rfc8388
Internet Engineering Task Force (IETF) J. Rabadan, Ed.
Request for Comments: 8388 S. Palislamovic
Category: Informational W. Henderickx
ISSN: 2070-1721 Nokia
A. Sajassi
Cisco
J. Uttaro
AT&T
May 2018
Usage and Applicability of BGP MPLS-Based Ethernet VPN
Abstract
This document discusses the usage and applicability of BGP MPLS-based
Ethernet VPN (EVPN) in a simple and fairly common deployment
scenario. The different EVPN procedures are explained in the example
scenario along with the benefits and trade-offs of each option. This
document is intended to provide a simplified guide for the deployment
of EVPN networks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8388.
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Copyright Notice
Copyright (c) 2018 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use Case Scenario Description and Requirements . . . . . . . 5
3.1. Service Requirements . . . . . . . . . . . . . . . . . . 5
3.2. Why EVPN Is Chosen to Address This Use Case . . . . . . . 7
4. Provisioning Model . . . . . . . . . . . . . . . . . . . . . 7
4.1. Common Provisioning Tasks . . . . . . . . . . . . . . . . 8
4.1.1. Non-Service-Specific Parameters . . . . . . . . . . . 8
4.1.2. Service-Specific Parameters . . . . . . . . . . . . . 9
4.2. Service-Interface-Dependent Provisioning Tasks . . . . . 9
4.2.1. VLAN-Based Service Interface EVI . . . . . . . . . . 10
4.2.2. VLAN Bundle Service Interface EVI . . . . . . . . . . 10
4.2.3. VLAN-Aware Bundling Service Interface EVI . . . . . . 10
5. BGP EVPN NLRI Usage . . . . . . . . . . . . . . . . . . . . . 11
6. MAC-Based Forwarding Model Use Case . . . . . . . . . . . . . 11
6.1. EVPN Network Startup Procedures . . . . . . . . . . . . . 12
6.2. VLAN-Based Service Procedures . . . . . . . . . . . . . . 12
6.2.1. Service Startup Procedures . . . . . . . . . . . . . 13
6.2.2. Packet Walk-Through . . . . . . . . . . . . . . . . . 13
6.3. VLAN Bundle Service Procedures . . . . . . . . . . . . . 17
6.3.1. Service Startup Procedures . . . . . . . . . . . . . 17
6.3.2. Packet Walk-Through . . . . . . . . . . . . . . . . . 18
6.4. VLAN-Aware Bundling Service Procedures . . . . . . . . . 18
6.4.1. Service Startup Procedures . . . . . . . . . . . . . 18
6.4.2. Packet Walk-Through . . . . . . . . . . . . . . . . . 19
7. MPLS-Based Forwarding Model Use Case . . . . . . . . . . . . 20
7.1. Impact of MPLS-Based Forwarding on the EVPN Network
Startup . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.2. Impact of MPLS-Based Forwarding on the VLAN-Based Service
Procedures . . . . . . . . . . . . . . . . . . . . . . . 21
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7.3. Impact of MPLS-Based Forwarding on the VLAN Bundle
Service Procedures . . . . . . . . . . . . . . . . . . . 22
7.4. Impact of MPLS-Based Forwarding on the VLAN-Aware Service
Procedures . . . . . . . . . . . . . . . . . . . . . . . 22
8. Comparison between MAC-Based and MPLS-Based Egress Forwarding
Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9. Traffic Flow Optimization . . . . . . . . . . . . . . . . . . 24
9.1. Control-Plane Procedures . . . . . . . . . . . . . . . . 24
9.1.1. MAC Learning Options . . . . . . . . . . . . . . . . 24
9.1.2. Proxy ARP/ND . . . . . . . . . . . . . . . . . . . . 25
9.1.3. Unknown Unicast Flooding Suppression . . . . . . . . 25
9.1.4. Optimization of Inter-Subnet Forwarding . . . . . . . 26
9.2. Packet Walk-Through Examples . . . . . . . . . . . . . . 27
9.2.1. Proxy ARP Example for CE2-to-CE3 Traffic . . . . . . 27
9.2.2. Flood Suppression Example for CE1-to-CE3 Traffic . . 27
9.2.3. Optimization of Inter-subnet Forwarding Example for
CE3-to-CE2 Traffic . . . . . . . . . . . . . . . . . 28
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative References . . . . . . . . . . . . . . . . . . 30
12.2. Informative References . . . . . . . . . . . . . . . . . 30
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 30
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
This document complements [RFC7432] by discussing the applicability
of the technology in a simple and fairly common deployment scenario,
which is described in Section 3.
After describing the topology and requirements of the use case
scenario, Section 4 will describe the provisioning model.
Once the provisioning model is analyzed, Sections 5, 6, and 7 will
describe the control-plane and data-plane procedures in the example
scenario for the two potential disposition/forwarding models: MAC-
based and MPLS-based models. While both models can interoperate in
the same network, each one has different trade-offs that are analyzed
in Section 8.
Finally, EVPN provides some potential traffic flow optimization tools
that are also described in Section 9 in the context of the example
scenario.
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2. Terminology
The following terminology is used:
VID: VLAN Identifier
CE: Customer Edge (device)
EVI: EVPN Instance
MAC-VRF: A Virtual Routing and Forwarding (VRF) table for Media
Access Control (MAC) addresses on a Provider Edge (PE) router.
ES: An Ethernet Segment is a set of links through which a CE is
connected to one or more PEs. Each ES is identified by an
Ethernet Segment Identifier (ESI) in the control plane.
CE-VIDs: The VLAN Identifier tags being used at CE1, CE2, and CE3 to
tag customer traffic sent to the service provider EVPN network.
CE1-MAC, CE2-MAC, and CE3-MAC: The source MAC addresses "behind"
each CE, respectively. These MAC addresses can belong to the
CEs themselves or to devices connected to the CEs.
CE1-IP, CE2-IP, and CE3-IP: The IP addresses associated with the
above MAC addresses
LACP: Link Aggregation Control Protocol
RD: Route Distinguisher
RT: Route Target
PE: Provider Edge (router)
AS: Autonomous System
PE-IP: The IP address of a given PE
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3. Use Case Scenario Description and Requirements
Figure 1 depicts the scenario that will be referenced throughout the
rest of the document.
+--------------+
| |
+----+ +----+ | | +----+ +----+
| CE1|-----| | | | | |---| CE3|
+----+ /| PE1| | IP/MPLS | | PE3| +----+
/ +----+ | Network | +----+
/ | |
/ +----+ | |
+----+/ | | | |
| CE2|-----| PE2| | |
+----+ +----+ | |
+--------------+
Figure 1: EVPN Use Case Scenario
There are three PEs and three CEs considered in this example: PE1,
PE2, and PE3, as well as CE1, CE2, and CE3. Broadcast domains must
be extended among the three CEs.
3.1. Service Requirements
The following service requirements are assumed in this scenario:
o Redundancy requirements:
- CE2 requires multihoming connectivity to PE1 and PE2, not only
for redundancy purposes but also for adding more upstream/
downstream connectivity bandwidth to/from the network.
- Fast convergence. For example, if the link between CE2 and PE1
goes down, a fast convergence mechanism must be supported so
that PE3 can immediately send the traffic to PE2, irrespective
of the number of affected services and MAC addresses.
o Service interface requirements:
- The service definition must be flexible in terms of CE-VID-to-
broadcast-domain assignment in the core.
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- The following three EVI services are required in this example:
EVI100 uses VLAN-based service interfaces in the three CEs with
a 1:1 VLAN-to-EVI mapping. The CE-VIDs at the three CEs can be
the same (for example, VID 100) or different at each CE (for
instance, VID 101 in CE1, VID 102 in CE2, and VID 103 in CE3).
