Internet DRAFT - draft-ietf-bess-evpn-usage
draft-ietf-bess-evpn-usage
BESS Workgroup J. Rabadan, Ed.
Internet Draft S. Palislamovic
W. Henderickx
Intended status: Informational Nokia
A. Sajassi
Cisco
J. Uttaro
AT&T
Expires: August 28, 2018 February 24, 2018
Usage and applicability of BGP MPLS based Ethernet VPN
draft-ietf-bess-evpn-usage-09
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 on the example
scenario, analyzing 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
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This Internet-Draft will expire on August 28, 2018.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Use-case scenario description and requirements . . . . . . . . 4
3.1. Service Requirements . . . . . . . . . . . . . . . . . . . 5
3.2. Why EVPN is chosen to address this use-case . . . . . . . . 6
4. Provisioning Model . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Common provisioning tasks . . . . . . . . . . . . . . . . . 7
4.1.1. Non-service specific parameters . . . . . . . . . . . . 7
4.1.2. Service specific parameters . . . . . . . . . . . . . . 8
4.2. Service interface dependent provisioning tasks . . . . . . 9
4.2.1. VLAN-based service interface EVI . . . . . . . . . . . 9
4.2.2. VLAN-bundle service interface EVI . . . . . . . . . . . 10
4.2.3. VLAN-aware bundling service interface EVI . . . . . . . 10
5. BGP EVPN NLRI usage . . . . . . . . . . . . . . . . . . . . . . 10
6. MAC-based forwarding model use-case . . . . . . . . . . . . . . 11
6.1. EVPN Network Startup procedures . . . . . . . . . . . . . . 11
6.2. VLAN-based service procedures . . . . . . . . . . . . . . . 12
6.2.1. Service startup procedures . . . . . . . . . . . . . . 12
6.2.2. Packet walkthrough . . . . . . . . . . . . . . . . . . 13
6.3. VLAN-bundle service procedures . . . . . . . . . . . . . . 16
6.3.1. Service startup procedures . . . . . . . . . . . . . . 16
6.3.2. Packet Walkthrough . . . . . . . . . . . . . . . . . . 17
6.4. VLAN-aware bundling service procedures . . . . . . . . . . 17
6.4.1. Service startup procedures . . . . . . . . . . . . . . 18
6.4.2. Packet Walkthrough . . . . . . . . . . . . . . . . . . 18
7. MPLS-based forwarding model use-case . . . . . . . . . . . . . 19
7.1. Impact of MPLS-based forwarding on the EVPN network
startup . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Impact of MPLS-based forwarding on the VLAN-based service
procedures . . . . . . . . . . . . . . . . . . . . . . . . 20
7.3. Impact of MPLS-based forwarding on the VLAN-bundle
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service procedures . . . . . . . . . . . . . . . . . . . . 21
7.4. Impact of MPLS-based forwarding on the VLAN-aware service
procedures . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Comparison between MAC-based and MPLS-based Egress Forwarding
Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. Traffic flow optimization . . . . . . . . . . . . . . . . . . . 23
9.1. Control Plane Procedures . . . . . . . . . . . . . . . . . 23
9.1.1. MAC learning options . . . . . . . . . . . . . . . . . 23
9.1.2. Proxy-ARP/ND . . . . . . . . . . . . . . . . . . . . . 24
9.1.3. Unknown Unicast flooding suppression . . . . . . . . . 25
9.1.4. Optimization of Inter-subnet forwarding . . . . . . . . 25
9.2. Packet Walkthrough Examples . . . . . . . . . . . . . . . . 26
9.2.1. Proxy-ARP example for CE2 to CE3 traffic . . . . . . . 26
9.2.2. Flood suppression example for CE1 to CE3 traffic . . . 26
9.2.3. Optimization of inter-subnet forwarding example for
CE3 to CE2 traffic . . . . . . . . . . . . . . . . . . 27
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Normative References . . . . . . . . . . . . . . . . . . . 29
12.2. Informative References . . . . . . . . . . . . . . . . . . 29
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 30
15. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
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.
