Internet DRAFT - draft-lapukhov-bgp-ecmp-considerations
draft-lapukhov-bgp-ecmp-considerations
IDR Working Group P. Lapukhov
Internet-Draft J. Tantsura
Intended status: Informational Nvidia
Expires: 30 June 2024 28 December 2023
Equal-Cost Multipath Considerations for BGP
draft-lapukhov-bgp-ecmp-considerations-12
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 30 June 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. AS-PATH attribute comparison . . . . . . . . . . . . . . . . 3
3. Multipath among eBGP-learned paths . . . . . . . . . . . . . 3
4. Multipath among iBGP learned paths . . . . . . . . . . . . . 4
5. Multipath among eBGP and iBGP paths . . . . . . . . . . . . . 5
6. Multipath with AIGP . . . . . . . . . . . . . . . . . . . . . 5
7. Best path advertisement . . . . . . . . . . . . . . . . . . . 5
8. Multipath and non-deterministic tie-breaking . . . . . . . . 5
9. Weighted equal-cost multipath . . . . . . . . . . . . . . . . 6
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.
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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 (counting an AS_SET as one hop regardless of how
many ASes are in the 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
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 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.
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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.
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 necessary 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.
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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 deployments 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
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 implementation 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
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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
logic, only associates additional load-sharing attributes with
equivalent paths.
10. Acknowledgements
We like to thank Diptanshu Singh, Jakob Heitz and Melchior Aelmans
for their reviews and valuable comments.
11. Informative References
[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>.
[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>.
[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>.
[I-D.ietf-idr-link-bandwidth]
Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", Work in Progress, Internet-Draft,
draft-ietf-idr-link-bandwidth-07, 5 March 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
link-bandwidth-07>.
[BGPMP] "BGP Best Path Selection Algorithm",
<http://www.cisco.com/c/en/us/support/docs/ip/border-
gateway-protocol-bgp/13753-25.html>.
Authors' Addresses
Petr Lapukhov
Nvidia
Email: plapukhov@nvidia.com
Jeff Tantsura
Nvidia
United States of America
Email: jefftant.ietf@gmail.com
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