Internet DRAFT - draft-ietf-bess-evpn-pref-df
draft-ietf-bess-evpn-pref-df
BESS Workgroup J. Rabadan, Ed.
Internet-Draft S. Sathappan
Updates: 8584 (if approved) Nokia
Intended status: Standards Track W. Lin
Expires: 11 April 2024 Juniper Networks
J. Drake
Independent
A. Sajassi
Cisco Systems
9 October 2023
Preference-based EVPN DF Election
draft-ietf-bess-evpn-pref-df-13
Abstract
The Designated Forwarder (DF) in Ethernet Virtual Private Networks
(EVPN) is defined as the Provider Edge (PE) router responsible for
sending Broadcast, Unknown unicast and Multicast traffic (BUM) 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 algorithm. While the 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 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 a Designated Forwarder Election algorithm that
meets the requirements of determinism and operation control. This
document updates RFC8584 by modifying the definition of the DF
Election Extended Community.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 11 April 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
1.2. Solution Requirements . . . . . . . . . . . . . . . . . . 3
1.3. Solution Overview . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language and Terminology . . . . . . . . . . . . 4
3. EVPN BGP Attributes Extensions . . . . . . . . . . . . . . . 5
4. Solution description . . . . . . . . . . . . . . . . . . . . 7
4.1. Use of the Highest-Preference and Lowest Preference
Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Use of the Highest-Preference or Lowest-Preference
algorithm in [RFC7432] Ethernet Segments . . . . . . . . 11
4.3. The Non-Revertive Capability . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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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 traffic (BUM) 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, or DF Alg as per [RFC8584].
While the Default Designated Forwarder Algorithm [RFC7432] or the
Highest Random Weight algorithm (HRW) [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 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
Designated Forwarder, assuming they are all operationally ready
to take over as 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 Designated
Forwarder, based on the DF Algorithms described in this document.
b. The extensions in this document work for [RFC7432] Ethernet
Segments and virtual Ethernet Segments, as defined in
[I-D.ietf-bess-evpn-virtual-eth-segment].
c. The user may force a PE to preempt the existing Designated
Forwarder for a given Ethernet Tag without re-configuring all the
PEs in the Ethernet Segment, by simply modifying the existing
administrative preference in that PE.
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d. The solution allows an option to NOT preempt the current
Designated Forwarder ("Don't Preempt" 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
[RFC7432] Designated Forwarder election procedures that are
always revertive (because the winner PE of the default Designated
Forwarder 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 that can be advertised in the DF
Election Extended Community Section 3. Carried with the new DF
Election Extended Community variants are a DF election preference
advertised for each PE, that influences which PE will become 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. An optional solution is discussed in
Section 4.2, for use in Ethernet segments that support large numbers
of Ethernet Tags and therefore need 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 Designated Forwarder Election Algorithm. This
is sometimes shortened to “Alg” in this document.
* DP - refers to the "Don't Preempt" (me) capability in the
Designated Forwarder Election extended community.
* ENNI - Ethernet Network to Network Interface.
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* ES and vES - Ethernet Segment and virtual Ethernet Segment.
* Ethernet A-D per EVI route - refers to [RFC7432] route type 1 or
Auto-Discovery per EVPN Instance route.
* 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 election. Note that any of the following may
be used to represent a Broadcast Domain: VIDs (including Q-in-Q
tags), configured IDs, VNI (VXLAN Network Identifiers), normalized
VID, I-SIDs (Service Instance Identifiers), 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 - refers to Operations And Maintenance protocols.
3. EVPN BGP Attributes Extensions
This solution reuses and extends the Designated Forwarder 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 is set to Highest-Preference or Lowest-Preference. This
document also defines a new capability referred to as the "Don't
Preempt" capability, that MAY be used with Highest-Preference or
Lowest-Preference DF Algorithms. The format of the DF Election
Extended Community that is used in this document 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:
* DF Algorithm can have the following values:
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- Alg 0 - Default Designated Forwarder Election algorithm, or
modulus-based algorithm as per [RFC7432].
