Equal-Cost Multipath Considerations for BGP
draft-lapukhov-bgp-ecmp-considerations-00

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Network Working Group                                        P. Lapukhov
Internet-Draft                                                  Facebook
Intended status: Informational                          October 31, 2016
Expires: May 4, 2017

              Equal-Cost Multipath Considerations for BGP
               draft-lapukhov-bgp-ecmp-considerations-00

Abstract

   BGP routing protocol defined in ([RFC4271]) employs tie-breaking
   logic to elect single best path among multiple possible.  At the same
   time, it has been common in virtually all BGP implementations to
   allow for "equal-cost multipath" (ECMP) election and programming of
   multiple next-hops in routing tables.  This documents summarizes some
   common considerations for the ECMP logic, with the intent of
   providing common reference on otherwise unstandardized feature.

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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
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

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

2.  AS-PATH attribute comparison

   A mandatory requirement is for all paths that are candidates for ECMP
   selection to have the same AS_PATH length, computed using the
   standard 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.
   Assuming that AS_PATH lengths computed in this fashion are the same,
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