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rfc:rfc4451

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
    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

McPherson & Gill Informational [Page 3] RFC 4451 BGP MED Considerations March 2006

    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

McPherson & Gill Informational [Page 4] RFC 4451 BGP MED Considerations March 2006

       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

McPherson & Gill Informational [Page 5] RFC 4451 BGP MED Considerations March 2006

 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

McPherson & Gill Informational [Page 6] RFC 4451 BGP MED Considerations March 2006

 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.

McPherson & Gill Informational [Page 7] RFC 4451 BGP MED Considerations March 2006

 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.

McPherson & Gill Informational [Page 8] RFC 4451 BGP MED Considerations March 2006

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.

McPherson & Gill Informational [Page 9] RFC 4451 BGP MED Considerations March 2006

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).

McPherson & Gill Informational [Page 10] RFC 4451 BGP MED Considerations March 2006

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.

McPherson & Gill Informational [Page 11] RFC 4451 BGP MED Considerations March 2006

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

McPherson & Gill Informational [Page 12] RFC 4451 BGP MED Considerations March 2006

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McPherson & Gill Informational [Page 13]

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