Internet DRAFT - draft-ietf-bess-evpn-unequal-lb
draft-ietf-bess-evpn-unequal-lb
BESS WorkGroup N. Malhotra, Ed.
Internet-Draft A. Sajassi
Intended status: Standards Track Cisco Systems
Expires: 9 June 2024 J. Rabadan
Nokia
J. Drake
Juniper
A. Lingala
ATT
S. Thoria
Cisco Systems
7 December 2023
Weighted Multi-Path Procedures for EVPN Multi-Homing
draft-ietf-bess-evpn-unequal-lb-21
Abstract
EVPN enables all-active multi-homing for a CE (Customer Equipment)
device connected to two or more PE (Provider Equipment) devices via a
LAG (Link Aggregation), such that bridged and routed traffic from
remote PEs to hosts attached to the Ethernet Segment can be equally
load balanced (it uses Equal Cost Multi Path) across the multi-homing
PEs. EVPN also enables multi-homing for IP subnets advertised in IP
Prefix routes, so that routed traffic from remote PEs to those IP
subnets can be load balanced. This document defines extensions to
EVPN procedures to optimally handle unequal access bandwidth
distribution across a set of multi-homing PEs in order to:
* provide greater flexibility, with respect to adding or removing
individual multi-homed PE-CE links.
* handle multi-homed PE-CE link failures that can result in unequal
PE-CE access bandwidth across a set of multi-homing PEs.
In order to achieve the above, it specifies signaling extensions and
procedures to:
* Loadbalance bridged and routed traffic across egress PEs in
proportion to PE-CE link bandwidth or a generalized weight
distribution.
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* Achieve BUM (Broadcast, UnknownUnicast, Multicast) DF (Designated
Forwarder) election distribution for a given ES (Ethernet Segment)
across the multi-homing PE set in proportion to PE-CE link
bandwidth. Section 6 of this document further updates RFC 8584,
draft-ietf-bess-evpn-per-mcast-flow-df-election and draft-ietf-
bess-evpn-pref-df in order for the DF election extension defined
in this document to work across different DF election algorithms.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 June 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. PE-CE Link Provisioning . . . . . . . . . . . . . . . . . 4
1.2. PE-CE Link Failures . . . . . . . . . . . . . . . . . . . 5
1.3. Design Requirement . . . . . . . . . . . . . . . . . . . 6
2. Requirements Language and Terminology . . . . . . . . . . . . 7
3. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 8
4. EVPN Link Bandwidth Extended Community . . . . . . . . . . . 8
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4.1. Encoding and Usage of EVPN Link Bandwidth Extended
Community . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Weighted Unicast Traffic Load-balancing to an Ethernet
Segment . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Egress PE Behavior . . . . . . . . . . . . . . . . . . . 10
5.2. Ingress PE Behavior . . . . . . . . . . . . . . . . . . . 10
6. Weighted BUM Traffic Load-Sharing across an Ethernet
Segment . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. The BW Capability in the DF Election Extended
Community . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. BW Capability and Default DF Election algorithm . . . . . 13
6.3. BW Capability and HRW DF Election algorithm (Type 1 and
4) . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3.1. BW Increment . . . . . . . . . . . . . . . . . . . . 14
6.3.2. HRW Hash Computations with BW Increment . . . . . . . 15
6.4. BW Capability and Preference DF Election algorithm . . . 16
6.5. Cost-Benefit Tradeoff on Link Failures . . . . . . . . . 17
7. Additional Considerations . . . . . . . . . . . . . . . . . . 17
7.1. Real-time Available Bandwidth . . . . . . . . . . . . . . 17
7.2. Weighted Load-balancing to Multi-homed Subnets . . . . . 17
7.3. Weighted Load-balancing without EVPN aliasing . . . . . . 18
7.4. EVPN IRB Multi-homing With Non-EVPN routing . . . . . . . 18
8. Operational Considerations . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. BGP-Link-Bandwidth-Extended-Community . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
In an EVPN IRB (Integrated Routing and Bridging) overlay network as
described in [RFC9135], with a CE multi-homed via a EVPN all-active
multi-homing, bridged and routed traffic from ingress PEs can be
equally load balanced (ECMPed) across the multi-homing egress PEs:
* ECMP Load-balancing for bridged unicast traffic is enabled via
aliasing and mass-withdraw procedures detailed in [RFC7432].
* ECMP Load-balancing for routed unicast traffic is enabled via
existing L3 ECMP mechanisms.
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* Load-sharing of bridged BUM (Broadcast, UnknownUnicast, Multicast)
traffic on local ports is enabled via EVPN DF election procedure
detailed in [RFC7432].
