Equal-Cost Multipath Considerations for BGP
draft-lapukhov-bgp-ecmp-considerations-05

Versions: 00 01 02 03 04 05                                             
IDR Working Group                                            P. Lapukhov
Internet-Draft                                                  Facebook
Intended status: Informational                               J. Tantsura
Expires: May 19, 2021                                       Apstra, Inc.
                                                       November 15, 2020


              Equal-Cost Multipath Considerations for BGP
               draft-lapukhov-bgp-ecmp-considerations-05

Abstract

   BGP (Border Gateway Protocol) [RFC4271] employs tie-breaking logic to
   select a single best path among multiple paths available, known as
   BGP best path selection.  At the same time, it has become a common
   practice to allow for "equal-cost multipath" (ECMP) selection and
   programming of multiple next-hops in routing tables.  This document
   summarizes some common considerations for the ECMP logic when BGP is
   used as the routing protocol, with the intent of providing common
   reference for otherwise unstandardized set of features.

Status of This Memo

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  AS-PATH attribute comparison  . . . . . . . . . . . . . . . .   2
   3.  Multipath among eBGP-learned paths  . . . . . . . . . . . . .   3
   4.  Multipath among iBGP learned paths  . . . . . . . . . . . . .   3
   5.  Multipath among eBGP and iBGP paths . . . . . . . . . . . . .   4
   6.  Multipath with AIGP . . . . . . . . . . . . . . . . . . . . .   5
   7.  Best path advertisement . . . . . . . . . . . . . . . . . . .   5
   8.  Multipath and non-deterministic tie-breaking  . . . . . . . .   5
   9.  Weighted equal-cost multipath . . . . . . . . . . . . . . . .   5
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   11. Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Section 9.1.2.2 of [RFC4271] defines step-by-step tie-breaking
   procedure for selecting a single "best-path" among multiple
   alternatives available for the same route.  In order to improve
   efficiency, in densely meshed symmetric network topologies is has
   become a common practice to allow selection of multiple "equal" paths
   for the same route.  Most commonly used approach is to abort the tie-
   breaking process after comparing IGP cost for the NEXT_HOP attribute
   and selecting either all eBGP or all iBGP paths that remained "equal"
   under the tie-breaking rules (see [BGPMP] for a vendor document
   explaining the logic).  Basically, the steps that compare the BGP
   identifiers and BGP peer IP addresses (steps (f) and (g) in
   [RFC4271]) are ignored for the purpose of multipath routing.  BGP
   implementations commonly have a configuration knob that specifies the
   maximum number of equal paths that are allowed be programmed in the
   routing table.  Commonnly, there's also a knob to enable multipath
   separately for iBGP-learned or eBGP-learned paths.

2.  AS-PATH attribute comparison

   The mandatory requirement for all paths that are considered as the
   candidates for ECMP selection is to have the same AS_PATH length,
   computed using the logic defined in [RFC4271] and [RFC5065], i.e.
   ignoring the AS_SET, AS_CONFED_SEQUENCE, and AS_CONFED_SET segment
   lengths.  The content of the latter attributes is used purely for
   loop detection and prevention.  Assuming that AS_PATHs length
   computed in this fashion are the same, many implementations require



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   that the content of AS_SEQUENCE segment MUST be the same among all
   the paths considered.  Two common configuration knobs to alter this
   behaviour are usually provided: One, to relax otherwise mandatory
   AS_SEQUENCE comparison rule, enforcing only the AS_PATH length rule,
   while ignoring the content of AS_SEQUENCE.  And another requiring
   that the first AS numbers in first AS_SEQUENCE segment found in
   AS_PATH (often referred to as "peer AS" number) be the same as the
   one found in best path (determined by running the full tie-breaking
   procedure).  This document refers to those two as "multipath as-path
   relaxed" and "multipath same peer-as".

