Equal-Cost Multipath Considerations for BGP
draft-lapukhov-bgp-ecmp-considerations-02

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Network Working Group                                        P. Lapukhov
Internet-Draft                                                  Facebook
Intended status: Informational                               J. Tantsura
Expires: January 2, 2020                                    Apstra, Inc.
                                                            July 1, 2019

              Equal-Cost Multipath Considerations for BGP
               draft-lapukhov-bgp-ecmp-considerations-02

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 is 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.

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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. Informative References  . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 it is
   common to allow the selection of multiple "equal cost" paths for the
   same route.  Typical approach is to abort the tie-breaking process
   after comparing IGP cost for the NEXT_HOP attribute and select either
   all eBGP or all iBGP paths that remained "equal" under the tie-
   breaking rules.  See [BGPMP] for a vendor document explaining the
   logic.  In a nutshell, 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.  Commonly,
   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
   routing loop prevention.  Assuming that AS_PATHs length computed in
   this fashion are the same, many implementations require that the
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