All of the above load balancing and DF election procedures implicitly
assume equal bandwidth distribution between the CE and the set of
egress PEs. Essentially, with this assumption of equal "access"
bandwidth distribution across all egress PEs, all remote traffic is
equally load balanced across the egress PEs. This assumption of
equal access bandwidth distribution can be restrictive with respect
to adding / removing links in a multi-homed LAG interface and may
also be easily broken on individual link failures. A solution to
handle unequal access bandwidth distribution across a set of egress
PEs is specified in this document. Primary motivation behind this
proposal is to enable greater flexibility with respect to adding /
removing member PE-CE links, as needed and to optimally handle PE-CE
link failures.
1.1. PE-CE Link Provisioning
+------------------------+
| Underlay Network Fabric|
+------------------------+
+-----+ +-----+
| PE1 | | PE2 |
+-----+ +-----+
\ /
\ ES-1 /
\ /
+\---/+
| \ / |
+--+--+
|
CE1
Figure 1
Consider CE1 that is dual-homed to egress PE1 and egress PE2 via EVPN
all-active multi-homing with single member links of equal bandwidth
to each PE (aka, equal access bandwidth distribution across PE1 and
PE2). If the provider wants to increase link bandwidth to CE1, it
must add a link to both PE1 and PE2 in order to maintain equal access
bandwidth distribution and inter-work with EVPN ECMP load balancing.
In other words, for a dual-homed CE, total number of CE links must be
provisioned in multiples of 2 (2, 4, 6, and so on). For a triple-
homed CE, number of CE links must be provisioned in multiples of
three (3, 6, 9, and so on). To generalize, for a CE that is multi-
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homed to "n" PEs, number of PE-CE physical links provisioned must be
an integral multiple of "n". This is restrictive in case of dual-
homing and very quickly becomes prohibitive in case of multi-homing.
Instead, a provider may wish to increase PE-CE bandwidth or number of
links in any link increments. As an example, for CE1 dual-homed to
egress PE1 and egress PE2 in all-active mode, provider may wish to
add a third link to only PE1 to increase total bandwidth for this CE
by 50%, rather than being required to increase access bandwidth by
100% by adding a link to each of the two PEs. While existing EVPN
based all-active load balancing procedures do not necessarily
preclude such asymmetric access bandwidth distribution among the PEs
providing redundancy, it may result in unexpected traffic loss due to
congestion in the access interface towards CE. This traffic loss is
due to the fact that PE1 and PE2 will continue to be treated as equal
cost paths at remote PEs, and as a result may attract approximately
equal amount of CE1 destined traffic, even when PE2 only has half the
bandwidth to CE1 as PE1. This may lead to congestion and traffic
loss on the PE2-CE1 link. If bandwidth distribution to CE1 across
PE1 and PE2 is 2:1, traffic from remote hosts must also be load
balanced across PE1 and PE2 in 2:1 manner.
1.2. PE-CE Link Failures
More importantly, unequal PE-CE bandwidth distribution described
above may occur during regular operation following a link failure,
even when PE-CE links were provisioned to provide equal bandwidth
distribution across multi-homing PEs.
+------------------------+
| Underlay Network Fabric|
+------------------------+
+-----+ +-----+
| PE1 | | PE2 |
+-----+ +-----+
\\ //
\\ ES-1 //
\\ /X
+\\---//+
| \\ // |
+---+---+
|
CE1
Figure 2
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Consider a CE1 that is multi-homed to egress PE1 and egress PE2 via a
LAG with two member links to each PE. On a PE2-CE1 physical link
failure, LAG represented by an Ethernet Segment ES-1 on PE2 stays up,
however, its bandwidth is cut in half. With existing ECMP
procedures, both PE1 and PE2 may continue to attract equal amount of
traffic from remote PEs, even when PE1 has double the bandwidth to
CE1. If bandwidth distribution to CE1 across PE1 and PE2 is 2:1,
traffic from remote hosts must also be load balanced across PE1 and
PE2 in 2:1 manner to avoid unexpected congestion and traffic loss on
PE2-CE1 links within the LAG. As an alternative, min-link on LAGs is
sometimes used to bring down the LAG interface on member link
failures. This however results in loss of available bandwidth in the
network, and is not ideal.
