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