BESS Workgroup J. Rabadan, Ed.
Internet-Draft S. Sathappan
Intended status: Standards Track Nokia
Expires: September 13, 2021 T. Przygienda
W. Lin
J. Drake
Juniper Networks
A. Sajassi
S. Mohanty
Cisco Systems
March 12, 2021
Preference-based EVPN DF Election
draft-ietf-bess-evpn-pref-df-07
Abstract
The Designated Forwarder (DF) in Ethernet Virtual Private Networks
(EVPN) is defined as the PE responsible for sending Broadcast,
Unknown unicast and Broadcast 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 DF 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 DF Election algorithm. While the Default Algorithm
provides an efficient and automated way of selecting the DF across
different Ethernet Tags in the ES, there are some use cases where a
more 'deterministic' and user-controlled method is required. At the
same time, Service Providers require an easy way to force an on-
demand DF switchover in order to carry out some maintenance tasks on
the existing DF or control whether a new active PE can preempt the
existing DF PE.
This document proposes a DF Election algorithm that meets the
requirements of determinism and operation control.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 September 13, 2021.
Copyright Notice
Copyright (c) 2021 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
1.2. Solution requirements . . . . . . . . . . . . . . . . . . 3
2. Requirements Language and Terminology . . . . . . . . . . . . 4
3. EVPN BGP Attributes Extensions . . . . . . . . . . . . . . . 5
4. Solution description . . . . . . . . . . . . . . . . . . . . 6
4.1. Use of the Highest-Preference Algorithm . . . . . . . . . 7
4.2. Use of the Lowest-Preference Algorithm . . . . . . . . . 9
4.3. Use of the Highest-Preference algorithm in [RFC7432]
Ethernet Segments . . . . . . . . . . . . . . . . . . . . 9
4.4. The Non-Revertive Capability . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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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, multicast and unknown
unicast traffic (BUM) to a multi-homed device/network in the case of
an all-active multi-homing ES or BUM and unicast traffic to a multi-
homed device or network in case of single-active multi-homing. The
DF is selected out of a candidate list of PEs that advertise the
Ethernet Segment Identifier (ESI) to the EVPN network and according
to the DF Election Algorithm, or DF Alg as per [RFC8584].
While the Default DF Alg [RFC7432] or HRW [RFC8584] provide an
efficient and automated way of selecting the DF across different
Ethernet Tags in the ES, there are some use-cases where a more
'deterministic' and user-controlled method is required. At the same
time, Service Providers require an easy way to force an on-demand DF
switchover in order to carry out some maintenance tasks on the
existing DF or control whether a new active PE can preempt the
existing DF PE.
This document proposes a new DF Alg and capability to address the
above needs.
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
DF, assuming they are all operationally ready to take over as DF.
b. This extension works for [RFC7432] Ethernet Segments and virtual
ES, as defined in [I-D.ietf-bess-evpn-virtual-eth-segment].
c. The user may force a PE to preempt the existing DF for a given
Ethernet Tag without re-configuring all the PEs in the ES.
d. The solution allows an option to NOT preempt the current DF, even
if the former DF PE comes back up after a failure. This is also
known as "non-revertive" behavior, as opposed to the [RFC7432] DF
election procedures that are always revertive.
e. The solution works for single-active and all-active multi-homing
Ethernet Segments.
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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
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
o AC - Attachment Circuit. An AC has an Ethernet Tag associated to
it.
o BUM - refers to the Broadcast, Unknown unicast and Multicast
traffic.
o DF, NDF and BDF - Designated Forwarder, Non-Designated Forwarder
and Backup Designated Forwarder.
o DF Alg or simply Alg - refers to Designated Forwarder Election
Algorithm.
o HRW - Highest Random Weight, as per [RFC8584].
o ES, vES and ESI - Ethernet Segment, virtual Ethernet Segment and
Ethernet Segment Identifier.
o EVI - EVPN Instance.
o ISID - refers to Service Instance Identifiers in Provider Backbone
Bridging (PBB) networks.
o MAC-VRF - A Virtual Routing and Forwarding table for Media Access
Control (MAC) addresses on a PE.
o BD - Broadcast Domain. An EVI may be comprised of one (VLAN-Based
or VLAN Bundle services) or multiple (VLAN-Aware Bundle services)
Broadcast Domains.
o EVC - Ethernet Virtual Circuit.
o DP - refers to the "Don't Preempt me" capability in the DF
Election extended community.
o OAM - refers to Operations And Maintenance protocols.
o Ethernet A-D per ES route - refers to [RFC7432] route type 1 or
Auto-Discovery per Ethernet Segment route.