1.3. Design Requirement
+-----------------------+
|Underlay Network Fabric|
+-----------------------+
+-----+ +-----+ +-----+ +-----+
| PE1 | | PE2 | ..... | PEx | | PEn |
+-----+ +-----+ +-----+ +-----+
\ \ // //
\ L1 \ L2 // Lx // Ln
\ \ // //
+-\-------\-----------//--------//-+
| \ \ ES-1 // // |
+----------------------------------+
|
CE
Figure 3
To generalize, if total link bandwidth to a CE is distributed across
"n" egress PEs, with Lx being the total bandwidth to PEx across all
links, traffic from ingress PEs to this CE must be load balanced
unequally across egress PE set [PE1, PE2, ....., PEn] such that,
fraction of total unicast and BUM flows destined for CE that are
serviced by egress PEx is:
Lx / [L1+L2+.....+Ln]
Figure 3 illustrates a scenario where egress PE1..PEn are attached to
a multi-homed Ethernet Segment, however this document generalizes
this requirement so that the unequal load balancing can be applied to
PEs attached to a vES or to a multi-homed subnet advertised by EVPN
IP Prefix routes.
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The solution specified below includes extensions to EVPN procedures
to achieve the above. Following assumption apply to procedure
described in this document:
* For procedures related to bridged unicast and BUM traffic, EVPN
all active multi-homing is assumed.
* Procedures related to bridged unicast and BUM traffic are
applicable to both aliasing and non-alaising mode as defined in
[RFC7432].
2. Requirements Language and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
* BW: BandWidth
* LAG: Link Aggregation Group
* ES: Ethernet Segment
* ESI: Ethernet Segment ID
* vES: Virtual Ethernet Segment
* EVI: Ethernet virtual Instance, this is a mac-vrf
* RT-1: EVPN Route Type 1 as defined in [RFC7432]
* RT-2: EVPN Route Type 2 as defined in [RFC7432]
* RT-5: EVPN Route Type 5 as defined in [RFC7432]
* Path-List: A forwarding object used to load-balance routed or
bridged traffic across multiple forwarding paths.
* Access Bandwidth: Bandwidth of PE-CE links in an Ethernet Segment
* Egress PE: In the context of an Ethernet Segment or a route, this
is the PE that advertises a locally attached Ethernet Segment RT-
1, or a locally attached host or prefix route (RT-2, RT-5)
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* Ingress PE: In the context of an Ethernet Segment or a route, this
is the receiving PE that learns remote Ethernet Segment RT-1 and/
or host and prefix routes (RT-2, RT-5) from the Egress PE
* IMET: Inclusive Multicast Route
* DF: Designated Forwarder
* BDF: Backup Designated Forwarder
* DCI: Data Center Interconnect Router
3. Solution Overview
In order to achieve weighted load balancing to an ES or vES for
overlay unicast traffic, Ethernet A-D per ES route (EVPN Route Type
1) is leveraged to signal the Ethernet Segment weight to ingress PEs.
Using Ethernet A-D per ES route to signal the Ethernet Segment weight
provides a mechanism that reacts to changes in access bandwidth or
number of access links in a service and host independent manner.
Ingress PEs computing the MAC path-lists based on global and aliasing
Ethernet A-D routes now have the ability to setup weighted load
balancing path-lists based on the ES access bandwidth or number of
links received from each egress PE that the ES is multi-homed to.
In order to achieve weighted load balancing of overlay BUM traffic,
EVPN ES route (Route Type 4) is leveraged to signal the ES weight to
egress PEs within an ES's redundancy group to influence per-service
DF election. Egress PEs in an ES redundancy group now have the
ability to do service carving in proportion to each egress PE's
relative ES weight.
Unequal load balancing to multi-homed subnets is achieved by
signaling the weight along with the IP Prefix routes advertised for
the subnet.
Procedures to accomplish this are described in greater detail next.
4. EVPN Link Bandwidth Extended Community
A new EVPN Link Bandwidth extended community is defined for the
solution specified in this document:
* This extended community is defined of type 0x06 (EVPN Extended
Community Sub-Types).
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* IANA has assigned sub-type value of 0x10 for the EVPN Link
bandwidth extended community, of type 0x06 (EVPN Extended
Community Sub-Types).
* EVPN Link Bandwidth extended community is defined as transitive.
4.1. Encoding and Usage of EVPN Link Bandwidth Extended Community
EVPN Link Bandwidth Extended Community value field is used to carry
total bandwidth of egress PE's all physical links in an ethernet
segment, expressed in Mbits/sec (MegabitsPerSecond) represented as an
unsigned integer. Note however that the load balancing algorithm
defined in this document uses ratio of Link Bandwidths. Hence, the
operator may choose a different unit or use the community as a
generalized weight that may be set to link count, locally configured
weight, or a value computed based on something other than link
bandwidth. In such case, the operator MUST ensure consistent usage
of the unit across all egress PEs in an ethernet segment. This may
involve multiple routing domains/Autonomous Systems.
