Internet Engineering Task Force (IETF) J. Rabadan, Ed.
Request for Comments: 9785 S. Sathappan
Updates: 8584 Nokia
Category: Standards Track W. Lin
ISSN: 2070-1721 Juniper Networks
J. Drake
Independent
A. Sajassi
Cisco Systems
May 2025
Preference-Based EVPN Designated Forwarder (DF) Election
Abstract
The Designated Forwarder (DF) in Ethernet Virtual Private Networks
(EVPNs) is defined as the Provider Edge (PE) router responsible for
sending Broadcast, Unknown Unicast, and Multicast (BUM) traffic to a
multi-homed device/network in the case of an All-Active multi-homing
Ethernet Segment (ES) or BUM and unicast in the case of Single-Active
multi-homing. The Designated Forwarder is selected out of a
candidate list of PEs that advertise the same Ethernet Segment
Identifier (ESI) to the EVPN network, according to the Default
Designated Forwarder Election default DF
election algorithm. While the Default Algorithm default algorithm provides an
efficient and automated way of selecting the Designated Forwarder
across different Ethernet Tags in the Ethernet Segment, there are
some use cases where a more "deterministic" and user-
controlled user-controlled
method is required. At the same time, Network Operators require an
easy way to force an on-demand Designated Forwarder switchover in
order to carry out some maintenance tasks on the existing Designated
Forwarder or control whether a new active PE can preempt the existing
Designated Forwarder PE.
This document proposes use of a Designated Forwarder Election DF election algorithm that meets the
requirements of determinism and operation control. This document
updates RFC 8584 by modifying the definition of the DF Election
Extended Community.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9785.
Copyright Notice
Copyright (c) 2025 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 described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Problem Statement
1.2. Solution Requirements
1.3. Solution Overview
2. Requirements Language and Terminology
3. EVPN BGP Attribute Extensions
4. Solution Description
4.1. Use of the Highest-Preference and Lowest-Preference
Algorithm
4.2. Use of the Highest-Preference or Lowest-Preference
Algorithm in Ethernet Segments
4.3. The Non-Revertive Capability
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
1.1. Problem Statement
[RFC7432] defines the Designated Forwarder (DF) in EVPN networks as
the PE responsible for sending Broadcast, Unknown Unicast, and
Multicast (BUM) traffic to a multi-homed device/network in the case
of an All-Active multi-homing Ethernet Segment or BUM and unicast
traffic to a multi-homed device or network in the case of Single-
Active multi-homing. The Designated Forwarder is selected out of a
candidate list of PEs that advertise the Ethernet Segment Identifier
(ESI) to the EVPN network and according to the Designated Forwarder
Election Algorithm DF election algorithm
or to DF Alg as per [RFC8584].
While the Default Designated Forwarder Algorithm [RFC7432] default DF algorithm or the Highest Random Weight (HRW)
algorithm [RFC8584] provide an efficient and automated way of
selecting the Designated Forwarder across different Ethernet Tags in
the Ethernet Segment, there are some use cases where a more user-controlled user-
controlled method is required. At the same time, Network Operators
require an easy way to force an on-demand Designated Forwarder
switchover in order to carry out some maintenance tasks on the
existing Designated Forwarder or control whether a new active PE can
preempt the existing Designated Forwarder PE.
1.2. Solution Requirements
The procedures described in this document meet the following
requirements:
a. The solution provides an administrative preference option so that
the user can control in what order the candidate PEs may become
the Designated Forwarder, assuming they are all operationally
ready to take over as the Designated Forwarder. The operator can
determine whether the Highest-Preference or Lowest-Preference PE
among the PEs in the Ethernet Segment will be elected as the
Designated Forwarder, based on the DF Algorithms algorithms described in
this document.
b. The extensions described in this document work for Ethernet
Segments [RFC7432] and virtual Ethernet Segments as defined in
[RFC9784].
c. The user may force a PE to preempt the existing Designated
Forwarder for a given Ethernet Tag without reconfiguring all the
PEs in the Ethernet Segment, by simply modifying the existing
administrative preference in that PE.
d. The solution allows an option to NOT preempt the current
Designated Forwarder (the "Don't Preempt" capability), Capability), even if
the former Designated Forwarder PE comes back up after a failure.
This is also known as "non-revertive" behavior, as opposed to the
Designated Forwarder
DF election procedures [RFC7432] that are always revertive
(because the winner PE of the default Designated
Forwarder DF election algorithm
always takes over as the operational Designated Forwarder).
e. The procedures described in this document support Single-Active
and All-Active multi-homing Ethernet Segments.