- Alg 1 - HRW algorithm as per [RFC8584].
- Alg 2 - Highest-Preference algorithm (this document
Section 4.1).
- Alg TBD - Lowest-Preference algorithm (this document
Section 4.1). TBD will be replaced by the allocated value at
the time of publication.
* Bitmap (2 octets) encodes "capabilities" [RFC8584], where this
document defines the "Don't Preempt" capability, 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 Designated Forwarder
Election Extended Community and it is defined by this
document): the D bit or 'Don't Preempt' bit (DP hereafter),
determines if the PE advertising the Ethernet Segment route
requests the remote PEs in the Ethernet Segment not to preempt
it as Designated Forwarder. The default value is DP=0, which
is compatible with the 'preempt' or 'revertive' behavior in the
Default DF Algorithm [RFC7432]. The DP capability is supported
by the Highest-Preference or Lowest-Preference DF Algorithms.
The procedures of the "Don't Preempt" capability for other DF
Algorithms are out of the scope of this document. The
procedures of the "Don't Preempt" capability for the Highest-
Preference and Lowest-Preference DF Algorithms are described in
Section 4.1.
- Bit 1: AC-DF or AC-Influenced Designated Forwarder Election is
described in [RFC8584]. When set to 1, it indicates the desire
to use AC-Influenced Designated Forwarder Election with the
rest of the PEs in the Ethernet Segment. The AC-DF capability
bit MAY be set along with the DP capability and the Highest-
Preference or Lowest-Preference DF Algorithms.
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* Designated Forwarder (DF) Preference (described in this document):
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
election depending on the DF Algorithm being used, as explained in
Section 4.1. The Designated Forwarder Preference field is
specific to DF Algorithms Highest-Preference and Lowest-
Preference, and this document does not define any meaning for
other algorithms. If the DF Algorithm is different from Highest-
Preference or Lowest-Preference, these two octets can be encoded
differently.
* RSV and Reserved fields (from bit 16 to bit 18, and from bit 40 to
47): when DF Algorithm is set to Highest-Preference or Lowest-
Preference algorithm, 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
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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 - that spans PE1 and PE2 - and CE2 is connected to
vES2, that is attached to PE1, PE2 and PE3.
If the algorithm chosen for vES1 and vES2 is DF Algorithm Highest-
Preference or Lowest-Preference, the PEs may become Designated
Forwarder irrespective of their IP address and based on the
administrative 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 Designated Forwarder in case of
multiple failures, the Highest-Preference or Lowest-Preference
algorithms can be used. Using 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 Designated Forwarder Election extended
community. These parameters are the Preference, Preemption
option (or "Don't Preempt" option) and DF Algorithm. We will
represent these parameters as (Pref,DP,Alg). For instance, vES1
(Pref,DP,Alg) is configured as (500,0,Highest-Preference) in PE1,
and (255,0,Highest-Preference) in PE2. vES2 is configured as
(100,0,Highest-Preference), (200,0,Highest-Preference) and
(300,0,Highest-Preference) in PE1, PE2 and PE3 respectively.
b. The PEs advertise an Ethernet Segment route for each virtual
Ethernet Segment, including the three parameters indicated in 'a'
above, in the Designated Forwarder Election Extended Community
(encoded as described in Section 3).
c. According to [RFC8584], each PE will run the Designated Forwarder
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 value in each Ethernet
Segment route, and assuming all the Ethernet Segment routes
(including the local route) are consistent in this DF
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 [RFC7432] Default Algorithm. The Highest-Preference
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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 will fall back to the Default Algorithm.