2. Terminology
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The following terminology is used:
o VID: VLAN Identifier.
o CE: Customer Edge device.
o EVI: EVPN Instance.
o MAC-VRF: A Virtual Routing and Forwarding table for Media Access
Control (MAC) addresses on a PE.
o Ethernet Segment (ES): set of links through which a customer site
(CE) is connected to one or more PEs. Each ES is identified by an
Ethernet Segment Identifier (ESI) in the control plane.
o CE-VIDs refer to the VLAN tag identifiers being used at CE1, CE2
and CE3 to tag customer traffic sent to the Service Provider E- VPN
network
o CE1-MAC, CE2-MAC and CE3-MAC refer to source MAC addresses "behind"
each CE respectively. Those MAC addresses can belong to the CEs
themselves or to devices connected to the CEs.
o CE1-IP, CE2-IP and CE3-IP refer to IP addresses associated to the
above MAC addresses.
o LACP: Link Aggregation Control Protocol.
o RD: Route Distinguisher.
o RT: Route Target.
o PE: Provider Edge router.
o AS: Autonomous System.
o PE-IP: it refers to the IP address of a given PE.
3. Use-case scenario description and requirements
Figure 1 depicts the scenario that will be referenced throughout the
rest of the document.
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+--------------+
| |
+----+ +----+ | | +----+ +----+
| 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, 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 multi-homing 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.
- The following three EVI services are required in this example:
EVI100 - It 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
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when the same CE-VID is used across the three CEs for EVI100 is
referred in [RFC7432] as the "Unique VLAN" EVPN case. This term
will be used throughout this document too.
EVI200 - It uses VLAN-bundle service interfaces in CE1, CE2 and
CE3, based on an N:1 VLAN-to-EVI mapping. The operator needs to
pre-configure 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 of keeping the separation
between users in different CE-VIDs.
EVI300 - It 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: 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
vs. 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 BUM (Broadcast, Unknown unicast, Multicast) 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.
3.2. Why EVPN is chosen to address this use-case
VPLS solutions based on [RFC4761], [RFC4762] and [RFC6074] cannot
meet the requirements in section 3, whereas EVPN can.
For example:
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o If CE2 has a single CE-VID (or a few CE-VIDs) the current VPLS
multi-homing 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
multi-homing solution required in this scenario to optimize the
upstream/downstream link utilization between CE2 and PE1-PE2.
o Also, EVPN provides a fast convergence solution that is independent
of the CE-VIDs in the multi-homed 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-flooding.
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).
4.1. Common provisioning tasks
Regardless of the service interface type (VLAN-based, VLAN-bundle or
VLAN-aware), the following sub-sections describe the parameters to be
provisioned in the three PEs.
4.1.1. Non-service specific parameters
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The multi-homing 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 multi-homing 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 to 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 NLRIs). In our example, a LAG is used
between CE2 and PE1-PE2 (since all-active multi-homing 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 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 Multi-homing type: the user must be able to provision the
multi-homing type to be used in the network. In our use-case, the
multi-homing 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 sent by PE1 and PE2 along with the Ethernet A-D route for
the CE2 ESI.
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: global identifier per EVI that is shared by all the
PEs part of the EVI, i.e. PE1, PE2 and PE3 will be provisioned with
EVI100, 200 and 300. The EVI identifier can be associated to (or be
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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 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. 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. Next section discusses the specifics of each
service interface type.
o CE-VID and port/LAG binding to EVI identifier or Ethernet Tag: see
section 4.2.
4.2. Service interface dependent provisioning tasks
Depending on the service interface type being used in the EVI, a
specific CE-VID binding provisioning must be specified.
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
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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, represented by an Ethernet Tag in the
control plane.
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.
+-------------------+---------------+---------------+
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| 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 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.
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 setup
among PE1, PE2 and PE3 so that the EVPN services can make use of
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
setup, 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 DF election: PE1 and PE2 will
advertise a BGP Ethernet Segment route for ESI12, where the ESI RD
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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 is 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.