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o Ethernet A-D per EVI route - refers to [RFC7432] route type 1 or
Auto-Discovery per EVPN Instance route.
o Ethernet Tag - used to represent a Broadcast Domain that is
configured on a given ES for the purpose of DF 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 ES. The Ethernet Tag value MUST be different
from zero.
3. EVPN BGP Attributes Extensions
This solution reuses and extends the DF Election Extended Community
defined in [RFC8584] that is advertised along with the ES route:
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 following fields are defined as follows:
o DF Alg can have the following values:
- Alg 0 - Default DF Election algorithm, or modulus-based
algorithm as per [RFC7432].
- Alg 1 - HRW algorithm as per [RFC8584].
- Alg 2 - Highest-Preference algorithm (this document).
- Alg TBD - Lowest-Preference algorithm (this document). TBD
will be replaced by the allocated value at the time of
publication.
o Bitmap (2 octets) can have the following values:
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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): D bit or 'Don't
Preempt' bit (DP hereafter), determines if the PE advertising
the ES route requests the remote PEs in the ES not to preempt
it as DF. The default value is DP=0, which is compatible with
the 'preempt' or 'revertive' behavior in the Default DF Alg
[RFC7432]. The DP capability is supported by Alg 2 and Alg
TBD, and MAY be used with DF Alg 0 or 1. The procedures of the
DP capability for DF Alg 0 or 1 are out of the scope of this
document.
- Bit 1: AC-DF or AC-Influenced DF Election, as explained in
[RFC8584]. When set to 1, it indicates the desire to use AC-
Influenced DF Election with the rest of the PEs in the ES. The
AC-DF capability bit MAY be set along with the DP capability
and DF Alg 2 or Alg TBD.
o DF Preference (defined in this document): defines a 2-octet value
that indicates the PE preference to become the DF in the ES. 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. The DF Preference field is specific
to DF Alg 2 and DF Alg TBD, and does not represent any Preference
value for other Algs. If the DF Alg is different than Alg 2 or
Alg TBD, these two octets can be encoded differently.
4. Solution description
Figure 3 illustrates an example that will be used in the description
of the solution.
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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 - that spans PE1 and PE2 - and CE2 is connected to
vES2, that is defined in PE1, PE2 and PE3.
If the algorithm chosen for vES1 and vES2 is Alg 2 or Alg TBD, i.e.,
Highest-Preference or Lowest-Preference, the PEs may become DF
irrespective of their IP address and based on an 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 Algorithm
Assuming the operator wants to control - in a flexible way - what PE
becomes the DF for a given vES and the order in which the PEs become
DF in case of multiple failures, the following procedure may be used:
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 option (or "Don't
Preempt Me" option) and DF Alg. We will represent these
parameters as (Pref,DP,Alg). Let's assume vES1 is configured as
(500,0,Highest-Pref) in PE1, and (255,0,Highest-Pref) in PE2.
vES2 is configured as (100,0,Highest-Pref), (200,0,Highest-Pref)
and (300,0,Highest-Pref) in PE1, PE2 and PE3 respectively.
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b. The PEs will advertise an ES route for each vES, including the 3
parameters in the DF Election Extended Community.
c. According to [RFC8584], each PE will run the DF election
algorithm upon expiration of the DF Wait timer. In this case,
each PE runs the Highest-Preference DF Alg for each ES as
follows:
- The PE will check the DF Alg value in each ES route, and
assuming all the ES routes are consistent in this DF Alg and
the value is 2 (Highest-Preference), the PE will run the
procedure in this section. Otherwise, the procedure will fall
back to [RFC7432] Default Alg.