* If all the PEs attached to the Ethernet Segment advertise
Highest-Preference Algorithm, each PE builds a list of
candidate PEs, ordered by Preference value from the
numerically highest value to lowest value. E.g., 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
Lowest-Preference Algorithm, then the candidate list is
ordered from the numerically lowest Preference value to the
highest Preference value. E.g., 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. E.g., if PE3 is going on maintenance and
the DF 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). 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 DF Algorithm Lowest-Preference too,
that is, 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 Designated Forwarder for vES2.
e. In case of equal Preference in two or more PEs in the Ethernet
Segment, the DP bit and the numerically lowest IP address of the
candidate PE(s) are used as tiebreakers. The procedures for the
use of the DP bit 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
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set, and then select the PE with the lowest IP address (if the DP
bit 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 the use of the DP bit and IP address tiebreakers follow:
* If vES1 parameters were (500,0,Highest-Preference) in PE1 and
(500,1,Highest-Preference) in PE2, PE2 would be elected due to
the DP bit. The same example applies if PE1 and PE2 advertise
Lowest-Preference DF Algorithm instead.
* If vES1 parameters were (500,0,Highest-Preference) in PE1 and
(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 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 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 value
in a dynamic way.
E.g., on PE1, if the DF Algorithm is Highest-Preference, ES1's
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.
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The 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 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 until its corresponding Ethernet A-D per ES and
Ethernet A-D per EVI routes are received, as described in [RFC8584].
The 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,
PE2 and PE3 Lowest-Preference for vES2. The use of one DF 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 [RFC7432]-based
Ethernet Segment or virtual Ethernet Segment as in
[I-D.ietf-bess-evpn-virtual-eth-segment], and also EVPN networks as
in [RFC8214], [RFC7623] or [RFC8365].
4.2. Use of the Highest-Preference or Lowest-Preference algorithm in
[RFC7432] 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 [RFC7432] definition of an
Ethernet Segment allows potentially up to thousands of Ethernet Tags
on the same Ethernet Segment. If this is the case, if 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
value and an administrative DF Algorithm, i.e., Highest-Preference or
Lowest-Preference Algorithm. However, the administrative DF
Algorithm (which is used to signal the DF Algorithm for the Ethernet
Segment) MAY be overridden to a different operational DF 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.
For instance:
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* Assuming ES3 is defined in PE1 and PE2, PE1 may be configured as
(500,0,Highest-Preference) for ES3 and PE2 as (100,0,Highest-
Preference). Both PEs will advertise the Ethernet Segment routes
for ES3 with the indicated parameters in the DF Election Extended
Community.
* In addition, assuming 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 Designated Forwarder for Ethernet
Tags 1-2000 (since they use the default Highest-Preference
Algorithm) and PE2 being 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 Section 1.2 (d), 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 Designated
Forwarder Election extended community. This option allows a non-
revertive behavior in the Designated Forwarder election.
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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
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 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
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
(please refer to 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 Highest-Preference Algorithm in
the Ethernet Segment, where local policy overrides the tiebreaker for
a given Ethernet Tag. The other cases above are a sub-set of this one
and the differences are explained.
1. A "Don't Preempt" 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, however, this
document does not enforce the consistency across all the PEs. In
case of inconsistency in the support of the DP capability in the
PEs of the same Ethernet Segment, non-revertive behavior is not
guaranteed. However, PEs supporting this capability still
attempt this procedure.
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2. We assume we want to avoid 'preemption' in all the PEs in the
Ethernet Segment, the three PEs are configured with the "Don't
Preempt" 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 DF 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
Designated Forwarder for Ethernet Tag-1 (due to the Highest-
Preference), and PE1 as Designated Forwarder for Ethernet Tag-2
(due to the Lowest-Preference).
4. If PE3's vES2 goes down (due to EVC failure - detected by OAM, or
port failure or 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 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 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 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
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Highest-PE over PE1, since, when comparing (Pref,DP,PE-IP),
(200,1,PE2-IP) wins over (100,1,PE1-IP). PE3 will select
PE1 as Lowest-PE over PE2, since (100,1,PE1-IP) wins over
(200,1,PE2-IP). Note that if there were only one remote PE
in the Ethernet Segment, 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
set in their Ethernet Segment routes. Depending on this
comparison PE3 sends the Ethernet Segment route with a
(Pref,DP) that may be different from its administrative
(Pref,DP):
- If PE3's Pref value is higher or equal than 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 or equal than 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 or equal than the
Highest-PE's and is not lower or equal than the Lowest-
PE's, PE3 will send the Ethernet Segment route with its
administrative (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).