1.3. Solution Overview
To provide a solution that satisfies the above requirements, we
introduce two new DF Algorithms algorithms that can be advertised in the DF
Election Extended Community (Section 3). Carried with the new DF
Election Extended Community variants is a DF election preference
advertised for each PE that influences which PE will become the DF
(Section 4.1). The advertised DF election preference can dynamically
vary from the administratively configured preference to provide non-
revertive behavior (Section 4.3). In Section 4.2, an optional
solution is discussed for use in Ethernet Segments that supports
large numbers of Ethernet Tags and therefore needs to balance load
among multiple DFs.
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.
AC: Attachment Circuit. An AC has an Ethernet Tag associated to it.
CE: Customer Equipment router
DF: Designated Forwarder
DF Alg: Refers to the Designated Forwarder Election Algorithm, DF election algorithm, which is sometimes
shortened to "Alg" in this document.
DP: Refers to the "Don't Preempt" (me) capability Capability in the DF Election
Extended Community.
ENNI: External Network-Network Interface
ES and vES: Ethernet Segment and virtual Ethernet Segment.
Ethernet A-D per EVI route: Refers to [RFC7432] route type Route Type 1 or Auto-Discovery
per EVPN Instance route. route [RFC7432].
EVC: Ethernet Virtual Circuit
EVI: EVPN Instance
Ethernet Tag: Used to represent a broadcast domain that is
configured on a given Ethernet Segment for the purpose of
Designated Forwarder DF
election. Note that any of the following may be used to represent
a broadcast domain: VLAN IDs (VIDs) (including Q-in-Q tags),
configured IDs, VXLAN Network Identifiers (VNIs), normalized VIDs,
Service Instance Identifiers (I-SIDs), etc., as long as the
representation of the broadcast domains is configured consistently
across the multi-homed PEs attached to that Ethernet Segment. The
Ethernet Tag value MUST NOT be zero.
HRW: Highest Random Weight, as per [RFC8584].
OAM: Operations, Administration, and Maintenance.
3. EVPN BGP Attribute Extensions
This solution reuses and extends the DF Election Extended Community
defined in [RFC8584] that is advertised along with the Ethernet
Segment route. It does so by replacing the last two reserved octets
of the DF Election Extended Community when the DF Algorithm algorithm is set to
Highest-Preference or Lowest-Preference. This document also defines
a new capability referred to as the "Don't Preempt" capability, Capability, which
MAY be used with Highest-Preference or Lowest-Preference DF Algorithms.
The format of the DF Election Extended Community used in this
document is as follows:
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=0x06 | Sub-Type(0x06)| RSV | DF Alg | Bitmap ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Bitmap | Reserved | DF Preference (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: DF Election Extended Community
Where the above fields are defined as follows:
* The DF Algorithm algorithm can have the following values:
- Alg 0 - Default Designated Forwarder Election DF election algorithm, or i.e., the modulus-based
algorithm as per [RFC7432].
- Alg 1 - HRW algorithm as per [RFC8584].
- Alg 2 - Highest-Preference Algorithm (Section 4.1).
- Alg 3 - Lowest-Preference Algorithm (Section 4.1).
* Bitmap (2 octets) encodes "capabilities" [RFC8584], whereas this
document defines the "Don't Preempt" capability, Capability, which is used to
indicate if a PE supports a non-revertive behavior:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D|A| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Bitmap Field in the DF Election Extended Community
- Bit 0 (corresponds to Bit 24 of the DF Election Extended
Community, and it is defined by this document): The D bit, or
'Don't Preempt' bit ("DP" hereafter),
"Don't Preempt" Capability, determines if the PE advertising
the Ethernet Segment route requests the remote PEs in the
Ethernet Segment to not preempt it as the Designated Forwarder.
The default value is DP=0, which is compatible with the
'preempt' or 'revertive' behavior in the Default default DF
Algorithm algorithm
[RFC7432]. The DP capability "Don't Preempt" Capability is supported by the
Highest-Preference or Lowest-Preference DF Algorithms. The
procedures of the "Don't Preempt" capability Capability for other DF
Algorithms
algorithms are out of the scope of this document. The
procedures of the "Don't Preempt" capability Capability for the Highest-
Preference and Lowest-Preference DF Algorithms are described in
Section 4.1.
- Bit 1: AC-Influenced DF (AC-DF) election is described in
[RFC8584]. When set to 1, it indicates the desire to use AC-DF
with the rest of the PEs in the Ethernet Segment. The AC-DF
capability bit MAY be set along with the DP capability "Don't Preempt"
Capability and Highest-Preference or Lowest-Preference DF
Algorithms.
