BESS Workgroup J. Rabadan, Ed.
Internet Draft S. Sathappan
Intended status: Standards Track Nokia
S. Boutros T. Przygienda
Individual W. Lin
J. Drake
Juniper Networks
A. Sajassi
S. Mohanty
Cisco Systems
Expires: April 25, 2019 October 22, 2018
Preference-based EVPN DF Election
draft-ietf-bess-evpn-pref-df-02
Abstract
The Designated Forwarder (DF) in (PBB-)EVPN networks is defined 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 in the 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, according to
the Default DF Election algorithm.
While the Default Algorithm provides an efficient and automated way
of selecting the DF across different EVIs or ISIDs 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 an extension to the Default DF election
procedures so that the above requirements can be met.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
2. Solution requirements . . . . . . . . . . . . . . . . . . . . . 3
3. EVPN BGP Attributes for Deterministic DF Election . . . . . . . 4
4. Solution description . . . . . . . . . . . . . . . . . . . . . 5
4.1 Use of the Preference algorithm . . . . . . . . . . . . . . 6
4.2 Use of the Preference algorithm in [RFC7432]
Ethernet-Segments . . . . . . . . . . . . . . . . . . . . . 7
4.3 The Non-Revertive option . . . . . . . . . . . . . . . . . . 8
5. Conventions used in this document . . . . . . . . . . . . . . . 11
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6. Security Considerations . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
8.2 Informative References . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 12
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
1. Problem Statement
[RFC7432] defines the Designated Forwarder (DF) in (PBB-)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 Alg [DF].
While the Default DF Election Algorithm provides an efficient and
automated way of selecting the DF across different EVIs or ISIDs 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 an extension to the current Default DF
election procedures [RFC7432] so that the above requirements can be
met.
2. Solution requirements
This document proposes an extension of the [RFC7432] Default DF
election Algorithm motivated by 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.
b) This extension works for [RFC7432] Ethernet Segments (ES) and
virtual ES, as defined in [vES].
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c) The user may force a PE to preempt the existing DF for a given
EVI/ISID 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.
3. EVPN BGP Attributes for Deterministic DF Election
This solution reuses and extends the DF Election Extended Community
defined in [DF] 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)| 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, modulo based DF election as per [RFC7432].
- Alg 1 - HRW algorithm as per [DF]
- Alg 2 - Preference algorithm (this document)
o Bitmap can have the following values:
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) - D bit or 'Don't Preempt' bit (DP hereafter),
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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 Alg [RFC7432]. The DP bit SHOULD be ignored if the DF
Alg is different than 2.
- Bit 1 - AC-DF or AC-Influenced DF Election, explained in [DF].
The AC-DF capability bit MAY be set along with the DP capability
and Alg 2.
o DF Preference 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 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 does not represent any
Preference value for other Algs.
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 - ES and Deterministic DF Election
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Figure 1 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, i.e. Preference-
based, 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 new defined procedures and how they are
applied in the use-case in Figure 1.
4.1 Use of the 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,Pref]
in PE1, and [255,0,Pref] in PE2. vES2 is configured as
[100,0,Pref], [200,0,Pref] and [300,0,Pref] in PE1, PE2 and PE3
respectively.
b) The PEs will advertise an ES route for each vES, including the 3
parameters in the DF Election Extended Community.
c) According to [RFC7432], each PE will wait for the DF timer to
expire before running the DF election algorithm. After the timer
expires, each PE runs the Preference-based DF election algorithm
as follows:
o The PE will check the DF Alg in each ES route, and assuming all
the ES routes are consistent in this DF Alg and the value is 2
(Preference-based), the PE will run the new extended procedure.
Otherwise, the procedure will fall back to [RFC7432] Default
Alg.
o In this extended procedure, each PE builds a list of candidate
PEs, ordered based on the 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) Note that, by default, the Highest-Preference is chosen for each
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ES or vES, however the ES configuration can be changed to the
Lowest-Preference algorithm as long as this option is consistent
in all the PEs in the ES. E.g. vES1 could have been explicitly
configured as Alg Preference-based with Lowest-Preference, in
which case, PE2 would have been the DF.
e) Assuming some maintenance tasks had to be executed on PE3, the
operator could set vES2's preference to e.g. 50 so that PE2 is
forced to take over as DF for vES2. Once the maintenance on PE3 is
over, the operator could decide to leave the existing preference
or configure the old preference back.
f) In case of equal Preference in two or more PEs in the ES, the tie-
breakers will be the DP bit and the lowest IP PE in that order.
For instance:
o If vES1 parameters were [500,0,Pref] in PE1 and [500,1,Pref] in
PE2, PE2 would be elected due to the DP bit.
o If vES1 parameters were [500,0,Pref] in PE1 and [500,0,Pref] in
PE2, PE1 would be elected, assuming PE1's IP address is lower
than PE2's.
g) The Preference is an administrative option that MUST be configured
on a per-ES basis from the management plane, but MAY 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, of 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.