* Note that, a PE will always send its DP capability set to zero
as long as the advertised Pref is the 'in-use' operational
Pref (as opposed to the 'administrative' Pref).
* This Ethernet Segment route update sent by PE3, with
(200,0,PE3-IP), will not cause any Designated Forwarder
switchover for any Ethernet Tag. PE2 will continue being
Designated Forwarder for Ethernet Tag-1. This is because the
DP bit will be used as a tiebreaker in the Designated
Forwarder 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.
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6. For any subsequent received update/withdraw in the Ethernet
Segment, the PEs will go through the process described in (5) to
select Highest 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 new Highest and Lowest-PEs (considering their
own active Ethernet Segment route) and then they will run the
Designated Forwarder Election.
* If a PE selects itself as new Highest 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).
* After running the Designated Forwarder 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, when a PE or Ethernet Segment
comes back and the PE advertises a Designated Forwarder 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.
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,
irrespective of the revertive or non-revertive behavior in the PE.
5. Security Considerations
This document describes a Designated Forwarder 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 such as HRW, the Designated
Forwarder Election is more automated and cannot be determined by
configuration. With Highest-Preference or Lowest-Preference as DF
Algorithm, an attacker may change the configuration of the Preference
value on a PE and Ethernet Segment, and impact the traffic going
through that PE and Ethernet Segment.
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The non-revertive capability described in this document may be seen
as a security improvement over the regular EVPN revertive Designated
Forwarder Election: an intentional link (or node) "flapping" on a PE
will only cause service disruption once, when the PE goes to 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
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 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 too.
6. IANA Considerations
This document solicits:
* The allocation of two new values in the "DF Alg" registry created
by [RFC8584] as follows:
Alg Name Reference
---- ----------------------------- -------------
2 Highest-Preference Algorithm This document
TBD Lowest-Preference Algorithm This document
* The allocation of a new value in the "DF Election Capabilities"
registry created by [RFC8584] for the 2-octet Bitmap field in the
DF Election Extended Community (Border gateway Protocol (BGP)
Extended Communities registry), as follows:
Bit Name Reference
---- ----------------------------- -------------
0 D (Don't Preempt) Capability This document
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* To update the reference of the "DF Election Extended Community"
field, in the EVPN Extended Community Sub-Types registry, as
follows:
Sub-Type Value Name Reference
-------------- ------------------------------ ---------------------------
0x06 DF Election Extended Community [RFC8584] and This Document
7. Acknowledgments
The authors would like to thank Kishore Tiruveedhula and Sasha
Vainshtein for their review and comments. Also thank you to Luc
Andre Burdet and Stephane Litkowski for their thorough review and
suggestions for a new DF Algorithm for lowest-preference.
8. 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
9. References
9.1. Normative References
[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>.
[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>.
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[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>.
[I-D.ietf-bess-evpn-virtual-eth-segment]
Sajassi, A., Brissette, P., Schell, R., Drake, J., and J.
Rabadan, "EVPN Virtual Ethernet Segment", Work in
Progress, Internet-Draft, draft-ietf-bess-evpn-virtual-
eth-segment-14, 23 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-bess-
evpn-virtual-eth-segment-14>.
9.2. Informative References
[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>.
Authors' Addresses
J. Rabadan (editor)
Nokia
520 Almanor Avenue
Sunnyvale, CA 94085
United States of America
Email: jorge.rabadan@nokia.com
S. Sathappan
Nokia
Email: senthil.sathappan@nokia.com
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W. Lin
Juniper Networks
Email: wlin@juniper.net
J. Drake
Independent
Email: je_drake@yahoo.com
A. Sajassi
Cisco Systems
Email: sajassi@cisco.com
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