* Designated Forwarder (DF) Preference: Defines a 2-octet value that
indicates the PE preference to become the Designated Forwarder in
the Ethernet Segment, as described in Section 4.1. The allowed
values are within the range 0-65535, and the default value MUST be
32767. This value is the midpoint in the allowed Preference range
of values, which gives the operator the flexibility of choosing a
significant number of values, above or below the default
Preference. A numerically higher or lower value of this field is
more preferred for Designated Forwarder DF election depending on the DF Algorithm algorithm being
used, as explained in Section 4.1. The Designated Forwarder
Preference field is specific to DF Algorithms Highest-Preference and Lowest-Preference, Lowest-
Preference Algorithms, and this document does not define any
meaning for other algorithms. If the DF Algorithm algorithm is different
from Highest-Preference or Lowest-Preference, these 2 octets can
be encoded differently.
* RSV and Reserved fields (from bit 16 to bit 18, and from bit 40 to
47): When the DF Algorithm algorithm is set to Highest-Preference or Lowest-
Preference, the values are set to zero when advertising the
Ethernet Segment route, and they are ignored when receiving the
Ethernet Segment route.
4. Solution Description
Figure 3 illustrates an example that will be used in the description
of the solution.
EVPN Network
+-------------------+
| +-------+ ENNI Aggregation
| <---ESI1,500 | PE1 | /\ +----Network---+
| <-----ESI2,100 | |===||=== |
| | |===||== \ vES1 | +----+
+-----+ | | \/ |\----------------+CE1 |
CE3--+ PE4 | +-------+ | \ ------------+ |
+-----+ | | \ / | +----+
| | | X |
| <---ESI1,255 +-----+============ \ |
| <-----ESI2,200 | PE2 |========== \ vES2 | +----+
| +-----+ | \ ----------+CE2 |
| | | --------------+ |
| +-----+ ----------------------+ |
| <-----ESI2,300 | PE3 +--/ | | +----+
| +-----+ +--------------+
--------------------+
Figure 3: Preference-Based DF Election
Figure 3 shows three PEs that are connecting EVCs coming from the
Aggregation Network to their EVIs in the EVPN network. CE1 is
connected to vES1, which spans PE1 and PE2, and CE2 is connected to
vES2, which is attached to PE1, PE2, and PE3.
If the algorithm chosen for vES1 and vES2 is the Highest-Preference
or Lowest-Preference DF Algorithm, the PEs may become the Designated
Forwarder irrespective of their IP address and based on the
administrative Preference preference value. The following sections provide some
examples of the procedures and how they are applied in the use case
of Figure 3.
4.1. Use of the Highest-Preference and Lowest-Preference Algorithm
Assuming the operator wants to control -- in a flexible way -- what
PE becomes the Designated Forwarder for a given virtual Ethernet
Segment and the order in which the PEs become a Designated Forwarder
in case of multiple failures, the Highest-Preference or Lowest-
Preference Algorithms can be used. Per the example in Figure 3,
these algorithms are used as follows:
a. vES1 and vES2 are now configurable with three optional parameters
that are signaled in the DF Election Extended Community. These
parameters are the Preference, Preemption (or "Don't Preempt") Preempt"
Capability) option, and DF Algorithm. algorithm. We will represent these
parameters as
(Pref,DP,Alg). (Pref, DP, Alg). For instance, vES1 (Pref,DP,Alg) (Pref, DP,
Alg) is configured as
(500,0,Highest-Preference) as:
(500, 0, Highest-Preference) in PE1 and as (255,0,Highest-
Preference) PE1,
(255, 0, Highest-Preference) in PE2.
vES2 is configured as (100,0,Highest-
Preference), (200,0,Highest-Preference), and (300,0,Highest-
Preference)
(100, 0, Highest-Preference) in PE1,
(200, 0, Highest-Preference) in PE2, and PE3, respectively.
(300, 0, Highest-Preference) in PE3.
b. The PEs advertise an Ethernet Segment route for each virtual
Ethernet Segment, including the three parameters indicated in (a)
above, in the DF Election Extended Community (encoded as
described in Section 3).
c. According to [RFC8584], each PE will run the Designated Forwarder DF election
algorithm upon expiration of the DF Wait timer. Each PE runs the
Highest-Preference or Lowest-Preference DF Algorithm for each
Ethernet Segment as follows:
* The PE will check the DF Algorithm algorithm value in each Ethernet
Segment route, and assuming all the Ethernet Segment routes
(including the local route) are consistent in this DF
Algorithm
algorithm (that is, all are configured for Highest-Preference
or Lowest-Preference, but not a mix), the PE runs the
procedure in this section. Otherwise, the procedure falls
back to the Default Algorithm default DF algorithm [RFC7432]. The Highest-
Preference and Lowest-Preference Algorithms are different
algorithms; therefore, if two PEs configured for Highest-
Preference and Lowest-Preference, respectively, are attached
to the same Ethernet Segment, the operational Designated
Forwarder Election Algorithm DF election
algorithm will fall back to the Default
Algorithm. default DF algorithm.