In order to facilitate this, as well as avoid interop issues because
of provisioning error, one octet in the extended community's six
octet 'value' field is used to explicitly signal if the weight
encoded in the remaining five octets is link bandwidth expressed in
Mbps or a generalized weight value. This results in the following
encoding for EVPN link bandwidth extended community:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Sub-Type | Value-Units | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Value-Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4
Value-Units is encoded as:
* 0x00: weight expressed using default units of Mbps
* 0x01: generalized weight expressed in something other than Mbps
Generalized weight units are intentionally left arbritrary to allow
for flexibility in its usage for different applications without
having to define new encoding for each non-default application.
Implementations MUST support the default units of Mbps, while support
of non-default generalized weight is considered optional.
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Additionally, following considerations apply to handling of this
extended community at the ingress PE:
* An ingress PE MUST check for consistent 'Value-Units' received in
the EVPN link bandwidth exteneded community from each egress PE in
an Ethernet Segment. In case of any inconsistency in 'Value-
Units' across egress PEs in an Ethernet Segment, this EVPN Link
Bandwidth extended community is to be ignored.
* An ingress PE MUST ensure that each route contains only a single
instance of this extended community sub-type. In case of more
than one instance, this EVPN Link Bandwidth extended community is
to be ignored.
5. Weighted Unicast Traffic Load-balancing to an Ethernet Segment
5.1. Egress PE Behavior
A PE that is part of an Ethernet Segment's redundancy group MUST
advertise an additional "EVPN link bandwidth" extended community with
Ethernet A-D per ES route (EVPN Route Type 1), that carries total
bandwidth of PE's physical links in an Ethernet Segment or a
generalized weight. New EVPN link bandwidth extended community
defined in this document is used for this purpose.
EVPN link bandwidth extended community MUST NOT be attached to per-
EVI RT-1 or to EVPN RT-2 as it is a physical ESI property and hence
advertised per-ESI.
5.2. Ingress PE Behavior
An ingress PE MUST ensure that the EVPN link bandwidth extended
community is recevied from all the egress PEs in an Ethernet Segment
and check for consistent 'Value-Units' received from each egress PE
in an Ethernet Segment. In case of missing EVPN Link Bandwidth
extended community or inconsistent 'Value-Units' from any of the
egress PEs in an Ethernet Segment, this EVPN Link Bandwidth extended
community is to be ignored by the ingress PE and ingress PE is to
follow regular ECMP forwarding to that Ethernet Segment. Ingress PE
MUST generate a syslog when the EVPN Link Bandwidth extended
community is ignored.
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Once consistency of 'Value-Units' is validated, ingress PE SHOULD use
the 'Value-Weight' received from each egress PE to compute a relative
(normalized) weight for each egress PE, per ES, and then use this
relative weight to compute a weighted path-list to be used for load
balancing, as opposed to using an ECMP path-list for load balancing
across the egress PE paths. Egress PE Weight and resulting weighted
path-list computation at ingress PEs is a local matter. An example
computation algorithm is shown below to illustrate the idea:
if,
L(x,y) : link bandwidth advertised by egress PE-x for ES-y
W(x,y) : normalized weight assigned to egress PE-x for ES-y
H(y) : Highest Common Factor (HCF) of [L(1,y), L(2,y), ....., L(n,y)]
then, the normalized weight assigned to egress PE-x for ES-y may be
computed as follows:
W(x,y) = L(x,y) / H(y)
For a MAC+IP route (EVPN Route Type 2) received with ES-y, ingress PE
may compute MAC and IP forwarding path-list weighted by the above
normalized weights.
As an example, for a CE multi-homed to PE-1, PE-2, PE-3 via 2, 1, and
1 GE physical links respectively, as part of a LAG represented by ES-
10:
L(1, 10) = 2000 Mbps
L(2, 10) = 1000 Mbps
L(3, 10) = 1000 Mbps
H(10) = 1000
Normalized weights assigned to each egress PE for ES-10 are as
follows:
W(1, 10) = 2000 / 1000 = 2.
W(2, 10) = 1000 / 1000 = 1.
W(3, 10) = 1000 / 1000 = 1.
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For a remote MAC+IP host route received with ES-10, forwarding load
balancing path-list may now be computed as: [PE-1, PE-1, PE-2, PE-3]
instead of [PE-1, PE-2, PE-3]. This now results in load balancing of
all traffic destined for ES-10 across the three egress PEs in
proportion to ES-10 bandwidth at each egress PE.