- In this Highest-Preference Alg, each PE builds a list of
candidate PEs, ordered by Preference. E.g. PE1 will build a
list of candidate PEs for vES1 ordered by the Preference, from
high to low: PE1>PE2. Hence PE1 will become the DF for vES1.
In the same way, PE3 becomes the DF for vES2.
d. Assuming some maintenance tasks had to be executed on, E.g., PE3,
the operator could set vES2's Preference to E.g., 50 so that PE2
is forced to take over as DF for vES2 (irrespective of the DP
capability). Once the maintenance task on PE3 is over, the
operator could decide to leave the existing preference or
configure the old preference back.
e. In case of equal Preference in two or more PEs in the ES, the DP
bit and the lowest IP of the candidate PEs are used as tie-
breakers. After selecting the PEs with the highest Preference
value, an implementation MUST first select the PE advertising the
DP bit 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 ES
route. Some examples of the use of the DP bit and IP address
tie-breakers follow:
- If vES1 parameters were (500,0,Highest-Pref) in PE1 and
(500,1,Highest-Pref) in PE2, PE2 would be elected due to the
DP bit.
- If vES1 parameters were (500,0,Highest-Pref) in PE1 and
(500,0,Highest-Pref) in PE2, PE1 would be elected, assuming
PE1's IP address is lower than PE2's.
f. The Preference is an administrative option that MUST be
configured on a per-ES basis from the management plane, but MAY
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also be dynamically changed based on the use of local policies.
For instance, on PE1, ES1's Preference can be lowered from 500 to
100 in case the bandwidth on the ENNI port is decreased a 50%
(that could happen if e.g. the 2-port LAG between PE1 and the
Aggregation Network loses one port). Policies 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. The default Preference value is 32767.
The Highest-Preference Alg MAY be used along with the AC-DF
capability. Assuming all the PEs in the ES are configured
consistently with Highest-Preference Alg and AC-DF capability, a
given PE in the ES is not considered as candidate for DF Election
until its corresponding Ethernet A-D per ES and Ethernet A-D per EVI
routes are not received, as described in [RFC8584].
The procedures in this document can be used in [RFC7432] based ES or
vES as in [I-D.ietf-bess-evpn-virtual-eth-segment], and including
EVPN networks as in [RFC8214], [RFC7623] or [RFC8365].
4.2. Use of the Lowest-Preference Algorithm
In addition to the Highest-Preference Alg described in Section 4.1
this document defines the Lowest-Preference Alg. In this case, and
using the example of vES1 in Figure 3, if the Lowest-Preference Alg
is configured in all the PEs in the ES, PE2 will be the DF due to its
lower Preference.
All the procedures described in Section 4.1 apply to the Lowest-
Preference Alg, only replacing the Highest-Preference tie-breaker
with the Lowest-Preference tie-breaker. The Highest-Preference and
Lowest-Preference Algs are different Algs, therefore if two PEs
configured for Highest-Preference and Lowest-Preference respectively,
are attached to the same ES, the operational DF Election Alg will
fall back to the Default Alg.
4.3. Use of the Highest-Preference algorithm in [RFC7432] Ethernet
Segments
While the Highest-Preference (or Lowest-Preference for that matter)
DF Alg described in Section 4.1 is typically used in virtual ES
scenarios where there is normally an individual Ethernet Tag per vES,
the existing [RFC7432] definition of an ES allows potentially up to
thousands of Ethernet Tags on the same ES. If this is the case, if
Highest-Preference (or Lowest-Preference) Alg is configured in all
the PEs of the ES, the same PE will be the elected DF for all the
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Ethernet Tags of the ES. A potential way to achive a more granular
load balancing is decribed below.
The ES is configured with an administrative Preference value and
E.g., Highest-Preference Alg, but then a range of Ethernet Tags can
be defined to use the Lowest-Preference depending on the desired
behavior. With this option, the PE will build a list of candidate
PEs ordered by Preference, however the DF for a given Ethernet Tag
will be determined by the local configuration.
