Equal-Cost Multipath Considerations for BGP
draft-lapukhov-bgp-ecmp-considerations-04

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IDR Working Group                                            P. Lapukhov
Internet-Draft                                                  Facebook
Intended status: Informational                               J. Tantsura
Expires: November 18, 2020                                  Apstra, Inc.
                                                            May 17, 2020

              Equal-Cost Multipath Considerations for BGP
               draft-lapukhov-bgp-ecmp-considerations-04

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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   This Internet-Draft will expire on November 18, 2020.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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Lapukhov & Tantsura     Expires November 18, 2020               [Page 1]
Internet-Draft   draft-lapukhov-bgp-ecmp-considerations         May 2020

   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