Please note that the pathlist computation algorithm above is for
illustration only. Weighted pathlist computation based on the
received EVPN link bandwidth extended community is a local
implementation choice. As an example, if the received link badwidth
values do not result in a good HCF H(y) in the computation method
above to be able to compute reasonable weights that can be programmed
in hardware, implementation MAY choose another approximation to
arrive at rounded integer weight values that can be programmed in
hardware.
Weighted path-list computation must only be done for an ES if EVPN
link bandwidth extended community is received from all of the egress
PE's advertising reachability to that ES via Ethernet A-D per ES
Route Type 1. In an unlikely event that EVPN link bandwidth extended
community is not received from one or more egress PEs, forwarding
path-list should be computed using regular ECMP semantics. Note that
a default weight cannot be assumed for an egress PE that does not
advertise its link bandwidth as the weight to be used in path-list
computation is relative.
If per-ES RT-1 is not advertised or withdrawn from any of the egress
PE(s), as per [RFC7432], egress PE is removed from the forwarding
path-list for that [EVI, ES]. Hence, the weighted path-list MUST be
re-computed.
In an unlikely scenario that per-[ES, EVI] RT-1 is not advertised
from any of the egress PE(s), as per [RFC7432], egress PE is not
included in the forwarding path-list for that [EVI, ES]. Hence, the
weighted path-list for the [EVI, ES] MUST be computed based only on
the weights received from egress PEs that advertised the per-[ES,
EVI] RT-1.
6. Weighted BUM Traffic Load-Sharing across an Ethernet Segment
Optionally, load sharing of per-service DF role, weighted by
individual egress PE's link-bandwidth share within a multi-homed ES
may also be achieved.
In order to do that, a new DF Election Capability [RFC8584] called
"BW" (Bandwidth Weighted DF Election) is defined. BW MAY be used
along with some DF Election Types, as described in the following
sections.
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6.1. The BW Capability in the DF Election Extended Community
[RFC8584] defines a new extended community for PEs within a
redundancy group to signal and agree on uniform DF Election Type and
Capabilities for each ES. This document requests IANA to allocate a
bit in the "DF Election capabilities" registry setup by [RFC8584]:
Bit 4: BW (Bandwidth Weighted DF Election)
ES routes advertised with the BW bit set will indicate the desire of
the advertising egress PE to consider the link-bandwidth in the DF
Election algorithm defined by the value in the "DF Type".
As per [RFC8584], all the egress PEs in the ES MUST advertise the
same Capabilities and DF Type, otherwise the PEs will fall back to
Default [RFC7432] DF Election procedure.
The BW Capability MAY be advertised with the following DF Types:
* Type 0: Default DF Election algorithm, as in [RFC7432]
* Type 1: HRW (Highest Random Weight) algorithm, as in [RFC8584]
* Type 2: Preference algorithm, as in [EVPN-DF-PREF]
* Type 4: HRW per-multicast flow DF Election, as in [EVPN-PER-MCAST-
FLOW-DF]
The following sections describe how the DF Election procedures are
modified for the above DF Types when the BW Capability is used.
6.2. BW Capability and Default DF Election algorithm
When all the PEs in the Ethernet Segment (ES) agree to use the BW
Capability with DF Type 0, the Default DF Election procedure as
defined in [RFC7432] is modified as follows:
* Each PE advertises a "EVPN Link Bandwidth" extended community
along with the ES route to signal the PE-CE link bandwidth (LBW)
for the ES.
* A receiving egress PE MUST use the ES link bandwidth extended
community received from each egress PE to compute a relative
weight for each egress PE in an Ethernet Segment.
* The DF Election procedure MUST now use this weighted list of
egress PEs to compute the per-VLAN Designated Forwarder, such that
the DF role is distributed in proportion to this normalized
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weight. As a result, a single PE may have multiple ordinals in
the DF candidate PE list and 'N' used in (V mod N) operation as
defined in [RFC7432] is modified to be total number of ordinals
instead of being total number of egress PEs in an Ethernet
Segment.
Considering the same example as in Section 5.2, the candidate PE list
for DF election is:
[PE-1, PE-1, PE-2, PE-3].
The DF for a given VLAN-a on ES-10 is now computed as (VLAN-a % 4).
This would result in the DF role being distributed across PE1, PE2,
and PE3 in portion to each PE's normalized weight for ES-10.
6.3. BW Capability and HRW DF Election algorithm (Type 1 and 4)
[RFC8584] introduces Highest Random Weight (HRW) algorithm (DF Type
1) for DF election in order to solve potential DF election skew
depending on Ethernet tag space distribution. [EVPN-PER-MCAST-FLOW-
DF] further extends HRW algorithm for per-multicast flow based hash
computations (DF Type 4). This section describes extensions to HRW
Algorithm for EVPN DF Election specified in [RFC8584] and in [EVPN-
PER-MCAST-FLOW-DF] in order to achieve DF election distribution that
is weighted by link bandwidth.