A single broadcast domain needs to be created for EVI100 in any
case; therefore, CE-VIDs will require translation at the egress
PEs if they are not consistent across the three CEs. The case
when the same CE-VID is used across the three CEs for EVI100 is
referred to in [RFC7432] as the "Unique VLAN" EVPN case. This
term will be used throughout this document too.
EVI200 uses VLAN bundle service interfaces in CE1, CE2, and CE3
based on an N:1 VLAN-to-EVI mapping. The operator needs to
preconfigure a range of CE-VIDs and its mapping to the EVI, and
this mapping should be consistent in all the PEs (no
translation is supported). A single broadcast domain is
created for the customer. The customer is responsible for
keeping the separation between users in different CE-VIDs.
EVI300 uses VLAN-aware bundling service interfaces in CE1, CE2,
and CE3. As in the EVI200 case, an N:1 VLAN-to-EVI mapping is
created at the ingress PEs; however, in this case, a separate
broadcast domain is required per CE-VID. The CE-VIDs can be
different (hence, CE-VID translation is required).
Note that in Section 4.2.1, only EVI100 is used as an example of
VLAN-based service provisioning. In Sections 6.2 and 7.2, 4k VLAN-
based EVIs (EVI1 to EVI4k) are used so that the impact of MAC versus
MPLS disposition models in the control plane can be evaluated. In
the same way, EVI200 and EVI300 will be described with a 4k:1 mapping
(CE-VIDs-to-EVI mapping) in Sections 6.3, 6.4, 7.3, and 7.4.
o Broadcast, Unknown Unicast, Multicast (BUM) optimization
requirements:
- The solution must support ingress replication or P2MP MPLS LSPs
on a per EVI service. For example, we can use ingress
replication for EVI100 and EVI200, assuming those EVIs will not
carry much BUM traffic. On the contrary, if EVI300 is
presumably carrying a significant amount of multicast traffic,
P2MP MPLS LSPs can be used for this service.
- The benefit of ingress replication compared to P2MP LSPs is
that the core routers will not need to maintain any multicast
states.
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3.2. Why EVPN Is Chosen to Address This Use Case
Virtual Private LAN Service (VPLS) solutions based on [RFC4761],
[RFC4762], and [RFC6074] cannot meet the requirements in Section 3,
whereas EVPN can.
For example:
o If CE2 has a single CE-VID (or a few CE-VIDs), the current VPLS
multihoming solutions (based on load-balancing per CE-VID or
service) do not provide the optimized link utilization required in
this example. EVPN provides the flow-based, load-balancing,
multihoming solution required in this scenario to optimize the
upstream/downstream link utilization between CE2 and PE1-PE2.
o EVPN provides a fast convergence solution that is independent of
the CE-VIDs in the multihomed PEs. Upon failure on the link
between CE2 and PE1, PE3 can immediately send the traffic to PE2
based on a single notification message being sent by PE1. This is
not possible with VPLS solutions.
o With regard to service interfaces and mapping to broadcast
domains, while VPLS might meet the requirements for EVI100 and
EVI200, the VLAN-aware bundling service interfaces required by
EVI300 are not supported by the current VPLS tools.
The rest of the document will describe how EVPN can be used to meet
the service requirements described in Section 3 and even optimize the
network further by:
o providing the user with an option to reduce (and even suppress)
ARP (Address Resolution Protocol) flooding; and
o supporting ARP termination and inter-subnet forwarding.
4. Provisioning Model
One of the requirements stated in [RFC7209] is the ease of
provisioning. BGP parameters and service context parameters should
be auto-provisioned so that the addition of a new MAC-VRF to the EVI
requires a minimum number of single-sided provisioning touches.
However, this is possible only in a limited number of cases. This
section describes the provisioning tasks required for the services
described in Section 3, i.e., EVI100 (VLAN-based service interfaces),
EVI200 (VLAN bundle service interfaces), and EVI300 (VLAN-aware
bundling service interfaces).
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4.1. Common Provisioning Tasks
Regardless of the service interface type (VLAN-based, VLAN bundle, or
VLAN-aware), the following subsections describe the parameters to be
provisioned in the three PEs.
4.1.1. Non-Service-Specific Parameters
The multihoming function in EVPN requires the provisioning of certain
parameters that are not service specific and that are shared by all
the MAC-VRFs in the node using the multihoming capabilities. In our
use case, these parameters are only provisioned or auto-derived in
PE1 and PE2 and are listed below:
o Ethernet Segment Identifier (ESI): Only the ESI associated with
CE2 needs to be considered in our example. Single-homed CEs such
as CE1 and CE3 do not require the provisioning of an ESI (the ESI
will be coded as zero in the BGP Network Layer Reachability
Information (NLRI)). In our example, a Link Aggregation Group
(LAG) is used between CE2 and PE1-PE2 (since all-active
multihoming is a requirement); therefore, the ESI can be auto-
derived from the LACP information as described in [RFC7432]. Note
that the ESI must be unique across all the PEs in the network;
therefore, the auto-provisioning of the ESI is recommended only in
case the CEs are managed by the operator. Otherwise, the ESI
should be manually provisioned (Type 0, as in [RFC7432]) in order
to avoid potential conflicts.
o ES-Import Route Target (ES-Import RT): This is the RT that will be
sent by PE1 and PE2, along with the ES route. Regardless of how
the ESI is provisioned in PE1 and PE2, the ES-Import RT must
always be auto-derived from the 6-byte MAC address portion of the
ESI value.
o Ethernet Segment Route Distinguisher (ES RD): This is the RD to be
encoded in the ES route, and it is the Ethernet Auto-Discovery
(A-D) route to be sent by PE1 and PE2 for the CE2 ESI. This RD
should always be auto-derived from the PE-IP address, as described
in [RFC7432].
o Multihoming type: The user must be able to provision the
multihoming type to be used in the network. In our use case, the
multihoming type will be set to all-active for the CE2 ESI. This
piece of information is encoded in the ESI Label extended
community flags and is sent by PE1 and PE2 along with the Ethernet
A-D route for the CE2 ESI.
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In addition, the same LACP parameters will be configured in PE1 and
PE2 for the ES so that CE2 can send frames to PE1 and PE2 as though
they were forming a single system.
4.1.2. Service-Specific Parameters
The following parameters must be provisioned in PE1, PE2, and PE3 per
EVI service:
o EVI Identifier: The global identifier per EVI that is shared by
all the PEs that are part of the EVI, i.e., PE1, PE2, and PE3 will
be provisioned with EVI100, 200, and 300. The EVI identifier can
be associated with (or be the same value as) the EVI default
Ethernet Tag (4-byte default broadcast domain identifier for the
EVI). The Ethernet Tag is different from zero in the EVPN BGP
routes only if the service interface type (of the source PE) is a
VLAN-aware bundle.
o EVI Route Distinguisher (EVI RD): This RD is a unique value across
all the MAC-VRFs in a PE. Auto-derivation of this RD might be
possible depending on the service interface type being used in the
EVI. The next section discusses the specifics of each service
interface type.
o EVI Route Target(s) (EVI RT): One or more RTs can be provisioned
per MAC-VRF. The RT(s) imported and exported can be equal or
different, just as the RT(s) in IP-VPNs. Auto-derivation of this
RT(s) might be possible depending on the service interface type
being used in the EVI. The next section discusses the specifics
of each service interface type.
o CE-VID and port/LAG binding to EVI identifier or Ethernet Tag: For
more information, please see Section 4.2.
