Network Working Group P. Lapukhov
Internet-Draft Facebook
Intended status: Informational October 30, 2017
Expires: May 3, 2018
Equal-Cost Multipath Considerations for BGP
draft-lapukhov-bgp-ecmp-considerations-01
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 all practical BGP implementations to
allow for "equal-cost multipath" (ECMP) path election and programming
of multiple next-hops in routing tables. This documents provides
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 selecting multiple "equivalent" paths
for the same prefix. Most commonly used approach is to stop 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,
many implementations require that content of AS_SEQUENCE segment MUST
be the same among all equivalent paths. Two common configuration
knobs are usually provided: one allowing only the length of AS_PATH
to be the same, and another requiring that the first AS numbers in
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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 algorithm). This
document refer to those two as "multipath as-path relaxed" and
"multipath same peer-as" knobs.
3. Multipath among eBGP-learned paths
Step (d) in Section 9.1.2.2 of [RFC4271] instructs to remove all iBGP
paths from considerations if an eBGP path is present in the candidate
set. This leaves the BGP process 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 advertising
neighbor. In this case, it is common for implementation 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.
In case when MED attribute is present in some paths, the set of
allowed multipath routes 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 the implementation provided a configuration knob to always
compare MED attributes across all paths, as recommended in [RFC4451].
In the latter case, the presence of MED attribute does not narrow the
candidate path set only to the same peer AS.
4. Multipath among iBGP learned paths
When all paths for a prefix are learned via iBGP, the tie-breaking
commonly occurs based on IGP metric of the NEXT_HOP attribute, since
in most cases iBGP is used along with an underlying IGP. It is
possible, in some implementations, 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 to as
"skip igp check" in this document. Notice that there is no standard
way for a BGP speaker to detect presence of such tunneling techniques
other than relying on 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 on every node (this also assumes the use of directly
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connected link IP 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.
Similar to the route-reflector scenario, the use of BGP
confederations assumes presence of an IGP for proper loop prevention
in multipath scenarios, and use the IGP metric as the final tie-
breaker for multipath routing. In addition to this, and similar to
eBGP case, implementation often require that equivalent paths belong
to the same peer member AS as the best-path. It is useful to have
two configuration knobs, one enabling "multipath same confederation
member peer-as" and another enabling less restrictive "confed as-path
multipath relaxed", which allows selecting multipath routes going 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, relying on IGP cost instead for loop
prevention.
In case if IGP is not present with BGP confederation deployment, and
similar to route-reflection case, it may be needed to consider
AS_CONFED_SEQUENCE length when selecting the equivalent routes,
effectively using it as a substitution for IGP metric. A separate
configuration knob is needed to allow this behavior.
Per [RFC5065] the path 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 model would work together. Therefore, this
document does not make recommendations or considers this 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 per
[RFC5065] is treated the same as iBGP from perspective of best-path
selection). In some case, allowing multipath routing between eBGP
and iBGP learned paths might be beneficial. This is only possible if
some sort of tunneling technique is used to reach both the eBGP and
iBGP path. If this feature is enabled, the equivalent routes are
selection by stopping the tie-breaking process prior at the MED
comparison step (c) in Section 9.1.2.2 of [RFC4271].
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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. 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
Event though multiple equivalent paths may be selected for
programming into the routing table, the BGP speaker always announces
single best-path to its peers, unless BGP "Add-Path" feature has been
enabled as described in [RFC7911]. 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 using
the oldest path rule to improve routing stability. 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 still be advertised, possibly affecting
the tie-breaking rules on down-stream device, for example if the
AS_PATH contents are different from previous.
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 the
"weights" associated with the multiple paths. The document defines
the applicability only in iBGP case, though there are implementations
that apply it to eBGP multipath as well. The proposal does not
change the equal-cost multipath selection logic, only associates
additional load-sharing attributes with equivalent paths.
10. Informative References
[BGPMP] "BGP Best Path Selection Algorithm",
<http://www.cisco.com/c/en/us/support/docs/ip/
border-gateway-protocol-bgp/13753-25.html>.
[I-D.ietf-idr-link-bandwidth]
Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", draft-ietf-idr-link-bandwidth-06
(work in progress), January 2013.
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[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>.
[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>.
Author's Address
Petr Lapukhov
Facebook
1 Hacker Way
Menlo Park, CA 94025
US
Email: petr@fb.com
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