* If all the PEs attached to the Ethernet Segment advertise the
Highest-Preference Algorithm, each PE builds a list of
candidate PEs, ordered by Preference preference value from the
numerically highest value to lowest value. For example, PE1
builds a list of candidate PEs for vES1 ordered by the
Preference, from high to low: <PE1, PE2> (since PE1's
preference is more preferred than PE2's). Hence, PE1 becomes
the Designated Forwarder for vES1. In the same way, PE3
becomes the Designated Forwarder for vES2.
* If all the PEs attached to the Ethernet Segment advertise the
Lowest-Preference Algorithm, then the candidate list is
ordered from the numerically lowest Preference preference value to the
highest Preference preference value. For example, PE1's ordered list for
vES1 is <PE2, PE1>. Hence, PE2 becomes the Designated
Forwarder for vES1. In the same way, PE1 becomes the
Designated Forwarder for vES2.
d. Assuming some maintenance tasks had to be executed on a PE, the
operator may want to make sure the PE is not the Designated
Forwarder for the Ethernet Segment so that the impact on the
service is minimized. For example, if PE3 is going on
maintenance and the DF Algorithm algorithm is Highest-Preference, the
operator could change vES2's Preference on PE3 from 300 to, e.g.,
50 (hence, the Ethernet Segment route from PE3 is updated with
the new preference value), so that PE2 is forced to take over as
Designated Forwarder for vES2 (irrespective of the DP
capability). "Don't
Preempt" Capability). Once the maintenance task on PE3 is over,
the operator could decide to leave the latest configured
preference value or configure the initial preference value back.
A similar procedure can be used for the Lowest-Preference DF
Algorithm too. For example, suppose the algorithm for vES2 is
Lowest-Preference, and PE1 (the DF) goes on maintenance mode.
The operator could change vES2's Preference on PE1 from 100 to,
e.g., 250, so that PE2 is forced to take over as the Designated
Forwarder for vES2.
e. In case of equal Preference in two or more PEs in the Ethernet
Segment, the DP bit "Don't Preempt" Capability and the numerically
lowest IP address of the candidate PE(s) are used as tiebreakers.
The procedures for the use of the DP bit "Don't Preempt" Capability are
specified in Section 4.3. If more than one PE is advertising
itself as the preferred Designated Forwarder, an implementation
MUST first select the PE advertising the DP bit "Don't Preempt"
Capability set, and then select the PE with the lowest IP address
(if the DP
bit "Don't Preempt" Capability selection does not yield a
unique candidate). The PE's IP address is the address used in
the candidate list, and it is derived from the Originating
Router's IP address of the Ethernet Segment route. In case PEs
use the Originating Router's IP address of different families, an
IPv4 address is always considered numerically lower than an IPv6
address. Some examples of using the DP bit "Don't Preempt" Capability
and IP address tiebreakers follow:
* If vES1 parameters were (500,0,Highest-Preference) (500, 0, Highest-Preference) in PE1
and
(500,1,Highest-Preference) (500, 1, Highest-Preference) in PE2, PE2 would be elected
due to the DP bit. "Don't Preempt" Capability. The same example
applies if PE1 and PE2 advertise the Lowest-Preference DF
Algorithm instead.
* If vES1 parameters were (500,0,Highest-Preference) (500, 0, Highest-Preference) in PE1
and
(500,0,Highest-Preference) (500, 0, Highest-Preference) in PE2, PE1 would be elected,
if PE1's IP address is lower than PE2's. Or PE2 would be
elected if PE2's IP address is lower than PE1's. The same
example applies if PE1 and PE2 advertise the Lowest-Preference DF
Algorithm instead.
f. The Preference is an administrative option that MUST be
configured on a per-Ethernet-Segment basis, and it is normally
configured from the management plane. The Preference preference value MAY
also be dynamically changed based on the use of local policies
that react to events on the PE. The following examples
illustrate the use of local policy to change the Preference preference value
in a dynamic way.
* On PE1, if the DF Algorithm algorithm is Highest-Preference, ES1's
Preference
preference value can be lowered from 500 to 100 in case the
bandwidth on the ENNI port is decreased by 50% (that could
happen if, e.g., the 2-port Link Aggregation Group between PE1
and the Aggregation Network loses one port).
* Local policy MAY also trigger dynamic Preference changes based
on the PE's bandwidth availability in the core, specific ports
going operationally down, etc.
* The definition of the actual local policies is out of scope of
this document.
Highest-Preference and Lowest-Preference Algorithms MAY be used along
with the AC-DF capability. Assuming all the PEs in the Ethernet
Segment are configured consistently with the Highest-Preference or
Lowest-Preference Algorithm and AC-DF capability, a given PE in the
Ethernet Segment is not considered as a candidate for Designated
Forwarder Election DF election
until its corresponding Ethernet A-D per ES and Ethernet A-D per EVI
routes are received, as described in [RFC8584].