4.2. Service-Interface-Dependent Provisioning Tasks
Depending on the service interface type being used in the EVI, a
given CE-VID binding provision must be specified.
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4.2.1. VLAN-Based Service Interface EVI
In our use case, EVI100 is a VLAN-based service interface EVI.
EVI100 can be a "unique-VLAN" service if the CE-VID being used for
this service in CE1, CE2, and CE3 is identical (for example, VID
100). In that case, the VID 100 binding must be provisioned in PE1,
PE2, and PE3 for EVI100 and the associated port or LAG. The MAC-VRF
RD and RT can be auto-derived from the CE-VID:
o The auto-derived MAC-VRF RD will be a Type 1 RD, as recommended in
[RFC7432], and it will be comprised of [PE-IP]:[zero-padded-VID];
where [PE-IP] is the IP address of the PE (a loopback address) and
[zero-padded-VID] is a 2-byte value where the low-order 12 bits
are the VID (VID 100 in our example) and the high-order 4 bits are
zero.
o The auto-derived MAC-VRF RT will be composed of [AS]:[zero-padded-
VID]; where [AS] is the Autonomous System that the PE belongs to
and [zero-padded-VID] is a 2- or 4-byte value where the low-order
12 bits are the VID (VID 100 in our example) and the high-order
bits are zero. Note that auto-deriving the RT implies supporting
a basic any-to-any topology in the EVI and using the same import
and export RT in the EVI.
If EVI100 is not a "unique-VLAN" instance, each individual CE-VID
must be configured in each PE, and MAC-VRF RDs and RTs cannot be
auto-derived; hence, they must be provisioned by the user.
4.2.2. VLAN Bundle Service Interface EVI
Assuming EVI200 is a VLAN bundle service interface EVI, and VIDs
200-250 are assigned to EVI200, the CE-VID bundle 200-250 must be
provisioned on PE1, PE2, and PE3. Note that this model does not
allow CE-VID translation and the CEs must use the same CE-VIDs for
EVI200. No auto-derived EVI RDs or EVI RTs are possible.
4.2.3. VLAN-Aware Bundling Service Interface EVI
If EVI300 is a VLAN-aware bundling service interface EVI, CE-VID
binding to EVI300 does not have to match on the three PEs (only on
PE1 and PE2, since they are part of the same ES). For example, PE1
and PE2 CE-VID binding to EVI300 can be set to the range 300-310 and
PE3 to 321-330. Note that each individual CE-VID will be assigned to
a different broadcast domain, which will be represented by an
Ethernet Tag in the control plane.
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Therefore, besides the CE-VID bundle range bound to EVI300 in each
PE, associations between each individual CE-VID and the corresponding
EVPN Ethernet Tag must be provisioned by the user. No auto-derived
EVI RDs/RTs are possible.
5. BGP EVPN NLRI Usage
[RFC7432] defines four different route types and four different
extended communities. However, not all the PEs in an EVPN network
must generate and process all the different routes and extended
communities. Table 1 shows the routes that must be exported and
imported in the use case described in this document. "Export", in
this context, means that the PE must be capable of generating and
exporting a given route, assuming there are no BGP policies to
prevent it. In the same way, "Import" means the PE must be capable
of importing and processing a given route, assuming the right RTs and
policies. "N/A" means neither import nor export actions are
required.
+-----------------+---------------+---------------+
| BGP EVPN Routes | PE1-PE2 | PE3 |
+-----------------+---------------+---------------+
| ES | Export/Import | N/A |
| A-D per ESI | Export/Import | Import |
| A-D per EVI | Export/Import | Import |
| MAC | Export/Import | Export/Import |
| Inclusive Mcast | Export/Import | Export/Import |
+-----------------+---------------+---------------+
Table 1: Base EVPN Routes and Export/Import Actions
PE3 is required to export only MAC and Inclusive Multicast (Mcast)
routes and be able to import and process A-D routes as well as MAC
and Inclusive Multicast routes. If PE3 did not support importing and
processing A-D routes per ESI and per EVI, fast convergence and
aliasing functions (respectively) would not be possible in this use
case.
6. MAC-Based Forwarding Model Use Case
This section describes how the BGP EVPN routes are exported and
imported by the PEs in our use case as well as how traffic is
forwarded assuming that PE1, PE2, and PE3 support a MAC-based
forwarding model. In order to compare the control- and data-plane
impact in the two forwarding models (MAC-based and MPLS-based) and
different service types, we will assume that CE1, CE2, and CE3 need
to exchange traffic for up to 4k CE-VIDs.
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6.1. EVPN Network Startup Procedures
Before any EVI is provisioned in the network, the following
procedures are required:
o Infrastructure setup: The proper MPLS infrastructure must be set
up among PE1, PE2, and PE3 so that the EVPN services can make use
of Point-to-Point (P2P) and P2MP LSPs. In addition to the MPLS
transport, PE1 and PE2 must be properly configured with the same
LACP configuration to CE2. Details are provided in [RFC7432].
Once the LAG is properly set up, the ESI for the CE2 Ethernet
Segment (for example, ESI12) can be auto-generated by PE1 and PE2
from the LACP information exchanged with CE2 (ESI Type 1), as
discussed in Section 4.1. Alternatively, the ESI can also be
manually provisioned on PE1 and PE2 (ESI Type 0). PE1 and PE2
will auto-configure a BGP policy that will import any ES route
matching the auto-derived ES-Import RT for ESI12.
o Ethernet Segment route exchange and Designated Forwarder (DF)
election: PE1 and PE2 will advertise a BGP Ethernet Segment route
for ESI12, where the ESI RD and ES-Import RT will be auto-
generated as discussed in Section 4.1.1. PE1 and PE2 will import
the ES routes of each other and will run the DF election algorithm
for any existing EVI (if any, at this point). PE3 will simply
discard the route. Note that the DF election algorithm can
support service carving so that the downstream BUM traffic from
the network to CE2 can be load-balanced across PE1 and PE2 on a
per-service basis.
At the end of this process, the network infrastructure is ready to
start deploying EVPN services. PE1 and PE2 are aware of the
existence of a shared Ethernet Segment, i.e., ESI12.
6.2. VLAN-Based Service Procedures
Assuming that the EVPN network must carry traffic among CE1, CE2, and
CE3 for up to 4k CE-VIDs, the service provider can decide to
implement VLAN-based service interface EVIs to accomplish it. In
this case, each CE-VID will be individually mapped to a different
EVI. While this means a total number of 4k MAC-VRFs are required per
PE, the advantages of this approach are the auto-provisioning of most
of the service parameters if no VLAN translation is needed (see
Section 4.2.1) and great control over each individual customer
broadcast domain. We assume in this section that the range of EVIs
from 1 to 4k is provisioned in the network.
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6.2.1. Service Startup Procedures
As soon as the EVIs are created in PE1, PE2, and PE3, the following
control-plane actions are carried out:
o Flooding tree setup per EVI (4k routes): Each PE will send one
Inclusive Multicast Ethernet Tag route per EVI (up to 4k routes
per PE) so that the flooding tree per EVI can be set up. Note
that ingress replication or P2MP LSPs can be optionally signaled
in the Provider Multicast Service Interface (PMSI) Tunnel
attribute and the corresponding tree can be created.
o Ethernet A-D routes per ESI (a set of routes for ESI12): A set of
A-D routes with a total list of 4k RTs (one per EVI) for ESI12
will be issued from PE1 and PE2 (it has to be a set of routes so
that the total number of RTs can be conveyed). As per [RFC7432],
each Ethernet A-D route per ESI is differentiated from the other
routes in the set by a different Route Distinguisher (ES RD).