6.3.1. BW Increment
A new variable called "bandwidth increment" is computed for each [PE,
ES] advertising the ES link bandwidth extended community as follows:
In the context of an ES,
L(i) = Link bandwidth advertised by PE(i) for this ES
Lmin = lowest link bandwidth advertised across all PEs for this ES
Bandwidth increment, "b(i)" for a given PE(i) advertising a link
bandwidth of L(i) is defined as an integer value computed as:
b(i) = L(i) / Lmin
As an example,
with L(1) = 10, L(2) = 10, L(3) = 20
bandwidth increment for each PE would be computed as:
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b(1) = 1, b(2) = 1, b(3) = 2
with L(1) = 10, L(2) = 10, L(3) = 10
bandwidth increment for each PE would be computed as:
b(1) = 1, b(2) = 1, b(3) = 1
Note that the bandwidth increment must always be an integer,
including, in an unlikely scenario of a PE's link bandwidth not being
an exact multiple of Lmin. If it computes to a non-integer value
(including as a result of link failure), it MUST be rounded down to
an integer.
6.3.2. HRW Hash Computations with BW Increment
HRW algorithm as described in [RFC8584] and in [EVPN-PER-MCAST-FLOW-
DF] computes a random hash value for each PE(i), where, (0 < i <= N),
PE(i) is the PE at ordinal i, and Address(i) is the IP address of
PE(i).
For 'N' PEs sharing an Ethernet segment, this results in 'N'
candidate hash computations. The PE that has the highest hash value
is selected as the DF.
We refer to this hash value as "affinity" in this document. Hash or
affinity computation for each PE(i) is extended to be computed one
per bandwidth increment associated with PE(i) instead of a single
affinity computation per PE(i).
PE(i) with b(i) = j, results in j affinity computations:
affinity(i, x), where 1 < x <= j
This essentially results in number of candidate HRW hash computations
for each PE that is directly proportional to that PE's relative
bandwidth within an ES and hence gives PE(i) a probability of being
DF in proportion to it's relative bandwidth within an ES.
As an example, consider an ES that is multi-homed to two PEs, PE1 and
PE2, with equal bandwidth distribution across PE1 and PE2. This
would result in a total of two candidate hash computations:
affinity(PE1, 1)
affinity(PE2, 1)
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Now, consider a scenario with PE1's link bandwidth as 2x that of PE2.
This would result in a total of three candidate hash computations to
be used for DF election:
affinity(PE1, 1)
affinity(PE1, 2)
affinity(PE2, 1)
which would give PE1 2/3 probability of getting elected as a DF, in
proportion to its relative bandwidth in the ES.
Depending on the chosen HRW hash function, affinity function MUST be
extended to include bandwidth increment in the computation.
For e.g.,
affinity function specified in [EVPN-PER-MCAST-FLOW-DF] MUST be
extended as follows to incorporate bandwidth increment j:
affinity(S,G,V, ESI, Address(i,j)) =
(1103515245.((1103515245.Address(i).j + 12345) XOR
D(S,G,V,ESI))+12345) (mod 2^31)
affinity or random function specified in [RFC8584] MUST be extended
as follows to incorporate bandwidth increment j:
affinity(v, Es, Address(i,j)) = (1103515245((1103515245.Address(i).j
+ 12345) XOR D(v,Es))+12345)(mod 2^31)
6.4. BW Capability and Preference DF Election algorithm
This section applies to ES'es where all the PEs in the ES agree use
the BW Capability with DF Type 2. The BW Capability modifies the
Preference DF Election procedure [EVPN-DF-PREF], by adding the LBW
value as a tie-breaker as follows:
Section 4.1, bullet (f) in [EVPN-DF-PREF] is updated to now consider
the LBW value as below:
f) In case of equal Preference in two or more PEs in the ES, the tie-
breakers will be the DP (Don't Preempt me) bit, the LBW value and the
lowest IP PE in that order. For instance:
* If vES1 parameters were [Pref=500,DP=0,LBW=1000] in PE1 and
[Pref=500,DP=1, LBW=2000] in PE2, PE2 would be elected due to the
DP bit.
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* If vES1 parameters were [Pref=500,DP=0,LBW=1000] in PE1 and
[Pref=500,DP=0, LBW=2000] in PE2, PE2 would be elected due to a
higher LBW, even if PE1's IP address is lower.
* The LBW exchanged value has no impact on the Non-Revertive option
described in [EVPN-DF-PREF].