For instance:
o 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).
o 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 will be configured with (Ethernet Tag-range,low),
E.g., (2001-4000, low).
o This will result in PE1 being DF for Ethernet Tags 1-2000 (since
they use the default Highest-Preference Alg) and PE2 being DF for
Ethernet Tags 2001-4000, due to the local policy overriding the
Highest-Preference Alg.
For Ethernet Segments attached to three or more PEs, any other logic
that provides a fair distribution of the DF function among the PEs is
valid, as long as that logic is consistent in all the PEs in the ES.
It is important to note that, when a local policy overrides the
Highest-Preference or Lowest-Preference signaled by all the PEs in
the ES, this local policy MUST be consistent in all the PEs of the
ES. If the local policy is inconsistent for a given Ethernet Tag in
the ES, black-holes or packet duplication may occur on that Ethernet
Tag.
4.4. The Non-Revertive Capability
As discussed in Section 1.2 (d), a capability to NOT preempt the
existing DF for a given Ethernet Tag is required and therefore added
to the DF Election extended community. This option will allow a non-
revertive behavior in the DF election.
Note that, when a given PE in an ES 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 DF comes back up without any controlled maintenance
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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 Alg, PE3
is the DF for ESI2.
If PE3 has a link, EVC or node failure, PE2 would take over as DF.
If/when PE3 comes back up again, PE3 will take over, causing some
unnecessary packet loss in the ES.
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:
o Highest-Preference Alg
o Highest-Preference Alg, where a local policy overrides the
Highest-Preference tie-breaker for a range of Ethernet Tags
o Lowest-Preference Alg
The procedure is described assuming Highest-Preference Alg in the ES,
where local policy overrides the tie-breaker for a given Ethernet
Tag, since this is the most complex case. The other two cases above
are a sub-set of this one and the differences will be explained
later.
1. A "Don't Preempt Me" capability is defined on a per-PE/per-ES
basis, as described in Section 3. If "Don't Preempt Me" is
disabled (default behavior), the advertised DP bit will be 0. If
"Don't Preempt Me" is enabled, the ES route will be advertised
with DP=1 ("Don't Preempt Me"). All the PEs in an ES 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 ES, non-revertive behavior is
not guaranteed. However, PEs supporting this capability will
still attempt this procedure.
2. We assume we want to avoid 'preemption' in all the PEs in the ES,
the three PEs are configured with the "Don't Preempt Me"
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 Alg 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
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will exchange the ES routes and select PE3 as DF for Ethernet
Tag-1 (due to the Highest-Preference), and PE1 as DF 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 DF 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 ES routes from
PE1 and PE2. This timer is applied between the INIT and the
DF_WAIT states in the DF Election Finite State Machine described
in [RFC8584]. PE3 will then:
- Select two "reference-PEs" among the ES routes in the vES, the
"Highest-PE" and the "Lowest-PE":
* 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 tie-breakers. PE3 will select PE2
as Highest-PE over PE1, since, when comparing (Pref,DP,PE-
IP), (200,1,PE2-IP) wins over (100,1,PE1-IP).
* 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 tie-breakers. 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 ES,
Lowest and Highest PE would be the same PE.
- Check its own administrative Pref and compares it with the one
of the Highest-PE and Lowest-PE that have DP=1 in their ES
routes. Depending on this comparison PE3 will send the ES
route with a (Pref,DP) that may be different from its
administrative (Pref,DP):
* If PE3's Pref value is higher than the Highest-PE's, PE3
will send the ES route with an 'in-use' operational Pref
equal to the Highest-PE's and DP=0.
* If PE3's Pref value is lower than the Lowest-PE's, PE3 will
send the ES route with an 'in-use' operational Preference
equal to the Lowest-PE's and DP=0.
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* If PE3's Pref value is neither higher nor lower than the
Highest-PE's or the Lowest-PE's respectively, PE3 will send
the ES 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 ES
route with an operational 'in-use' (Pref,DP)=(200,0).