This set will also include ESI Label extended communities with the
active-standby flag set to zero (all-active multihoming type) and
an ESI Label different from zero (used for split-horizon
functions). These routes will be imported by the three PEs, since
the RTs match the locally configured EVI RTs. The A-D routes per
ESI will be used for fast convergence and split-horizon functions,
as discussed in [RFC7432].
o Ethernet A-D routes per EVI (4k routes): An A-D route per EVI will
be sent by PE1 and PE2 for ESI12. Each individual route includes
the corresponding EVI RT and an MPLS Label to be used by PE3 for
the aliasing function. These routes will be imported by the three
PEs.
6.2.2. Packet Walk-Through
Once the services are set up, the traffic can start flowing.
Assuming there are no MAC addresses learned yet and that MAC learning
at the access is performed in the data plane in our use case, this is
the process followed upon receiving frames from each CE (for example,
EVI1).
BUM frame example from CE1:
a. An ARP request with CE-VID=1 is issued from source MAC CE1-MAC
(MAC address coming from CE1 or from a device connected to CE1)
to find the MAC address of CE3-IP.
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b. Based on the CE-VID, the frame is identified to be forwarded in
the MAC-VRF-1 (EVI1) context. A source MAC lookup is done in the
MAC FIB, and the sender's CE1-IP is looked up in the proxy ARP
table within the MAC-VRF-1 (EVI1) context. If CE1-MAC/CE1-IP are
unknown in both tables, three actions are carried out (assuming
the source MAC is accepted by PE1):
1. the forwarding state is added for the CE1-MAC associated with
the corresponding port and CE-VID;
2. the ARP request is snooped and the tuple CE1-MAC/CE1-IP is
added to the proxy ARP table; and
3. a BGP MAC Advertisement route is triggered from PE1
containing the EVI1 RD and RT, ESI=0, Ethernet-Tag=0, and
CE1-MAC/CE1-IP, along with an MPLS Label assigned to MAC-
VRF-1 from the PE1 Label space. Note that depending on the
implementation, the MAC FIB and proxy ARP learning processes
can independently send two BGP MAC advertisements instead of
one (one containing only the CE1-MAC and another one
containing CE1-MAC/CE1-IP).
Since we assume a MAC forwarding model, a label per MAC-VRF is
normally allocated and signaled by the three PEs for MAC
Advertisement routes. Based on the RT, the route is imported by
PE2 and PE3, and the forwarding state plus the ARP entry are
added to their MAC-VRF-1 context. From this moment on, any ARP
request from CE2 or CE3 destined to CE1-IP can be directly
replied to by PE1, PE2, or PE3, and ARP flooding for CE1-IP is
not needed in the core.
c. Since the ARP frame is a broadcast frame, it is forwarded by PE1
using the Inclusive Multicast Tree for EVI1 (CE-VID=1 tag should
be kept if translation is required). Depending on the type of
tree, the label stack may vary. For example, assuming ingress
replication, the packet is replicated to PE2 and PE3 with the
downstream allocated labels and the P2P LSP transport labels. No
other labels are added to the stack.
d. Assuming PE1 is the DF for EVI1 on ESI12, the frame is locally
replicated to CE2.
e. The MPLS-encapsulated frame gets to PE2 and PE3. Since PE2 is
non-DF for EVI1 on ESI12, and there is no other CE connected to
PE2, the frame is discarded. At PE3, the frame is
de-encapsulated and the CE-VID is translated, if needed, and
forwarded to CE3.
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Any other type of BUM frame from CE1 would follow the same
procedures. BUM frames from CE3 would follow the same procedures
too.
BUM frame example from CE2:
a. An ARP request with CE-VID=1 is issued from source MAC CE2-MAC to
find the MAC address of CE3-IP.
b. CE2 will hash the frame and will forward it to, for example, PE2.
Based on the CE-VID, the frame is identified to be forwarded in
the EVI1 context. A source MAC lookup is done in the MAC FIB and
the sender's CE2-IP is looked up in the proxy ARP table within
the MAC-VRF-1 context. If both are unknown, three actions are
carried out (assuming the source MAC is accepted by PE2):
1. the forwarding state is added for the CE2-MAC associated with
the corresponding LAG/ESI and CE-VID;
2. the ARP request is snooped and the tuple CE2-MAC/CE2-IP is
added to the proxy ARP table; and
3. a BGP MAC Advertisement route is triggered from PE2
containing the EVI1 RD and RT, ESI=12, Ethernet-Tag=0, and
CE2-MAC/CE2-IP, along with an MPLS Label assigned from the
PE2 Label space (one label per MAC-VRF). Again, depending on
the implementation, the MAC FIB and proxy ARP learning
processes can independently send two BGP MAC advertisements
instead of one.
Note that since PE3 is not part of ESI12, it will install the
forwarding state for CE2-MAC as long as the A-D routes for ESI12
are also active on PE3. On the contrary, PE1 is part of ESI12,
therefore PE1 will not modify the forwarding state for CE2-MAC if
it has previously learned CE2-MAC locally attached to ESI12.
Otherwise, it will add the forwarding state for CE2-MAC
associated with the local ESI12 port.
c. Assuming PE2 does not have the ARP information for CE3-IP yet,
and since the ARP is a broadcast frame and PE2 is the non-DF for
EVI1 on ESI12, the frame is forwarded by PE2 in the Inclusive
Multicast Tree for EVI1, thus adding the ESI Label for ESI12 at
the bottom of the stack. The ESI Label has been previously
allocated and signaled by the A-D routes for ESI12. Note that,
as per [RFC7432], if the result of the CE2 hashing is different
and the frame is sent to PE1, PE1 should add the ESI Label too
(PE1 is the DF for EVI1 on ESI12).
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d. The MPLS-encapsulated frame gets to PE1 and PE3. PE1
de-encapsulates the Inclusive Multicast Tree Label(s) and, based
on the ESI Label at the bottom of the stack, it decides to not
forward the frame to the ESI12. It will pop the ESI Label and
will replicate it to CE1, since CE1 is not part of the ESI
identified by the ESI Label. At PE3, the Inclusive Multicast
Tree Label is popped and the frame forwarded to CE3. If a P2MP
LSP is used as the Inclusive Multicast Tree for EVI1, PE3 will
find an ESI Label after popping the P2MP LSP Label. The ESI
Label will simply be popped, since CE3 is not part of ESI12.
Unicast frame example from CE3 to CE1:
a. A unicast frame with CE-VID=1 is issued from source MAC CE3-MAC
and destination MAC CE1-MAC (we assume PE3 has previously
resolved an ARP request from CE3 to find the MAC of CE1-IP and
has added CE3-MAC/CE3-IP to its proxy ARP table).
b. Based on the CE-VID, the frame is identified to be forwarded in
the EVI1 context. A source MAC lookup is done in the MAC FIB
within the MAC-VRF-1 context and this time, since we assume
CE3-MAC is known, no further actions are carried out as a result
of the source lookup. A destination MAC lookup is performed next
and the label stack associated with the MAC CE1-MAC is found
(including the label associated with MAC-VRF-1 in PE1 and the P2P
LSP Label to get to PE1). The unicast frame is then encapsulated
and forwarded to PE1.
c. At PE1, the packet is identified to be part of EVI1 and a
destination MAC lookup is performed in the MAC-VRF-1 context.
The labels are popped and the frame is forwarded to CE1 with
CE-VID=1.
Unicast frames from CE1 to CE3 or from CE2 to CE3 follow the same
procedures described above.