6.5. Cost-Benefit Tradeoff on Link Failures
While incorporating link bandwidth into the DF election process
provides optimal BUM traffic distribution across the ES links, it
also implies that DF elections are re-adjusted on link failures or
bandwidth changes. If the operator does not wish to have this level
of churn in their DF election, then they should not advertise the BW
capability. Not advertising BW capability may result in less than
optimal BUM traffic distribution while still retaining the ability to
allow an ingress PE to do weighted ECMP for its unicast traffic to a
set of egress PEs.
7. Additional Considerations
7.1. Real-time Available Bandwidth
PE-CE link bandwidth availability may sometimes vary in real-time
disproportionately across PE-CE links within a multi-homed ES due to
various factors such as flow based hashing combined with fat flows
and unbalanced hashing. Reacting to real-time available bandwidth is
at this time outside the scope of this document.
7.2. Weighted Load-balancing to Multi-homed Subnets
EVPN Link bandwidth extended community may also be used to achieve
unequal load-balancing of prefix routed traffic by including this
extended community in EVPN Route Type 5. When included in EVPN RT-5,
its value is to be interpreted as egress PE's relative weight for the
prefix included in this RT-5. Ingress PE will then compute the
forwarding path-list for the prefix route using weighted paths
received from each egress PE. EVPN Link bandwidth extended community
MUST be encoded with "Value-Units = 0x01" to signal a generalized
weight associated with the advertising PE.
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7.3. Weighted Load-balancing without EVPN aliasing
[RFC7432] defines per-[ES, EVI] RT-1 based EVPN aliasing procedure as
an optional propcedure. In an unlikely scenario where an EVPN
implementation does not support EVPN aliasing procedures, MAC
forwarding path-list at the ingress PE is computed based on per-ES
RT-1 and RT-2 routes received from egress PEs instead of per-ES RT-1
and per-[ES, EVI] RT-1 from egress PEs. In such a case, only the
weights received via per-ES RT-1 from the egress PEs included in the
MAC path-list are to be considered for weighted path-list
computation.
7.4. EVPN IRB Multi-homing With Non-EVPN routing
EVPN-LAG based multi-homing on an IRB gateway may also be deployed
together with non-EVPN routing, such as global routing or an L3VPN
routing control plane. Key property that differentiates this set of
use cases from EVPN IRB use cases discussed earlier is that EVPN
control plane is used only to enable LAG interface based multi-homing
and not as an overlay VPN control plane. Applicability of weighted
ECMP procedures specified in this document to these set of use cases
is an area of further consideration beyond the scope of this
document.
8. Operational Considerations
* In order for the solution specified in this document to function
correctly, implementation SHOULD ensure that EVPN Link Bandwidth
Extended Comminiuty is being advertised with same "Value-Units"
across all PEs.
* Further, when a generalized weight option is used with "Value-
Units = 0x1", implementation SHOULD ensure that the weights are
assigned to each PE in a consistent manner.
* Implementation SHOULD alert the users via syslog when an
inconsistency in "Value-Units" is detected across the PE set for a
given ESI or prefix.
* Implementation SHOULD also alert users via syslog if an
unreasonable discrepancy is detected across advertised BW or
weights from different PEs, such that the implementation is unable
to compute a weighted pathlist that can be programmed in hardware.
This could likely result from inconsistent units of weight used by
different PEs.
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* Operators MAY monitor the traffic flow distribution and DF
election distribution across the egress PE set to ensure that the
implementation is working as expected.
9. Security Considerations
Security considerations discussed in [RFC7432] and [RFC8584] apply to
this document. Methods described in this document further extend
signaling of multi-homed devices using ESI LAG. They are hence
subject to same considerations if the control plane or data plane was
to be compromised. As an example, if control plane is compromised,
signaling of heavily skewed Link Bandwidth Attributes could result in
all traffic to be directed towards one PE resulting in its host
facing links to be overloaded. Exposure to such an attack is limited
by suggested syslogs discussed in Operational Consideration section.
Considerations for protecting control and data plane described in
[RFC7432] are equally applicable to signaling of Link Bandwidth
Attribute defined in this document.
10. IANA Considerations
[RFC8584] defines a new extended community for PEs within a
redundancy group to signal and agree on uniform DF Election Type and
Capabilities for each ES. This document requests IANA to allocate a
bit in the "DF Election capabilities" registry setup by [RFC8584]
with the following suggested bit number:
Bit 4: BW (Bandwidth Weighted DF Election)
A new EVPN Link Bandwidth extended community is defined to signal
local ES link bandwidth to ingress PEs. This extended community is
defined of type 0x06 (EVPN Extended Community Sub-Types). IANA has
assigned a sub-type value of 0x10 for the EVPN Link bandwidth
extended community, of type 0x06 (EVPN Extended Community Sub-Types).