3.  Multipath among eBGP-learned paths

   Step (d) in Section 9.1.2.2 of [RFC4271] mandates, in presence of an
   eBGP path to remove all iBGP paths from the the ECMP candidates set.
   This leaves the BGP tie-breaking procedure with just eBGP paths.  At
   this point, the mandatory BGP NEXT_HOP attribute value most commonly
   belongs to the IP subnet that the BGP speaker shares with the
   advertising neighbor.  In this case, it is common for implementations
   to treat all NEXT_HOP values as having the same "internal cost" to
   reach them per the guidance of step (e) of Section 9.1.2.2.  In some
   cases, either static routing or an IGP routing protocol could be
   running between the BGP speakers peering over eBGP session.  An
   implementation may use the metric discovered from the above sources
   to perform tie-breaking even for eBGP paths.

   Notice that in case when, in some paths MED attribute is present, the
   set of multipath routes allowed will most likely be reduced to the
   ones coming from the same peer AS, per step (c) of Section 9.1.2.2.
   This is unless an implementation provides a configuration knob to
   always compare MED attributes across all paths, as recommended by
   [RFC4451].  In the latter case, the presence of MED attribute does
   not automatically reduce the candidate path set to the same peer AS
   only.

4.  Multipath among iBGP learned paths

   When all paths for a prefix are learned via iBGP, since in most cases
   iBGP is used along with an underlying IGP, the tie-breaking commonly
   occurs based on IGP metric of the NEXT_HOP attribute.  In some
   implementations, it is however possible to ignore the IGP cost as
   well, if all of the paths are reachable via some kind of tunneling
   mechanism, such as MPLS [RFC3031].  This is enabled via a knob
   referred in this document as "skip igp check" . Notice that there is
   no standard way for a BGP speaker to detect presence of such
   tunneling techniques other than relying on the configuration
   settings.




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   When iBGP is deployed with BGP route-reflectors per [RFC4456] the
   path attribute list may include the CLUSTER_LIST attribute.  Most
   implementations commonly ignore it for the purpose of ECMP route
   selection, assuming that IGP cost along should be sufficient for loop
   prevention.  This assumption may not hold when IGP is not deployed,
   and instead iBGP session are configured to reset the NEXT_HOP
   attribute to self on every node (this also assumes the use of
   directly connected link addresses for session formation).  In this
   case, ignoring CLUSTER_LIST length might lead to routing loops.  It
   is therefore recommended for implementations to have a knob that
   enables accounting for CLUSTER_LIST length when performing multipath
   route selection.  In this case, CLUSTER_LIST attribute length should
   be effectively used to replace the IGP metric.

   Similarly to the route-reflector scenario, the use of BGP
   confederations in multipath scenarios assumes presence of an IGP for
   proper loop prevention and use the IGP metric as the final tie-
   breaker for multipath routing.  In addition to that, and similar to
   eBGP case, implementations often require that in order to be
   considered equal, paths under consideration must belong to the same
   peer member AS as the best-path.  It is useful to have the following
   two configuration knobs, one enabling "multipath same confederation
   member peer-as" and another enabling less restrictive "confed as-path
   multipath relaxed" rule, that allow selecting multipath routes
   reachable via any confederation member peer AS.  As mentioned above,
   the AS_CONFED_SEQUENCE value length is usually ignored for the
   purpose of AS_PATH length comparison, for the loop prevention relying
   instead on the IGP cost .

   In cases, when IGP is not present with BGP confederation deployment,
   and similar to route-reflection case, it may be nessesary to consider
   AS_CONFED_SEQUENCE length when selecting the equivalent routes,
   effectively using it as a substitution for an IGP metric.  A separate
   configuration knob is needed to allow this behavior.

   Per [RFC5065] paths learned over BGP intra-confederation peering
   sessions are treated as iBGP.  There is no specification or
   operational document that defines how a mixed iBGP route-reflector
   and confederation based deplyments would work together.  Therefore,
   this document does not make recommendations for the above case.