- Note that, a PE will always send DP=0 as long as the
advertised Pref is the 'in-use' operational Pref (as opposed
to the 'administrative' Pref).
- This ES route update sent by PE3, with (200,0,PE3-IP), will
not cause any DF switchover for any Ethernet Tag. PE2 will
continue being DF for Ethernet Tag-1. This is because the DP
bit will be used as a tie-breaker in the DF election. That
is, if a PE has two candidate PEs with the same Pref, it will
pick up the one with DP=1. There are no DF changes for
Ethernet Tag-2 either.
6. For any subsequent update/withdraw in the ES, the PEs will go
through the process described in (5) to select Highest and
Lowest-PEs. For instance, if PE2 fails, upon receiving PE2's ES
route withdrawal, PE3 and PE1 will go through the selection of
new Highest and Lowest-PEs (considering their own active ES
route) and then they will run the DF 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 ES route update with its administrative Pref and DP
values. In the example, PE3 will be the new Highest-PE,
therefore it will send an ES route update with
(Pref,DP)=(300,1).
- After running the DF Election, PE3 will become the new DF for
Ethernet Tag-1. No changes will occur for Ethernet Tag-2.
If the ES uses Highest-Preference Alg (for all the Ethernet Tags, no
local policy), the PEs only need to select the "Highest-PE" as the
"reference-PE" (i.e., no need to select the "Lowest-PE"). If the ES
uses Lowest-Preference Alg for all the Ethernet Tags, the PEs only
need to select the "Lowest-PE" as the "reference-PE". The rest of
the procedure remains the same.
Note that, irrespective of the DP bit, when a PE or ES comes back and
the PE advertises a DF Election Alg different than the one configured
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in the rest of the PEs in the ES, all the PEs in the ES MUST fall
back to the Default [RFC7432] Alg.
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 ES
after running the DF Election, irrespective of the revertive or non-
revertive behavior in the PE.
5. Security Considerations
This document describes a DF Election Algorithm that provides
absolute control (by configuration) over what PE is the DF 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
ES may change the behavior of the network. In other DF Algs such as
HRW, the DF Election is more automated and cannot be determined by
configuration.
The non-revertive capability described in this document may be seen
as a security improvement over the regular EVPN revertive DF
Election: an intentional link (or node) "flapping" on a PE will only
cause service disruption once, when the PE goes to NDF state.
The document also describes how a local policy can override the
Highest-Preference Alg for a range of Ethernet Tags in the ES. If
the local policy is not consistent across all PEs in the ES and there
is an Ethernet Tag that ends up with an inconsistent use of Highest-
Preference or Lowest-Preference in different PEs, black-holing or
packet duplication may occur for that Ethernet Tag.
6. IANA Considerations
This document solicits the allocation of the following values:
o DF Alg = 2 in the [RFC8584] "DF Alg" registry, with name "Highest-
Preference Algorithm".
o DF Alg = TBD in the same "DF Alg" registry, with name "Lowest-
Preference Algorithm".
o Bit 0 in the [RFC8584] DF Election Capabilities registry, with
name "D (Don't Preempt) Capability" for Non-revertive ES.
7. Acknowledgments
The authors would like to thank Kishore Tiruveedhula for his review
and comments. Also thank you to Luc Andre Burdet and Stephane
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Litkowski for their thorough review and suggestions for a new DF Alg
for lowest-preference.
8. Contributors
In addition to the authors listed, the following individuals also
contributed to this document:
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>.
[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", draft-ietf-bess-
evpn-virtual-eth-segment-06 (work in progress), March
2020.
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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
777 Middlefield Road
Mountain View, CA 94043
USA
Email: jorge.rabadan@nokia.com
S. Sathappan
Nokia
Email: senthil.sathappan@nokia.com
T. Przygienda
Juniper Networks
Email: prz@juniper.net
W. Lin
Juniper Networks
Email: wlin@juniper.net
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J. Drake
Juniper Networks
Email: jdrake@juniper.net
A. Sajassi
Cisco Systems
Email: sajassi@cisco.com
S. Mohanty
Cisco Systems
Email: satyamoh@cisco.com
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