Unicast frame example from CE3 to CE2:
a. A unicast frame with CE-VID=1 is issued from source MAC CE3-MAC
and destination MAC CE2-MAC (we assume PE3 has previously
resolved an ARP request from CE3 to find the MAC of CE2-IP).
b. Based on the CE-VID, the frame is identified to be forwarded in
the MAC-VRF-1 context. We assume CE3-MAC is known. A
destination MAC lookup is performed next and PE3 finds CE2-MAC
associated with PE2 on ESI12, an Ethernet Segment for which PE3
has two active A-D routes per ESI (from PE1 and PE2) and two
active A-D routes for EVI1 (from PE1 and PE2). Based on a
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hashing function for the frame, PE3 may decide to forward the
frame using the label stack associated with PE2 (label received
from the MAC Advertisement route) or the label stack associated
with PE1 (label received from the A-D route per EVI for EVI1).
Either way, the frame is encapsulated and sent to the remote PE.
c. At PE2 (or PE1), the packet is identified to be part of EVI1
based on the bottom label, and a destination MAC lookup is
performed. At either PE (PE2 or PE1), the FIB lookup yields a
local ESI12 port to which the frame is sent.
Unicast frames from CE1 to CE2 follow the same procedures.
6.3. VLAN Bundle Service Procedures
Instead of using VLAN-based interfaces, the operator can choose to
implement VLAN bundle interfaces to carry the traffic for the 4k
CE-VIDs among CE1, CE2, and CE3. If that is the case, the 4k CE-VIDs
can be mapped to the same EVI (for example, EVI200) at each PE. The
main advantage of this approach is the low control-plane overhead
(reduced number of routes and labels) and easiness of provisioning at
the expense of no control over the customer broadcast domains, i.e.,
a single Inclusive Multicast Tree for all the CE-VIDs and no CE-VID
translation in the provider network.
6.3.1. Service Startup Procedures
As soon as the EVI200 is created in PE1, PE2, and PE3, the following
control-plane actions are carried out:
o Flooding tree setup per EVI (one route): Each PE will send one
Inclusive Multicast Ethernet Tag route per EVI (hence, only one
route per PE) so that the flooding tree per EVI can be set up.
Note that ingress replication or P2MP LSPs can optionally be
signaled in the PMSI Tunnel attribute and the corresponding tree
can be created.
o Ethernet A-D routes per ESI (one route for ESI12): A single A-D
route for ESI12 will be issued from PE1 and PE2. This route will
include a single RT (RT for EVI200), an ESI Label extended
community with the active-standby flag set to zero (all-active
multihoming type), and an ESI Label different from zero (used by
the non-DF for split-horizon functions). This route will be
imported by the three PEs, since the RT matches the locally
configured EVI200 RT. The A-D routes per ESI will be used for
fast convergence and split-horizon functions, as described in
[RFC7432].
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o Ethernet A-D routes per EVI (one route): An A-D route (EVI200)
will be sent by PE1 and PE2 for ESI12. This route includes the
EVI200 RT and an MPLS Label to be used by PE3 for the aliasing
function. This route will be imported by the three PEs.
6.3.2. Packet Walk-Through
The packet walk-through for the VLAN bundle case is similar to the
one described for EVI1 in the VLAN-based case except for the way the
CE-VID is handled by the ingress PE and the egress PE:
o No VLAN translation is allowed and the CE-VIDs are kept untouched
from CE to CE, i.e., the ingress CE-VID must be kept at the
imposition PE and at the disposition PE.
o The frame is identified to be forwarded in the MAC-VRF-200 context
as long as its CE-VID belongs to the VLAN bundle defined in the
PE1/PE2/PE3 port to CE1/CE2/CE3. Our example is a special VLAN
bundle case since the entire CE-VID range is defined in the ports;
therefore, any CE-VID would be part of EVI200.
Please refer to Section 6.2.2 for more information about the control-
plane and forwarding-plane interaction for BUM and unicast traffic
from the different CEs.
6.4. VLAN-Aware Bundling Service Procedures
The last potential service type analyzed in this document is VLAN-
aware bundling. When this type of service interface is used to carry
the 4k CE-VIDs among CE1, CE2, and CE3, all the CE-VIDs will be
mapped to the same EVI (for example, EVI300). The difference,
compared to the VLAN bundle service type in the previous section, is
that each incoming CE-VID will also be mapped to a different
"normalized" Ethernet Tag in addition to EVI300. If no translation
is required, the Ethernet Tag will match the CE-VID. Otherwise, a
translation between CE-VID and Ethernet Tag will be needed at the
imposition PE and at the disposition PE. The main advantage of this
approach is the ability to control customer broadcast domains while
providing a single EVI to the customer.
6.4.1. Service Startup Procedures
As soon as the EVI300 is created in PE1, PE2, and PE3, the following
control-plane actions are carried out:
o Flooding tree setup per EVI per Ethernet Tag (4k routes): Each PE
will send one Inclusive Multicast Ethernet Tag route per EVI and
per Ethernet Tag (hence, 4k routes per PE) so that the flooding
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tree per customer broadcast domain can be set up. Note that
ingress replication or P2MP LSPs can optionally be signaled in the
PMSI Tunnel attribute and the corresponding tree be created. In
the described use case, since all the CE-VIDs and Ethernet Tags
are defined on the three PEs, multicast tree aggregation might
make sense in order to save forwarding states.
o Ethernet A-D routes per ESI (one route for ESI12): A single A-D
route for ESI12 will be issued from PE1 and PE2. This route will
include a single RT (RT for EVI300), an ESI Label extended
community with the active-standby flag set to zero (all-active
multihoming type), and an ESI Label different than zero (used by
the non-DF for split-horizon functions). This route will be
imported by the three PEs, since the RT matches the locally
configured EVI300 RT. The A-D routes per ESI will be used for
fast convergence and split-horizon functions, as described in
[RFC7432].
o Ethernet A-D routes per EVI: A single A-D route (EVI300) may be
sent by PE1 and PE2 for ESI12 in case no CE-VID translation is
required. This route includes the EVI300 RT and an MPLS Label to
be used by PE3 for the aliasing function. This route will be
imported by the three PEs. Note that if CE-VID translation is
required, an A-D per EVI route is required per Ethernet Tag (4k).
6.4.2. Packet Walk-Through
The packet walk-through for the VLAN-aware case is similar to the one
described before. Compared to the other two cases, VLAN-aware
services allow for CE-VID translation and for an N:1 CE-VID to EVI
mapping. Both things are not supported at once in either of the two
other service interfaces. Some differences compared to the packet
walk-through described in Section 6.2.2 are as follows:
o At the ingress PE, the frames are identified to be forwarded in
the EVI300 context as long as their CE-VID belong to the range
defined in the PE port to the CE. In addition to it, CE-VID=x is
mapped to a "normalized" Ethernet-Tag=y at the MAC-VRF-300 (where
x and y might be equal if no translation is needed). Qualified
learning is now required (a different bridge table is allocated
within MAC-VRF-300 for each Ethernet Tag). Potentially, the same
MAC could be learned in two different Ethernet Tag bridge tables
of the same MAC-VRF.
o Any new locally learned MAC on the MAC-VRF-300/Ethernet-Tag=y
interface is advertised by the ingress PE in a MAC Advertisement
route using the now Ethernet Tag field (Ethernet-Tag=y) so that
the remote PE learns the MAC associated with the MAC-VRF-300/
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Ethernet-Tag=y FIB. Note that the Ethernet Tag field is not used
in advertisements of MACs learned on VLAN-based or VLAN-bundle
service interfaces.
o At the ingress PE, BUM frames are sent to the corresponding
flooding tree for the particular Ethernet Tag they are mapped to.