EVPN Link Bandwidth extended community is defined as transitive.
IANA is requested to set up a registry called "Value-Units" for the
1-octet field in the EVPN Link Bandwidth Extended Community. New
registrations will be made through the "RFC Required" procedure
defined in [RFC8126]. The following are suggested initial values in
that registry exist:
Value Name Reference
---- ---------------- -------------
0 Weight in units of Mbps This document
1 Generalized Weight This document
2-255 Unassigned
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11. Acknowledgements
Authors would like to thank Satya Mohanty for valuable review and
inputs with respect to HRW and weighted HRW algorithm refinements
specified in this document. Authors would also like to thank Bruno
Decraene and Sergey Fomin for valuable review and comments.
12. Contributors
Satya Ranjan Mohanty
Cisco Systems
US
Email: satyamoh@cisco.com
13. References
13.1. Normative References
[EVPN-DF-PREF]
Rabadan, J., Sathappan, S., Przygienda, T., Lin, W.,
Drake, J., Sajassi, A., Mohanty, S., and , "Preference-
based EVPN DF Election", Work in Progress, Internet-Draft,
draft-ietf-bess-evpn-pref-df-06, 19 June 2020,
<https://tools.ietf.org/html/draft-ietf-bess-evpn-pref-df-
06.txt>.
[EVPN-PER-MCAST-FLOW-DF]
Sajassi, A., mishra, m., Thoria, S., Rabadan, J., and J.
Drake, "Per multicast flow Designated Forwarder Election
for EVPN", Work in Progress, Internet-Draft, draft-ietf-
bess-evpn-per-mcast-flow-df-election-04, 31 August 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-bess-evpn-
per-mcast-flow-df-election-04.txt>.
[EVPN-VIRTUAL-ES]
Sajassi, A., Brissette, P., Schell, R., Drake, J.,
Rabadan, J., and , "EVPN Virtual Ethernet Segment", Work
in Progress, Internet-Draft, draft-ietf-bess-evpn-virtual-
eth-segment-06, 9 March 2020,
<https://tools.ietf.org/html/draft-ietf-bess-evpn-virtual-
eth-segment-06.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC7814] Xu, X., Jacquenet, C., Raszuk, R., Boyes, T., and B. Fee,
"Virtual Subnet: A BGP/MPLS IP VPN-Based Subnet Extension
Solution", RFC 7814, DOI 10.17487/RFC7814, March 2016,
<https://tools.ietf.org/html/rfc7814>.
[RFC8584] Rabadan, J., Ed., Mohanty, R., Sajassi, N., Drake, A.,
Nagaraj, K., and S. Sathappan, "Framework for Ethernet VPN
Designated Forwarder Election Extensibility", RFC 8584,
DOI 10.17487/RFC8584, April 2019,
<https://www.rfc-editor.org/info/rfc8584>.
13.2. Informative References
[BGP-LINK-BW]
Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", Work in Progress, Internet-Draft,
draft-ietf-idr-link-bandwidth-07, March 2019,
<https://tools.ietf.org/html/draft-ietf-idr-link-
bandwidth-07.txt>.
[RFC9135] Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in EVPN",
RFC 9135, DOI 10.17487/RFC9135, October 2021,
<https://www.rfc-editor.org/rfc/rfc9135>.
Appendix A. BGP-Link-Bandwidth-Extended-Community
Link bandwidth extended community described in [BGP-LINK-BW] for
layer 3 VPNs was considered for re-use here. This Link bandwidth
extended community is however defined in [BGP-LINK-BW] as optional
non-transitive. Since it is not possible to change deployed behavior
of extended community defined in [BGP-LINK-BW], it was decided to
define a new one. In inter-AS scenarios, link-bandwidth needs to be
signaled to eBGP neighbors. When signaled across AS boundary, this
extended community can be used to achieve optimal load-balancing
towards egress PEs in a different AS. This is applicable both when
next-hop is changed or unchanged across AS boundaries.
Authors' Addresses
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Neeraj Malhotra (editor)
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
Email: nmalhotr@cisco.com
Ali Sajassi
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
Email: sajassi@cisco.com
Jorge Rabadan
Nokia
777 E. Middlefield Road
Mountain View, CA 94043
United States of America
Email: jorge.rabadan@nokia.com
John Drake
Juniper
Email: jdrake@juniper.net
Avinash Lingala
ATT
200 S. Laurel Avenue
Middletown, CA 07748
United States of America
Email: ar977m@att.com
Samir Thoria
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
Email: sthoria@cisco.com
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