Highest-Preference and Lowest-Preference DF Algorithms can be used in
different virtual Ethernet Segments on the same PE. For instance,
PE1 and PE2 can use Highest-Preference for vES1 and PE1, and PE2 and
PE3 can use Lowest-Preference for vES2. The use of one DF Algorithm algorithm
over the other is the operator's choice. The existence of both
provides flexibility and full control to the operator.
The procedures in this document can be used in an Ethernet Segment as
defined in [RFC7432] or a virtual Ethernet Segment per [RFC9784] and
also in EVPN networks as described in [RFC8214], [RFC7623], or
[RFC8365].
4.2. Use of the Highest-Preference or Lowest-Preference Algorithm in
Ethernet Segments
While the Highest-Preference or Lowest-Preference DF Algorithm described
in Section 4.1 is typically used in virtual Ethernet Segment
scenarios where there is normally an individual Ethernet Tag per
virtual Ethernet Segment, the existing definition of an Ethernet
Segment [RFC7432] allows potentially up to thousands of Ethernet Tags
on the same Ethernet Segment. If this is the case, and if the
Highest-Preference or Lowest-Preference Algorithm is configured in
all the PEs of the Ethernet Segment, the same PE will be the elected
Designated Forwarder for all the Ethernet Tags of the Ethernet
Segment. A potential way to achieve a more granular load balancing
is described below.
The Ethernet Segment is configured with an administrative Preference preference
value and an administrative DF Algorithm, algorithm, i.e., Highest-Preference or
Lowest-Preference Algorithm. However, the administrative DF
Algorithm
algorithm (which is used to signal the DF Algorithm algorithm for the Ethernet
Segment) MAY be overridden to a different operational DF Algorithm algorithm
for a range of Ethernet Tags. With this option, the PE builds a list
of candidate PEs ordered by Preference; however, the Designated
Forwarder for a given Ethernet Tag will be determined by the locally
overridden DF Algorithm. algorithm.
For instance:
* Assuming ES3 is defined in PE1 and PE2, PE1 may be configured as
(500,0,Highest-Preference)
(500, 0, Highest-Preference) and PE2 as (100,0,Highest-Preference) (100, 0, Highest-
Preference) for ES3. Both PEs will advertise the Ethernet Segment
routes for ES3 with the indicated parameters in the DF Election
Extended Community.
* In addition, assuming there are VLAN-based service interfaces and
that the PEs are attached to all Ethernet Tags in the range
1-4000, both PE1 and PE2 may be configured with (Ethernet Tag-
range, Lowest-Preference), e.g., (2001-4000, Lowest-Preference).
* This will result in PE1 being the Designated Forwarder for
Ethernet Tags 1-2000 (since they use the default Highest-
Preference Algorithm) and PE2 being the Designated Forwarder for
Ethernet Tags 2001-4000, due to the local policy overriding the
Highest-Preference Algorithm.
While the above logic provides a perfect load-balancing distribution
of Ethernet Tags per Designated Forwarder when there are only two
PEs, for Ethernet Segments attached to three or more PEs, there would
be only two Designated Forwarder PEs for all the Ethernet Tags. Any
other logic that provides a fair distribution of the Designated
Forwarder function among the three or more PEs is valid, as long as
that logic is consistent in all the PEs in the Ethernet Segment. It
is important to note that, when a local policy overrides the Highest-
Preference or Lowest-Preference signaled by all the PEs in the
Ethernet Segment, this local policy MUST be consistent in all the PEs
of the Ethernet Segment. If the local policy is inconsistent for a
given Ethernet Tag in the Ethernet Segment, packet drops or packet
duplication may occur on that Ethernet Tag. For all these reasons,
the use of virtual Ethernet Segments is RECOMMENDED for cases where
more than two PEs per Ethernet Segment exist and a good load-
balancing distribution per Ethernet Tag of the Designated Forwarder
function is desired.
4.3. The Non-Revertive Capability
As discussed in item d of Section 1.2, a capability to NOT preempt
the existing Designated Forwarder (for all the Ethernet Tags in the
Ethernet Segment) is required and therefore added to the DF Election
Extended Community. This option allows a non-revertive behavior in
the Designated Forwarder DF election.
Note that when a given PE in an Ethernet Segment is taken down for
maintenance operations, before bringing it back, the Preference may
be changed in order to provide a non-revertive behavior. The DP bit "Don't
Preempt" Capability and the mechanism explained in this section will
be used for those cases when a former Designated Forwarder comes back
up without any controlled maintenance operation, and the non-revertive non-
revertive option is desired in order to avoid service impact.
In Figure 3, we assume that based on the Highest-Preference
Algorithm, PE3 is the Designated Forwarder for ESI2.