Each individual Ethernet Tag can have a different flooding tree
within the same EVI300. For instance, Ethernet-Tag=y can use
ingress replication to get to the remote PEs, whereas Ethernet-
Tag=z can use a P2MP LSP.
o At the egress PE, Ethernet-Tag=y (for a given broadcast domain
within MAC-VRF-300) can be translated to egress CE-VID=x. That is
not possible for VLAN bundle interfaces. It is possible for VLAN-
based interfaces, but it requires a separate MAC-VRF per CE-VID.
7. MPLS-Based Forwarding Model Use Case
EVPN supports an alternative forwarding model, usually referred to as
the MPLS-based forwarding or disposition model, as opposed to the
MAC-based forwarding or disposition model described in Section 6.
Using the MPLS-based forwarding model instead of the MAC-based model
might have an impact on the following:
o the number of forwarding states required; and
o the FIB where the forwarding states are handled (MAC FIB or MPLS
Label FIB (LFIB)).
The MPLS-based forwarding model avoids the destination MAC lookup at
the egress PE MAC FIB at the expense of increasing the number of
next-hop forwarding states at the egress MPLS LFIB. This also has an
impact on the control plane and the label allocation model, since an
MPLS-based disposition PE must send as many routes and labels as
required next-hops in the egress MAC-VRF. This concept is equivalent
to the forwarding models supported in IP-VPNs at the egress PE, where
an IP lookup in the IP-VPN FIB may or may not be necessary depending
on the available next-hop forwarding states in the LFIB.
The following subsections highlight the impact on the control- and
data-plane procedures described in Section 6 when an MPLS-based
forwarding model is used.
Note that both forwarding models are compatible and interoperable in
the same network. The implementation of either model in each PE is a
local decision to the PE node.
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7.1. Impact of MPLS-Based Forwarding on the EVPN Network Startup
The MPLS-based forwarding model has no impact on the procedures
explained in Section 6.1.
7.2. Impact of MPLS-Based Forwarding on the VLAN-Based Service
Procedures
Compared to the MAC-based forwarding model, the MPLS-based forwarding
model has no impact in terms of the number of routes when all the
service interfaces are based on VLAN. The differences for the use
case described in this document are summarized in the following list:
o Flooding tree setup per EVI (4k routes per PE): There is no impact
when compared to the MAC-based model.
o Ethernet A-D routes per ESI (one set of routes for ESI12 per PE):
There is no impact compared to the MAC-based model.
o Ethernet A-D routes per EVI (4k routes per PE/ESI): There is no
impact compared to the MAC-based model.
o MAC Advertisement routes: Instead of allocating and advertising
the same MPLS Label for all the new MACs locally learned on the
same MAC-VRF, a different label must be advertised per CE next-hop
or MAC so that no MAC FIB lookup is needed at the egress PE. In
general, this means that a different label (at least per CE) must
be advertised, although the PE can decide to implement a label per
MAC if more granularity (hence, less scalability) is required in
terms of forwarding states. For example, if CE2 sends traffic
from two different MACs to PE1, CE2-MAC1, and CE2-MAC2, the same
MPLS Label=x can be re-used for both MAC advertisements, since
they both share the same source ESI12. It is up to the PE1
implementation to use a different label per individual MAC within
the same ES (even if only one label per ESI is enough).
o PE1, PE2, and PE3 will not add forwarding states to the MAC FIB
upon learning new local CE MAC addresses on the data plane but
will rather add forwarding states to the MPLS LFIB.
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7.3. Impact of MPLS-Based Forwarding on the VLAN Bundle Service
Procedures
Compared to the MAC-based forwarding model, the MPLS-based forwarding
model has no impact in terms of number of routes when all the service
interfaces are VLAN bundle type. The differences for the use case
described in this document are summarized in the following list:
o Flooding tree setup per EVI (one route): There is no impact
compared to the MAC-based model.
o Ethernet A-D routes per ESI (one route for ESI12 per PE): There is
no impact compared to the MAC-based model.
o Ethernet A-D routes per EVI (one route per PE/ESI): There is no
impact compared to the MAC-based model since no VLAN translation
is required.
o MAC Advertisement routes: Instead of allocating and advertising
the same MPLS Label for all the new MACs locally learned on the
same MAC-VRF, a different label must be advertised per CE next-hop
or MAC so that no MAC FIB lookup is needed at the egress PE. In
general, this means that a different label (at least per CE) must
be advertised, although the PE can decide to implement a label per
MAC if more granularity (hence, less scalability) is required in
terms of forwarding states. It is up to the PE1 implementation to
use a different label per individual MAC within the same ES (even
if only one label per ESI is enough).
o PE1, PE2, and PE3 will not add forwarding states to the MAC FIB
upon learning new local CE MAC addresses on the data plane, but
will rather add forwarding states to the MPLS LFIB.
7.4. Impact of MPLS-Based Forwarding on the VLAN-Aware Service
Procedures
Compared to the MAC-based forwarding model, the MPLS-based forwarding
model has no impact in terms of the number of A-D routes when all the
service interfaces are of the VLAN-aware bundle type. The
differences for the use case described in this document are
summarized in the following list:
o Flooding tree setup per EVI (4k routes per PE): There is no impact
compared to the MAC-based model.
o Ethernet A-D routes per ESI (one route for ESI12 per PE): There is
no impact compared to the MAC-based model.
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o Ethernet A-D routes per EVI (1 route per ESI or 4k routes per PE/
ESI): PE1 and PE2 may send one route per ESI if no CE-VID
translation is needed. However, 4k routes are normally sent for
EVI300, one per <ESI, Ethernet Tag ID> tuple. This allows the
egress PE to find out all the forwarding information in the MPLS
LFIB and even support Ethernet Tag to CE-VID translation at the
egress.
o MAC Advertisement routes: Instead of allocating and advertising
the same MPLS Label for all the new MACs locally learned on the
same MAC-VRF, a different label must be advertised per CE next-hop
or MAC so that no MAC FIB lookup is needed at the egress PE. In
general, this means that a different label (at least per CE) must
be advertised, although the PE can decide to implement a label per
MAC if more granularity (hence, less scalability) is required in
terms of forwarding states. It is up to the PE1 implementation to
use a different label per individual MAC within the same ES. Note
that the Ethernet Tag will be set to a non-zero value for the MAC
Advertisement routes. The same MAC address can be announced with
a different Ethernet Tag value. This will make the advertising PE
install two different forwarding states in the MPLS LFIB.
o PE1, PE2, and PE3 will not add forwarding states to the MAC FIB
upon learning new local CE MAC addresses on the data plane but
will rather add forwarding states to the MPLS LFIB.
8. Comparison between MAC-Based and MPLS-Based Egress Forwarding Models
Both forwarding models are possible in a network deployment, and each
one has its own trade-offs.
Both forwarding models can save A-D routes per EVI when VLAN-aware
bundling services are deployed and no CE-VID translation is required.
While this saves a significant amount of routes, customers normally
require CE-VID translation; hence, we assume an A-D per EVI route per
<ESI, Ethernet Tag> is needed.
The MAC-based model saves a significant amount of MPLS Labels
compared to the MPLS-based forwarding model. All the MACs and A-D
routes for the same EVI can signal the same MPLS Label, saving labels
from the local PE space. A MAC FIB lookup at the egress PE is
required in order to do so.
The MPLS-based forwarding model can save forwarding states at the
egress PEs if labels per next-hop CE (as opposed to per MAC) are
implemented. No egress MAC lookup is required. Also, a different
label per next-hop CE per MAC-VRF is consumed, as opposed to a single
label per MAC-VRF.
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Table 2 summarizes the resource implementation details of both
models.