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 setup. Note that
ingress replication or P2MP LSPs can optionally be signaled in the
PMSI Tunnel attribute and the corresponding tree 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 multi-homing 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
EVI RTs locally configured. The A-D routes per ESI will be used for
fast convergence and split-horizon functions, as discussed in
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[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 walkthrough
Once the services are setup, 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 (example for
EVI1).
(1) 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.
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 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) Forwarding state is added for CE1-MAC associated to 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 ARP entry are added to
their MAC-VRF-1 context. From this moment on, any ARP request from
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CE2 or CE3 destined to CE1-IP, can be directly replied 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, CE-
VID translated if needed and forwarded to CE3.
Any other type of BUM frame from CE1 would follow the same
procedures. BUM frames from CE3 would follow the same procedures too.
(2) 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 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) Forwarding state is added for CE2-MAC associated to 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.
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Note that, since PE3 is not part of ESI12, it will install
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 learnt CE2-MAC locally attached to ESI12.
Otherwise it will add forwarding state for CE2-MAC associated to
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 the non-DF for EVI1 on
ESI12, the frame is forwarded by PE2 in the Inclusive multicast
tree for EVI1, 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 sent to PE1,
PE1 should add the ESI label too (PE1 is the DF for EVI1 on
ESI12).
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 though, 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 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.
(3) 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 to the MAC CE1-MAC is found (including
the label associated to 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
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labels are popped and the frame forwarded to CE1 with CE-VID=1.
Unicast frames from CE1 to CE3 or from CE2 to CE3 follow the same
procedures described above.
(4) 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 to
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 hashing function for the
frame, PE3 may decide to forward the frame using the label stack
associated to PE2 (label received from the MAC advertisement
route) or the label stack associated to 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
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Inclusive Multicast Ethernet Tag route per EVI (hence only one
route per PE) so that the flooding tree per EVI can be setup. Note
that ingress replication or P2MP LSPs can optionally be signaled
in the PMSI Tunnel attribute and the corresponding tree 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
multi-homing 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 EVI200 RT
locally configured. 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 (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 Walkthrough
The packet walkthrough 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
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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
tree per customer broadcast domain can be setup. 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
multi-homing 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 EVI300 RT
locally configured. 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 Walkthrough
The packet walkthrough 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
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other service interfaces. Some differences compared to the packet
walkthrough described in section 6.2.2 are:
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 now the Ethernet-Tag field (Ethernet-Tag=y) so that
the remote PE learns the MAC associated to the MAC-VRF-
300/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
MPLS-based forwarding or disposition model as opposed to the
MAC-based forwarding or disposition model described in section 6.
Using MPLS-based forwarding model instead of MAC-based model might
have an impact on:
o The number of forwarding states required.
o The FIB where the forwarding states are handled: MAC FIB or MPLS
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
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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 might be necessary or not depending on
the available next-hop forwarding states in the LFIB.
The following sub-sections highlight the impact on the control and
data plane procedures described in section 6 when and 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.
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 number of routes, when all the
service interfaces are VLAN-based. 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): no impact compared
to the MAC-based model.
o Ethernet A-D routes per ESI (one set of routes for ESI12 per PE):
no impact compared to the MAC-based model.
o Ethernet A-D routes per EVI (4k routes per PE/ESI): 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 learnt 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
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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 Segment (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.
7.3. 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): no impact compared to the
MAC-based model.
o Ethernet A-D routes per ESI (one route for ESI12 per PE): no impact
compared to the MAC-based model.
o Ethernet A-D routes per EVI (one route per PE/ESI): 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 learnt 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 Segment (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 number of A-D routes when all the
service interfaces are VLAN-aware bundle type. The differences for
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the use-case described in this document are summarized in the
following list:
o Flooding tree setup per EVI (4k routes per PE): no impact compared
to the MAC-based model.
o Ethernet A-D routes per ESI (one route for ESI12 per PE): no impact
compared to the MAC-based model.