If PE3 has a link, EVC, or node failure, PE2 would take over as the
Designated Forwarder. If/when PE3 comes back up again, PE3 will take
over, causing some unnecessary packet loss in the Ethernet Segment.
The following procedure avoids preemption upon failure recovery (see
Figure 3). The procedure supports a non-revertive mode that can be
used along with:
* Highest-Preference Algorithm
* Lowest-Preference Algorithm
* Highest-Preference or Lowest-Preference Algorithm, where a local
policy overrides the Highest-/Lowest-Preference tiebreaker for a
range of Ethernet Tags (Section 4.2)
The procedure is described, assuming the Highest-Preference Algorithm
in the Ethernet Segment, where local policy overrides the tiebreaker
for a given Ethernet Tag. The other cases above are a subset of this
one, and the differences are explained.
1. A "Don't Preempt" capability Capability is defined on a per-PE / per-
Ethernet-Segment basis, as described in Section 3. If "Don't
Preempt" is disabled (default behavior), the PE sets DP to zero
and advertises it in an Ethernet Segment route. If "Don't
Preempt" is enabled, the Ethernet Segment route from the PE
indicates the desire of not being preempted by the other PEs in
the Ethernet Segment. All the PEs in an Ethernet Segment should
be consistent in their configuration of the DP capability; "Don't Preempt"
Capability; however, this document does not enforce the
consistency across all the PEs. In case of inconsistency in the
support of the DP
capability "Don't Preempt" Capability in the PEs of the same
Ethernet Segment, non-revertive behavior is not guaranteed.
However, PEs supporting this capability still attempt this
procedure.
2. Assuming we want to avoid 'preemption' in all the PEs in the
Ethernet Segment, the three PEs are configured with the "Don't
Preempt" capability. Capability. In this example, we assume ESI2 is
configured as 'DP=enabled' in the three PEs.
3. We also assume vES2 is attached to Ethernet Tag-1 and Ethernet
Tag-2. vES2 uses Highest-Preference as the DF Algorithm, algorithm, and a
local policy is configured in the three PEs to use Lowest-
Preference for Ethernet Tag-2. When vES2 is enabled in the three
PEs, the PEs will exchange the Ethernet Segment routes and select
PE3 as the Designated Forwarder for Ethernet Tag-1 (due to the
Highest-Preference) and PE1 as the Designated Forwarder for
Ethernet Tag-2 (due to the Lowest-Preference).
4. If PE3's vES2 goes down (due to an EVC failure (as detected by
OAM protocols), a port failure, or a node failure), PE2 will
become the Designated Forwarder for Ethernet Tag-1. No changes
will occur for Ethernet Tag-2.
5. When PE3's vES2 comes back up, PE3 will start a boot-timer (if
booting up) or hold-timer (if the port or EVC recovers). That
timer will allow some time for PE3 to receive the Ethernet
Segment routes from PE1 and PE2. This timer is applied between
the INIT and the DF_WAIT states in the Designated Forwarder
Election DF election Finite State
Machine described in [RFC8584]. PE3 will then:
* Select a "reference-PE" among the Ethernet Segment routes in
the virtual Ethernet Segment. If the Ethernet Segment uses
the Highest-Preference Algorithm, select a "Highest-PE". If
it uses the Lowest-Preference Algorithm, select a "Lowest-PE".
If a local policy is in use, to override the Highest-/Lowest-
Preference for a range of Ethernet Tags (as discussed in
Section 4.2), it is necessary to select both a Highest-PE and
a Lowest-PE. They are selected as follows:
- The Highest-PE is the PE with higher Preference, using the
DP bit
"Don't Preempt" Capability first (with DP=1 being better)
and, after that, the lower PE-IP address as tiebreakers.
- The Lowest-PE is the PE with lower Preference, using the DP
bit
"Don't Preempt" Capability first (with DP=1 being better)
and, after that, the lower PE-IP address as tiebreakers.
- In our example, the Highest-Preference Algorithm is used,
with a local policy to override it to use Lowest-Preference
for a range of Ethernet Tags. Therefore, PE3 selects a
Highest-PE and a Lowest-PE. PE3 will select PE2 as the
Highest-PE over PE1, because when comparing (Pref,DP,PE- (Pref, DP, PE-
IP), (200,1,PE2-IP) (200, 1, PE2-IP) wins over (100,1,PE1-IP). (100, 1, PE1-IP). PE3 will
select PE1 as the Lowest-PE over PE2, because
(100,1,PE1-IP) (100, 1,
PE1-IP) wins over (200,1,PE2-IP). (200, 1, PE2-IP). Note that if there
were only one remote PE in the Ethernet Segment, the Lowest
and Highest PE would be the same PE.