+-----------------------------+-----------------+------------------+
| Resources | MAC-Based Model | MPLS-Based Model |
+-----------------------------+-----------------+------------------+
| MPLS Labels Consumed | 1 per MAC-VRF | 1 per CE/EVI |
| Egress PE Forwarding States | 1 per MAC | 1 per Next-Hop |
| Egress PE Lookups | 2 (MPLS+MAC) | 1 (MPLS) |
+-----------------------------+-----------------+------------------+
Table 2: Resource Comparison between MAC-Based and MPLS-Based Models
The egress forwarding model is an implementation local to the egress
PE and is independent of the model supported on the rest of the PEs;
i.e., in our use case, PE1, PE2, and PE3 could have either egress
forwarding model without any dependencies.
9. Traffic Flow Optimization
In addition to the procedures described across Sections 3 through 8,
EVPN [RFC7432] procedures allow for optimized traffic handling in
order to minimize unnecessary flooding across the entire
infrastructure. Optimization is provided through specific ARP
termination and the ability to block unknown unicast flooding.
Additionally, EVPN procedures allow for intelligent, close to the
source, inter-subnet forwarding and solves the commonly known
suboptimal routing problem. Besides the traffic efficiency, ingress-
based inter-subnet forwarding also optimizes packet forwarding rules
and implementation at the egress nodes as well. Details of these
procedures are outlined in Sections 9.1 and 9.2.
9.1. Control-Plane Procedures
9.1.1. MAC Learning Options
The fundamental premise of [RFC7432] is the notion of a different
approach to MAC address learning compared to traditional IEEE 802.1
bridge learning methods; specifically, EVPN differentiates between
data and control-plane-driven learning mechanisms.
Data-driven learning implies that there is no separate communication
channel used to advertise and propagate MAC addresses. Rather, MAC
addresses are learned through IEEE-defined bridge learning procedures
as well as by snooping on DHCP and ARP requests. As different MAC
addresses show up on different ports, the Layer 2 (L2) FIB is
populated with the appropriate MAC addresses.
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Control-plane-driven learning implies a communication channel that
could be either a control-plane protocol or a management-plane
mechanism. In the context of EVPN, two different learning procedures
are defined: local and remote procedures.
o Local learning defines the procedures used for learning the MAC
addresses of network elements locally connected to a MAC-VRF.
Local learning could be implemented through all three learning
procedures: control plane, management plane, and data plane.
However, the expectation is that for most of the use cases, local
learning through the data plane should be sufficient.
o Remote learning defines the procedures used for learning MAC
addresses of network elements remotely connected to a MAC-VRF,
i.e., far-end PEs. Remote learning procedures defined in
[RFC7432] advocate using only control-plane learning, BGP
specifically. Through the use of BGP EVPN NLRIs, the remote PE
has the capability of advertising all the MAC addresses present in
its local FIB.
9.1.2. Proxy ARP/ND
In EVPN, MAC addresses are advertised via the MAC/IP Advertisement
route, as discussed in [RFC7432]. Optionally, an IP address can be
advertised along with the MAC address advertisement. However, there
are certain rules put in place in terms of IP address usage: if the
MAC/IP Route contains an IP address, this particular IP address
correlates directly with the advertised MAC address. Such
advertisement allows us to build a proxy ARP / Neighbor Discovery
(ND) table populated with the IP<->MAC bindings received from all the
remote nodes.
Furthermore, based on these bindings, a local MAC-VRF can now provide
proxy ARP/ND functionality for all ARP requests and ND solicitations
directed to the IP address pool learned through BGP. Therefore, the
amount of unnecessary L2 flooding (ARP/ND requests/solicitations in
this case) can be further reduced by the introduction of proxy ARP/ND
functionality across all EVI MAC-VRFs.
9.1.3. Unknown Unicast Flooding Suppression
Given that all locally learned MAC addresses are advertised through
BGP to all remote PEs, suppressing flooding of any unknown unicast
traffic towards the remote PEs is a feasible network optimization.
The assumption in the use case is made that any network device that
appears on a remote MAC-VRF will somehow signal its presence to the
network. This signaling can be done through, for example, gratuitous
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ARPs. Once the remote PE acknowledges the presence of the node in
the MAC-VRF, it will do two things: install its MAC address in its
local FIB and advertise this MAC address to all other BGP speakers
via EVPN NLRI. Therefore, we can assume that any active MAC address
is propagated and learned through the entire EVI. Given that MAC
addresses become prepopulated -- once nodes are alive on the network
-- there is no need to flood any unknown unicast towards the remote
PEs. If the owner of a given destination MAC is active, the BGP
route will be present in the local RIB and FIB, assuming that the BGP
import policies are successfully applied; otherwise, the owner of
such destination MAC is not present on the network.
It is worth noting that unknown unicast flooding must not be
suppressed unless (at least) one of the following two statements is
given: a) control- or management-plane learning is performed
throughout the entire EVI for all the MACs or b) all the EVI-attached
devices signal their presence when they come up (Gratuitous ARP
(GARP) packets or similar).
9.1.4. Optimization of Inter-Subnet Forwarding
In a scenario in which both L2 and L3 services are needed over the
same physical topology, some interaction between EVPN and IP-VPN is
required. A common way of stitching the two service planes is
through the use of an Integrated Routing and Bridging (IRB)
interface, which allows for traffic to be either routed or bridged
depending on its destination MAC address. If the destination MAC
address is the one from the IRB interface, traffic needs to be passed
through a routing module and potentially be either routed to a remote
PE or forwarded to a local subnet. If the destination MAC address is
not the one from the IRB interface, the MAC-VRF follows standard
bridging procedures.
A typical example of EVPN inter-subnet forwarding would be a scenario
in which multiple IP subnets are part of a single or multiple EVIs,
and they all belong to a single IP-VPN. In such topologies, it is
desired that inter-subnet traffic can be efficiently routed without
any tromboning effects in the network. Due to the overlapping
physical and service topology in such scenarios, all inter-subnet
connectivity will be locally routed through the IRB interface.
In addition to optimizing the traffic patterns in the network, local
inter-subnet forwarding also greatly optimizes the amount of
processing needed to cross the subnets. Through EVPN MAC
advertisements, the local PE learns the real destination MAC address
associated with the remote IP address and the inter-subnet forwarding
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can happen locally. When the packet is received at the egress PE, it
is directly mapped to an egress MAC-VRF and bypasses any egress
IP-VPN processing.
Please refer to [EVPN-INTERSUBNET] for more information about the IP
inter-subnet forwarding procedures in EVPN.
9.2. Packet Walk-Through Examples
Assuming that the services are set up according to Figure 1 in
Section 3, the following flow optimization processes will take place
in terms of creating, receiving, and forwarding packets across the
network.
9.2.1. Proxy ARP Example for CE2-to-CE3 Traffic
Using Figure 1 in Section 3, consider EVI400 residing on PE1, PE2,
and PE3 connecting CE2 and CE3 networks. Also, consider that PE1 and
PE2 are part of the all-active multihoming ES for CE2, and that PE2
is elected designated forwarder for EVI400. We assume that all the
PEs implement the proxy ARP functionality in the MAC-VRF-400 context.
In this scenario, PE3 will not only advertise the MAC addresses
through the EVPN MAC Advertisement route but also IP addresses of
individual hosts (i.e., /32 prefixes) behind CE3. Upon receiving the
EVPN routes, PE1 and PE2 will install the MAC addresses in the MAC-
VRF-400 FIB and, based on the associated received IP addresses, PE1
and PE2 can now build a proxy ARP table within the context of MAC-
VRF-400.