4.2 Use of the Preference algorithm in [RFC7432] Ethernet-Segments
While the Preference-based DF Alg described in section 4.1 is
typically used in virtual ES scenarios where there is normally an
individual EVI per vES, the existing [RFC7432] definition of ES
allows potentially up to thousands of EVIs on the same ES. If this is
the case, and the operator still wants to control who the DF is for a
given EVI, the use of the Preference-based DF Alg can also provide
the desired level of load balancing.
In this type of scenarios, the ES is configured with an
administrative Preference value, but then a range of EVI/ISIDs can be
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defined to use the Highest-Preference or the Lowest-Preference
depending on the desired behavior. With this option, the PE will
build a list of candidate PEs ordered by the Preference, however the
DF for a given EVI/ISID 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,Preference] for ES3 and PE2 as [100,0,Preference].
o In addition, assuming vlan-based service interfaces, the PEs will
be configured with (vlan/ISID-range,high_or_low), e.g. (1-
2000,high) and (2001-4000, low).
o This will result in PE1 being DF for EVI/ISIDs 1-2000 and PE2 being
DF for EVI/ISIDs 2001-4000.
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.
4.3 The Non-Revertive option
As discussed in section 2(d), an option to NOT preempt the existing
DF for a given EVI/ISID 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
operation, and the non-revertive option is desired in order to avoid
service impact.
In Figure 1, we assume that based on the Highest-Pref, 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 1):
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1) A new "Don't Preempt Me" parameter 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").
2) Assuming we want to avoid 'preemption', the three PEs are
configured with the "Don't Preempt Me" option. Note that each PE
individually MAY be configured with different preemption value. In
this example, we assume ESI2 is configured as 'DP=enabled' in the
three PEs.
3) Assuming EVI1 uses Highest-Pref in vES2 and EVI2 uses Lowest-Pref,
when vES2 is enabled in the three PEs, the PEs will exchange the
ES routes and select PE3 as DF for EVI1 (due to the Highest-Pref
type), and PE1 as DF for EVI2 (due to the Lowest-Pref).
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 EVI1. No
changes will occur for EVI2.
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. PE3 will then:
o 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.
o 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
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[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.
- 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 EVI/ISID. PE2 will continue being
DF for EVI1. 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 EVI2 either.
6) Subsequently, if PE2 fails, upon receiving PE2's ES route
withdrawal, PE3 and PE1 will go through the process described in
(5) to select new Highest and Lowest-PEs (considering their own
active ES route) and then they will run the DF Election.
o 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].
o After running the DF Election, PE3 will become the new DF for
EVI1. No changes will occur for EVI2.
Note that, irrespective of the DP bit, when a PE or ES comes back and
the PE advertises a DF Election Alg different than 2 (Preference
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algorithm), the rest of the PEs in the ES MUST fall back to the
Default [RFC7432] Alg.
5. Conventions used in this document
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.
6. Security Considerations
This section will be added in future versions.
7. IANA Considerations
This document solicits the allocation of DF Alg = 2 in the registry
created by [DF] for the DF Alg field, and the DP bit (Bit 0) in the
[DF] Bitmap registry.
8. References
8.1 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>.
[DF] Rabadan J., Mohanty S. et al. "Framework for EVPN Designated
Forwarder Election Extensibility", draft-ietf-bess-evpn-df-election-
framework-05, work-in-progress, October, 2018.
[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>.
8.2 Informative References
[vES] Sajassi et al. "EVPN Virtual Ethernet Segment", draft-ietf-
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bess-evpn-virtual-eth-segment-00, work-in-progress, June, 2018.
9. Acknowledgments
The authors would like to thank Kishore Tiruveedhula for his review
and comments.
10. Contributors
In addition to the authors listed, the following individuals also
contributed to this document:
Kiran Nagaraj, Nokia
Vinod Prabhu, Nokia
Selvakumar Sivaraj, Juniper
11. Authors' Addresses
Jorge Rabadan
Nokia
777 E. Middlefield Road
Mountain View, CA 94043 USA
Email: jorge.rabadan@nokia.com
Senthil Sathappan
Alcatel-Lucent
Email: senthil.sathappan@nokia.com
Tony Przygienda
Juniper Networks, Inc.
Email: prz@juniper.net
John Drake
Juniper Networks, Inc.
Email: jdrake@juniper.net
Wen Lin
Juniper Networks, Inc.
Email: wlin@juniper.net
Ali Sajassi
Cisco Systems, Inc.
Email: sajassi@cisco.com
Satya Ranjan Mohanty
Cisco Systems, Inc.
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Email: satyamoh@cisco.com
Sami Boutros
Email: boutros.sami@gmail.com
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