* Check its own administrative Pref and compare it with the one
of the Highest-PE and Lowest-PE that have the DP capability "Don't Preempt"
Capability set in their Ethernet Segment routes. Depending on
this comparison, PE3 sends the Ethernet Segment route with a
(Pref,DP)
(Pref, DP) that may be different from its administrative
(Pref,DP):
(Pref, DP):
- If PE3's Pref value is higher than or equal to the Highest-
PE's, PE3 will send the Ethernet Segment route with an 'in-
use' operational Pref equal to the Highest-PE's and DP=0.
- If PE3's Pref value is lower than or equal to the Lowest-
PE's, PE3 will send the Ethernet Segment route with an 'in-
use' operational Preference equal to the Lowest-PE's and
DP=0.
- If PE3's Pref value is not higher than or equal to the
Highest-PE's and is not lower than or equal to the Lowest-
PE's, PE3 will send the Ethernet Segment route with its
administrative (Pref,DP)=(300,1). (Pref, DP)=(300, 1).
- In this example, PE3's administrative Pref=300 is higher
than the Highest-PE with DP=1, that is, PE2 (Pref=200).
Hence, PE3 will inherit PE2's preference and send the
Ethernet Segment route with an operational 'in-use'
(Pref,DP)=(200,0). (Pref,
DP)=(200, 0).
* Note that a PE will always send Send its DP capability "Don't Preempt" Capability set to zero zero, as long as
the advertised Pref is the 'in-use' operational Pref (as
opposed to the 'administrative' Pref).
* Not trigger any Designated Forwarder changes for Ethernet Tag-
1. This Ethernet Segment route update sent by PE3, with
(200,0,PE3-IP), (200,
0, PE3-IP), will not cause any Designated Forwarder switchover
for any Ethernet Tag. PE2 will continue being the
Designated Forwarder for Ethernet Tag-1. This is because the
DP bit "Don't Preempt"
Capability will be used as a tiebreaker in the Designated
Forwarder DF election.
That is, if a PE has two candidate PEs with the same Pref, it
will pick the one with DP=1. There are no Designated
Forwarder changes for Ethernet Tag-2 either.
6. For any subsequent received update/withdraw in the Ethernet
Segment, the PEs will go through the process described in (5) to
select the Highest-PEs and Lowest-PEs, now considering themselves
as candidates. For instance, if PE2 fails upon receiving PE2's
Ethernet Segment route withdrawal, PE3 and PE1 will go through
the selection of the new Highest-PEs and Lowest-PEs (considering
their own active Ethernet Segment route), and then they will run
the Designated Forwarder Election. DF election.
* If a PE selects itself as the new Highest-PE or Lowest-PE and
it was not before, the PE will then compare its operational
'in-use' Pref with its administrative Pref. If different, the
PE will send an Ethernet Segment route update with its
administrative Pref and DP values. In the example, PE3 will
be the new Highest-PE; therefore, it will send an Ethernet
Segment route update with (Pref,DP)=(300,1). (Pref, DP)=(300, 1).
* After running the Designated Forwarder Election, DF election, PE3 will become the new
Designated Forwarder for Ethernet Tag-1. No changes will
occur for Ethernet Tag-2.
Note that, irrespective of the DP bit, "Don't Preempt" Capability, when a PE
or Ethernet Segment comes back and the PE advertises a Designated Forwarder Election
Algorithm DF election
algorithm different from the one configured in the rest of the PEs in
the Ethernet Segment, all the PEs in the Ethernet Segment MUST fall
back to the Default [RFC7432] Algorithm. default DF algorithm [RFC7432].
This document does not modify the use of the P and B bits in the
Ethernet A-D per EVI routes [RFC8214] advertised by the PEs in the
Ethernet Segment after running the Designated Forwarder Election, DF election, irrespective of the
revertive or non-revertive behavior in the PE.
5. Security Considerations
This document describes a Designated Forwarder Election Algorithm DF election algorithm that provides
absolute control (by configuration) over what PE is the Designated
Forwarder for a given Ethernet Tag. While this control is desired in
many situations, a malicious user that gets access to the
configuration of a PE in the Ethernet Segment may change the behavior
of the network. In other DF Algorithms algorithms such as HRW, the Designated
Forwarder Election DF election
is more automated and cannot be determined by configuration. If the
DF Algorithm algorithm is Highest-Preference or Lowest-
Preference, Lowest-Preference, an attacker
may change the configuration of the
Preference preference value on a PE and
Ethernet Segment to impact the traffic going through that PE and
Ethernet Segment.
The non-revertive capability described in this document may be seen
as a security improvement over the regular EVPN revertive Designated
Forwarder Election: DF
election: an intentional link (or node) "flapping" on a PE will only
cause service disruption once, when the PE goes to Non-
Designated Non-Designated
Forwarder state. However, an attacker who gets access to the
configuration of a PE in the Ethernet Segment will be able to disable
the non-revertive behavior, by advertising a conflicting DF election
algorithm and thereby forcing fallback to the Default default DF algorithm.
