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Network Working Group                                       D. McPhersonRequest for Comments: 4451                          Arbor Networks, Inc.Category: Informational                                          V. Gill                                                                     AOL                                                              March 2006BGP MULTI_EXIT_DISC (MED) ConsiderationsStatus 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 2006Table of Contents1. Introduction ....................................................32. Specification of Requirements ...................................32.1. About the MULTI_EXIT_DISC (MED) Attribute ..................32.2. MEDs and Potatoes ..........................................53. Implementation and Protocol Considerations ......................63.1. MULTI_EXIT_DISC Is an Optional Non-Transitive Attribute ....63.2. MED Values and Preferences .................................63.3. Comparing MEDs between Different Autonomous Systems ........73.4. MEDs, Route Reflection, and AS Confederations for BGP ......73.5. Route Flap Damping and MED Churn ...........................83.6. Effects of MEDs on Update Packing Efficiency ...............93.7. Temporal Route Selection ...................................94. Deployment Considerations ......................................104.1. Comparing MEDs between Different Autonomous Systems .......104.2. Effects of Aggregation on MEDs ............................115. Security Considerations ........................................116. Acknowledgements ...............................................117. References .....................................................127.1. Normative References ......................................127.2. Informative References ....................................12McPherson & Gill             Informational                      [Page 2]

RFC 4451                 BGP MED Considerations               March 20061.  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 theMcPherson & 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).  SeeSection 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 notMcPherson & 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, manyMcPherson & 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 rewriteMcPherson & 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 insection 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 20063.6.  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 20064.  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 20064.2.  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 20067.  References7.1.  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.comMcPherson & Gill             Informational                     [Page 12]

RFC 4451                 BGP MED Considerations               March 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).McPherson & Gill             Informational                     [Page 13]