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