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 normally sent for EVI300,
one per <ESI, Ethernet-Tag ID> tuple. This will allow 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 learnt 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 Segment. 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
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
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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.
Table 2 summarizes the resource implementation details of both
models.
+-----------------------------+----------------+----------------+
| Resources | MAC-based | MPLS-based |
| | Model | 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 sub-
optimal 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
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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 L2 FIB is populated with
the appropriate MAC addresses.
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, i.e. 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 as well as data plane.
However, the expectation is that for most of the use cases, local
learning through 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; specifically BGP.
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/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.
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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
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 learnt through the entire EVI. Given that MAC
addresses become pre-populated - 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 are
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 (GARPs 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 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 of 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 of the IRB, 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.
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In addition to optimizing the traffic patterns in the network, local
inter-subnet forwarding also optimizes greatly 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
can happen locally. When the packet is received at the egress PE, it
is directly mapped to an egress MAC-VRF, bypassing 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 Walkthrough Examples
Assuming that the services are setup 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 EVI 400 residing on PE1, PE2
and PE3 connecting CE2 and CE3 networks. Also, consider that PE1 and
PE2 are part of the all-active multi-homing 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
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Using Figure 1 in section 3, consider EVI 500 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 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 pre-populated 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 EVI 500, 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 EVI 500 context, or will 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
pre-populated 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 be flooded and consume
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, EVI 600 and EVI 60. Each IP subnet is associated
to a different MAC-VRF context. Thus there is a single subnet, subnet
600, between CE1 and CE3 that is established through EVI 600.
Similarly, there is another subnet, subnet 60, between CE2 and CE3
that is established through EVI 60. 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)
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after adding the corresponding EVPN label to the MPLS label stack
(downstream label allocation from PE3 for EVI 60).
When it comes to crossing the subnet boundaries, the ingress PE
implements local inter-subnet forwarding. For example, when a node
behind CE2 (EVI 60) sends a packet to a node behind CE1 (EVI 600) the
destination IP address will be in the subnet 600, but the destination
MAC address will be the address of source node's default gateway,
which in this case will be an IRB interface on PE1 (connecting EVI 60
to IP-VPN 666). Once PE1 sees the traffic destined to its own MAC
address, it will route the packet to EVI 600, i.e. it will change the
source MAC address to the one of the IRB interface in EVI 600 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 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 EVI 600, 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 EVI 60 on CE2 to EVI 600
on CE3, there is no need for IP-VPN processing on the egress PE3.
Traffic is forwarded either to the EVI 600 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 WG address secure route origin
authentication (e.g., RFCs 6480-93) and route advertisement security
(e.g., RFCs 8205-11). 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,
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and thus the protections offered by the SIDR WG RFCs are not
applicable in this context.
11. IANA Considerations
No IANA considerations are needed.
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, <http://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, <http://www.rfc-
editor.org/info/rfc7432>.
12.2. Informative References
[EVPN-INTERSUBNET] Sajassi et al., "IP Inter-subnet forwarding in
EVPN", draft-ietf-bess-evpn-inter-subnet-forwarding-03.txt
[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, <http://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,
<http://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, <http://www.rfc-editor.org/info/rfc6074>.
13. Acknowledgments
The authors want to thank Giles Heron for his detailed review of the
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document. We also thank Stefan Plug, and Eric Wunan for their
comments.
14. Contributors
In addition to the authors listed on the front page, the following
co-authors have also contributed to this document:
Florin Balus
Keyur Patel
Aldrin Isaac
Truman Boyes
15. Authors' Addresses
Jorge Rabadan
Nokia
777 E. Middlefield Road
Mountain View, CA 94043 USA
Email: jorge.rabadan@nokia.com
Senad Palislamovic
Nokia
Email: senad.palislamovic@nokia.com
Wim Henderickx
Nokia
Email: wim.henderickx@nokia.com
Ali Sajassi
Cisco
Email: sajassi@cisco.com
James Uttaro
AT&T
Email: uttaro@att.com
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