From the forwarding perspective, when a node behind CE2 sends a frame
destined to a node behind CE3, it will first send an ARP request to,
for example, PE2 (based on the result of the CE2 hashing). Assuming
that PE2 has populated its proxy ARP table for all active nodes
behind the CE3, and that the IP address in the ARP message matches
the entry in the table, PE2 will respond to the ARP request with the
actual MAC address on behalf of the node behind CE3.
Once the nodes behind CE2 learn the actual MAC address of the nodes
behind CE3, all the MAC-to-MAC communications between the two
networks will be unicast.
9.2.2. Flood Suppression Example for CE1-to-CE3 Traffic
Using Figure 1 in Section 3, consider EVI500 residing on PE1 and PE3
connecting CE1 and CE3 networks. Consider that both PE1 and PE3 have
disabled unknown unicast flooding for this specific EVI context.
Once the network devices behind CE3 come online, they will learn
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their MAC addresses and create local FIB entries for these devices.
Note that local FIB entries could also be created through either a
control or management plane between PE and CE as well. Consequently,
PE3 will automatically create EVPN Type 2 MAC Advertisement routes
and advertise all locally learned MAC addresses. The routes will
also include the corresponding MPLS Label.
Given that PE1 automatically learns and installs all MAC addresses
behind CE3, its MAC-VRF FIB will already be prepopulated with the
respective next-hops and label assignments associated with the MAC
addresses behind CE3. As such, as soon as the traffic sent by CE1 to
nodes behind CE3 is received into the context of EVI500, PE1 will
push the MPLS Label(s) onto the original Ethernet frame and send the
packet to the MPLS network. As usual, once PE3 receives this packet,
and depending on the forwarding model, PE3 will either do a next-hop
lookup in the EVI500 context or just forward the traffic directly to
the CE3. In the case that PE1 MAC-VRF-500 does not have a MAC entry
for a specific destination that CE1 is trying to reach, PE1 will drop
the frame since unknown unicast flooding is disabled.
Based on the assumption that all the MAC entries behind the CEs are
prepopulated through gratuitous ARP and/or DHCP requests, if one
specific MAC entry is not present in the MAC-VRF-500 FIB on PE1, the
owner of that MAC is not alive on the network behind the CE3; hence,
the traffic can be dropped at PE1 instead of flooding and consuming
network bandwidth.
9.2.3. Optimization of Inter-subnet Forwarding Example for CE3-to-CE2
Traffic
Using Figure 1 in Section 3, consider that there is an IP-VPN 666
context residing on PE1, PE2, and PE3, which connects CE1, CE2, and
CE3 into a single IP-VPN domain. Also consider that there are two
EVIs present on the PEs, EVI600 and EVI60. Each IP subnet is
associated with a different MAC-VRF context. Thus, there is a single
subnet (subnet 600) between CE1 and CE3 that is established through
EVI600. Similarly, there is another subnet (subnet 60) between CE2
and CE3 that is established through EVI60. Since both subnets are
part of the same IP-VPN, there is a mapping of each EVI (or
individual subnet) to a local IRB interface on the three PEs.
If a node behind CE2 wants to communicate with a node on the same
subnet seating behind CE3, the communication flow will follow the
standard EVPN procedures, i.e., FIB lookup within the PE1 (or PE2)
after adding the corresponding EVPN label to the MPLS Label stack
(downstream label allocation from PE3 for EVI60).
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When it comes to crossing the subnet boundaries, the ingress PE
implements local inter-subnet forwarding. For example, when a node
behind CE2 (EVI60) sends a packet to a node behind CE1 (EVI600), the
destination IP address will be in the subnet 600, but the destination
MAC address will be the address of the source node's default gateway,
which in this case will be an IRB interface on PE1 (connecting EVI60
to IP-VPN 666). Once PE1 sees the traffic destined to its own MAC
address, it will route the packet to EVI600, i.e., it will change the
source MAC address to the one of the IRB interface in EVI600 and
change the destination MAC address to the address belonging to the
node behind CE1, which is already populated in the MAC-VRF-600 FIB,
either through data- or control-plane learning.
An important optimization to be noted is the local inter-subnet
forwarding in lieu of IP-VPN routing. If the node from subnet 60
(behind CE2) is sending a packet to the remote end-node on subnet 600
(behind CE3), the mechanism in place still honors the local inter-
subnet (inter-EVI) forwarding.
In our use case, therefore, when the node from subnet 60 behind CE2
sends traffic to the node on subnet 600 behind CE3, the destination
MAC address is the PE1 MAC-VRF-60 IRB MAC address. However, once the
traffic locally crosses EVIs to EVI600 (via the IRB interface on
PE1), the source MAC address is changed to that of the IRB interface
and the destination MAC address is changed to the one advertised by
PE3 via EVPN and already installed in MAC-VRF-600. The rest of the
forwarding through PE1 is using the MAC-VRF-600 forwarding context
and label space.
Another very relevant optimization is due to the fact that traffic
between PEs is forwarded through EVPN rather than through IP-VPN. In
the example described above for traffic from EVI60 on CE2 to EVI600
on CE3, there is no need for IP-VPN processing on the egress PE3.
Traffic is forwarded either to the EVI600 context in PE3 for further
MAC lookup and next-hop processing or directly to the node behind
CE3, depending on the egress forwarding model being used.
10. Security Considerations
Please refer to the "Security Considerations" section in [RFC7432].
The standards produced by the SIDR Working Group address secure route
origin authentication (e.g., RFCs 6480 through 6493) and route
advertisement security (e.g., RFCs 8205 through 8211). They protect
the integrity and authenticity of IP address advertisements and ASN/
IP prefix bindings. This document and [RFC7432] use BGP to convey
other info (e.g., MAC addresses); thus, the protections offered by
the SIDR WG RFCs are not applicable in this context.
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11. IANA Considerations
This document has no IANA actions.
12. References
12.1. Normative References
[RFC7209] Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
Henderickx, W., and A. Isaac, "Requirements for Ethernet
VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
<https://www.rfc-editor.org/info/rfc7209>.
[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>.
12.2. Informative References
[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<https://www.rfc-editor.org/info/rfc4761>.
[RFC4762] Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
<https://www.rfc-editor.org/info/rfc4762>.
[RFC6074] Rosen, E., Davie, B., Radoaca, V., and W. Luo,
"Provisioning, Auto-Discovery, and Signaling in Layer 2
Virtual Private Networks (L2VPNs)", RFC 6074,
DOI 10.17487/RFC6074, January 2011,
<https://www.rfc-editor.org/info/rfc6074>.
[EVPN-INTERSUBNET]
Sajassi, A., Salam, S., Thoria, S., Drake, J., Rabadan,
J., and L. Yong, "Integrated Routing and Bridging in
EVPN", Work in Progress, draft-ietf-bess-evpn-inter-
subnet-forwarding-03, February 2017.
Acknowledgments
The authors want to thank Giles Heron for his detailed review of the
document. We also thank Stefan Plug and Eric Wunan for their
comments.
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Contributors
The following people contributed substantially to the content of this
document and should be considered coauthors:
Florin Balus
Keyur Patel
Aldrin Isaac
Truman Boyes
Authors' Addresses
Jorge Rabadan (editor)
Nokia
777 E. Middlefield Road
Mountain View, CA 94043
United States America
Email: jorge.rabadan@nokia.com
Senad Palislamovic
Nokia
Email: senad.palislamovic@nokia.com
Wim Henderickx
Nokia
Copernicuslaan 50
2018 Antwerp
Belgium
Email: wim.henderickx@nokia.com
Ali Sajassi
Cisco
Email: sajassi@cisco.com
James Uttaro
AT&T
Email: uttaro@att.com
Rabadan, et al. Informational [Page 31]
ERRATA