The document also describes how a local policy can override the
Highest-Preference or Lowest-Preference Algorithms for a range of
Ethernet Tags in the Ethernet Segment. If the local policy is not
consistent across all PEs in the Ethernet Segment and there is an
Ethernet Tag that ends up with an inconsistent use of Highest-
Preference or Lowest-Preference in different PEs, packet drop or
packet duplication may occur for that Ethernet Tag.
Finally, the two Designated Forwarder Election Algorithms DF election algorithms specified in this document
(Highest-Preference and Lowest-Preference) do not change the way the
PEs share their Ethernet Segment information, compared to the
algorithms in [RFC7432] and [RFC8584]. Therefore, the security
considerations in [RFC7432] and [RFC8584] apply to this document as
well.
6. IANA Considerations
Per this document, IANA has:
* Allocated two new values in the "DF Alg" registry created by
[RFC8584], as follows:
+=====+==============================+===========+
| Alg | Name | Reference |
+=====+==============================+===========+
| 2 | Highest-Preference Algorithm | RFC 9785 |
+-----+------------------------------+-----------+
| 3 | Lowest-Preference Algorithm | RFC 9785 |
+-----+------------------------------+-----------+
Table 1
* Allocated a new value in the "DF Election Capabilities" registry
created by [RFC8584] for the 2-octet Bitmap field in the DF
Election Extended Community (under the "Border Gateway Protocol
(BGP) Extended Communities" registry group), as follows:
+=====+==============================+===========+
| Bit | Name | Reference |
+=====+==============================+===========+
| 0 | D (Don't Preempt) Capability | RFC 9785 |
+-----+------------------------------+-----------+
Table 2
* Listed this document as an additional reference for the DF
Election Extended Community field in the "EVPN Extended Community
Sub-Types" registry, as follows:
+================+================================+==============+
| Sub-Type Value | Name | Reference |
+================+================================+==============+
| 0x06 | DF Election | [RFC8584] |
| | Extended Community | and RFC 9785 |
+----------------+--------------------------------+--------------+
Table 3
7. References
7.1. Normative References
[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>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8584] Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake,
J., 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>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9784] Sajassi, A., Brissette, P., Schell, R., Drake, J., and J.
Rabadan, "EVPN Virtual Ethernet Segment", RFC 9784,
DOI 10.17487/9784, May 2025,
<https://www.rfc-editor.org/info/rfc9784>.
7.2. Informative References
[RFC7623] Sajassi, A., Ed., Salam, S., Bitar, N., Isaac, A., and W.
Henderickx, "Provider Backbone Bridging Combined with
Ethernet VPN (PBB-EVPN)", RFC 7623, DOI 10.17487/RFC7623,
September 2015, <https://www.rfc-editor.org/info/rfc7623>.
[RFC8214] Boutros, S., Sajassi, A., Salam, S., Drake, J., and J.
Rabadan, "Virtual Private Wire Service Support in Ethernet
VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017,
<https://www.rfc-editor.org/info/rfc8214>.
[RFC8365] Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
Uttaro, J., and W. Henderickx, "A Network Virtualization
Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
DOI 10.17487/RFC8365, March 2018,
<https://www.rfc-editor.org/info/rfc8365>.
[RFC7623] Sajassi, A., Ed., Salam, S., Bitar, N., Isaac, A., and W.
Henderickx, "Provider Backbone Bridging Combined with
Ethernet VPN (PBB-EVPN)", RFC 7623, DOI 10.17487/RFC7623,
September 2015, <https://www.rfc-editor.org/info/rfc7623>.
Acknowledgements
The authors would like to thank Kishore Tiruveedhula and Sasha
Vainshtein for their reviews and comments. Also, thank you to Luc
André Burdet and Stephane Litkowski for their thorough reviews and
suggestions for a new Lowest-Preference DF Algorithm.
Contributors
In addition to the authors listed, the following individuals also
contributed to this document:
Tony Przygienda
Juniper
Satya Mohanty
Cisco
Kiran Nagaraj
Nokia
Vinod Prabhu
Nokia
Selvakumar Sivaraj
Juniper
Sami Boutros
VMWare
Authors' Addresses
Jorge Rabadan (editor)
Nokia
520 Almanor Avenue
Sunnyvale, CA 94085
United States of America
Email: jorge.rabadan@nokia.com
Senthil Sathappan
Nokia
Email: senthil.sathappan@nokia.com
Wen Lin
Juniper Networks
Email: wlin@juniper.net
John Drake
Independent
Email: je_drake@yahoo.com
Ali Sajassi
Cisco Systems
Email: sajassi@cisco.com