5.  Multipath among eBGP and iBGP paths

   The best-path selection algorithm explicitly prefers eBGP paths over
   iBGP (or learned from BGP confederation member AS, which is, as per
   [RFC5065] treated the same as iBGP from perspective of best-path
   selection).  In some cases however, it might be beneficial to allow
   multipath routing between eBGP and iBGP learned paths.  This is only



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   possible if some sort of tunneling technique is used to reach both
   the eBGP and iBGP paths.  If this feature is enabled, the equal
   routes are selected prior to the MED comparison step (c) in
   Section 9.1.2.2 [RFC4271].

6.  Multipath with AIGP

   AIGP attribute defined in [RFC7311] must be used for best-path
   selection prior to running any logic of Section 9.1.2.2 [RFC4271].
   Only the paths with minimal value of AIGP metric are eligible for
   further consideration of tie-breaking rules.  The rest of multipath
   selection logic remains the same.

7.  Best path advertisement

   Unless BGP "Add-Path" feature as described in [RFC7911] is enabled
   and even though multiple equal paths may be selected for programming
   into the routing table, a BGP speaker announces to its peers single
   best-path only.  The unique best-path is elected among the multi-path
   set using the standard tie-breaking rules.

8.  Multipath and non-deterministic tie-breaking

   Some implementations may implement non-standard tie-breaking logic,
   for example using the oldest path rule, IETF reference - [RFC5004], a
   vendor implementaion example [BGPMP].  This is generally not
   recommended, and may interact with multi-path route selection on
   downstream BGP speakers.  That is, after a route flap that affects
   the best-path upstream, the original best path would not be
   recovered, and the older path would still be advertised, possibly
   affecting the tie-breaking rules on down-stream device if for
   example, the AS_PATH contents are different from previous.  Another
   side effect of using non-standard tie-breaking could be increased
   number of BGP Next-Hop sets for Prefixes learned from eBGP neighbors
   and advertised downstream towards iBGP Neighbors.  This could
   potentially cause ECMP group/entry tables to overrun (depending on a
   platform) as the prefixes will be less coalesced.

9.  Weighted equal-cost multipath

   The proposal in [I-D.ietf-idr-link-bandwidth] defines conditions
   where iBGP multipath feature might inform the routing table of
   "weights" associated with the multiple external paths.
   [I-D.ietf-idr-link-bandwidth] defines the weight extended community
   attribute as non-transitive, considers the applicability for iBGP
   only, though there are implementations that apply it to eBGP as well.
   The proposal does not change the equal-cost multipath selection




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   logic, only associates additional load-sharing attributes with
   equivalent paths.

10.  Acknowledgements

   We like to thank Diptanshu Singh for their reviews and valuable
   comments.

11.  Informative References

   [BGPMP]    "BGP Best Path Selection Algorithm",
              <http://www.cisco.com/c/en/us/support/docs/ip/border-
              gateway-protocol-bgp/13753-25.html>.

   [I-D.ietf-idr-link-bandwidth]
              Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
              Extended Community", draft-ietf-idr-link-bandwidth-07
              (work in progress), March 2018.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4451]  McPherson, D. and V. Gill, "BGP MULTI_EXIT_DISC (MED)
              Considerations", RFC 4451, DOI 10.17487/RFC4451, March
              2006, <https://www.rfc-editor.org/info/rfc4451>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [RFC5004]  Chen, E. and S. Sangli, "Avoid BGP Best Path Transitions
              from One External to Another", RFC 5004,
              DOI 10.17487/RFC5004, September 2007,
              <https://www.rfc-editor.org/info/rfc5004>.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <https://www.rfc-editor.org/info/rfc5065>.




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   [RFC7311]  Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro,
              "The Accumulated IGP Metric Attribute for BGP", RFC 7311,
              DOI 10.17487/RFC7311, August 2014,
              <https://www.rfc-editor.org/info/rfc7311>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

Authors' Addresses

   Petr Lapukhov
   Facebook
   1 Hacker Way
   Menlo Park, CA  94025
   US

   Email: petr@fb.com


   Jeff Tantsura
   Apstra, Inc.
   333 Middlefield Rd #200
   Menlo Park, CA  94025
   US

   Email: jefftant.ietf@gmail.com























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