BGP MPLS Based Ethernet VPN
draft-ietf-l2vpn-evpn-04
The information below is for an old version of the document.
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| Authors | Ali Sajassi , Rahul Aggarwal , Wim Henderickx , Florin Balus , Aldrin Isaac , Jim Uttaro | ||
| Last updated | 2014-01-15 (Latest revision 2013-07-14) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-l2vpn-evpn-04
Network Working Group A. Sajassi
INTERNET-DRAFT Cisco
Category: Standards Track
R. Aggarwal
N. Bitar Arktan
Verizon
W. Henderickx
S. Boutros F. Balus
K. Patel Alcatel-Lucent
S. Salam
Cisco Aldrin Isaac
Bloomberg
J. Drake
R. Shekhar J. Uttaro
Juniper Networks AT&T
Expires: January 15, 2014 July 15, 2013
BGP MPLS Based Ethernet VPN
draft-ietf-l2vpn-evpn-04
Status of this Memo
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Copyright and License Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Abstract
This document describes procedures for BGP MPLS based Ethernet VPNs
(EVPN).
Table of Contents
1. Specification of requirements . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. BGP MPLS Based EVPN Overview . . . . . . . . . . . . . . . . . 6
6. Ethernet Segment . . . . . . . . . . . . . . . . . . . . . . . 7
7. Ethernet Tag . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 VLAN Based Service Interface . . . . . . . . . . . . . . . . 9
7.2 VLAN Bundle Service Interface . . . . . . . . . . . . . . . 9
7.2.1 Port Based Service Interface . . . . . . . . . . . . . . 10
7.3 VLAN Aware Bundle Service Interface . . . . . . . . . . . . 10
7.3.1 Port Based VLAN Aware Service Interface . . . . . . . . 10
8. BGP EVPN NLRI . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Ethernet Auto-Discovery Route . . . . . . . . . . . . . . . 11
8.2. MAC Advertisement Route . . . . . . . . . . . . . . . . . 12
8.3. Inclusive Multicast Ethernet Tag Route . . . . . . . . . . 12
8.4 Ethernet Segment Route . . . . . . . . . . . . . . . . . . . 13
8.5 ESI Label Extended Community . . . . . . . . . . . . . . . . 13
8.6 ES-Import Route Target . . . . . . . . . . . . . . . . . . . 14
8.7 MAC Mobility Extended Community . . . . . . . . . . . . . . 14
8.8 Default Gateway Extended Community . . . . . . . . . . . . . 15
9. Multi-homing Functions . . . . . . . . . . . . . . . . . . . . 15
9.1 Multi-homed Ethernet Segment Auto-Discovery . . . . . . . . 15
9.1.1 Constructing the Ethernet Segment Route . . . . . . . . 15
9.2 Fast Convergence . . . . . . . . . . . . . . . . . . . . . . 16
9.2.1 Constructing the Ethernet A-D Route per Ethernet
Segment . . . . . . . . . . . . . . . . . . . . . . . . 16
9.2.1.1. Ethernet A-D Route Targets . . . . . . . . . . . . 17
9.3 Split Horizon . . . . . . . . . . . . . . . . . . . . . . . 17
9.3.1 ESI Label Assignment . . . . . . . . . . . . . . . . . . 18
9.3.1.1 Ingress Replication . . . . . . . . . . . . . . . . 18
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9.3.1.2. P2MP MPLS LSPs . . . . . . . . . . . . . . . . . . 19
9.4 Aliasing and Backup-Path . . . . . . . . . . . . . . . . . . 20
9.4.1 Constructing the Ethernet A-D Route per EVI . . . . . . 21
9.4.1.1 Ethernet A-D Route Targets . . . . . . . . . . . . . 22
9.5 Designated Forwarder Election . . . . . . . . . . . . . . . 22
9.6. Interoperability with Single-homing PEs . . . . . . . . . . 24
10. Determining Reachability to Unicast MAC Addresses . . . . . . 25
10.1. Local Learning . . . . . . . . . . . . . . . . . . . . . . 25
10.2. Remote learning . . . . . . . . . . . . . . . . . . . . . 26
10.2.1. Constructing the BGP EVPN MAC Address Advertisement . 26
10.2.2 Route Resolution . . . . . . . . . . . . . . . . . . . 28
11. ARP and ND . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1 Default Gateway . . . . . . . . . . . . . . . . . . . . . . 29
12. Handling of Multi-Destination Traffic . . . . . . . . . . . . 30
12.1. Construction of the Inclusive Multicast Ethernet Tag
Route . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.2. P-Tunnel Identification . . . . . . . . . . . . . . . . . 31
13. Processing of Unknown Unicast Packets . . . . . . . . . . . . 32
13.1. Ingress Replication . . . . . . . . . . . . . . . . . . . 33
13.2. P2MP MPLS LSPs . . . . . . . . . . . . . . . . . . . . . . 33
14. Forwarding Unicast Packets . . . . . . . . . . . . . . . . . . 34
14.1. Forwarding packets received from a CE . . . . . . . . . . 34
14.2. Forwarding packets received from a remote PE . . . . . . . 35
14.2.1. Unknown Unicast Forwarding . . . . . . . . . . . . . . 35
14.2.2. Known Unicast Forwarding . . . . . . . . . . . . . . . 35
15. Load Balancing of Unicast Frames . . . . . . . . . . . . . . . 36
15.1. Load balancing of traffic from an PE to remote CEs . . . . 36
15.1.1 Single-Active Redundancy Mode . . . . . . . . . . . . . 36
15.1.2 All-Active Redundancy Mode . . . . . . . . . . . . . . 37
15.2. Load balancing of traffic between an PE and a local CE . . 38
15.2.1. Data plane learning . . . . . . . . . . . . . . . . . 38
15.2.2. Control plane learning . . . . . . . . . . . . . . . . 39
16. MAC Mobility . . . . . . . . . . . . . . . . . . . . . . . . . 39
16.1. MAC Duplication Issue . . . . . . . . . . . . . . . . . . 41
16.2. Sticky MAC addresses . . . . . . . . . . . . . . . . . . . 41
17. Multicast & Broadcast . . . . . . . . . . . . . . . . . . . . 41
17.1. Ingress Replication . . . . . . . . . . . . . . . . . . . 41
17.2. P2MP LSPs . . . . . . . . . . . . . . . . . . . . . . . . 42
17.2.1. Inclusive Trees . . . . . . . . . . . . . . . . . . . 42
18. Convergence . . . . . . . . . . . . . . . . . . . . . . . . . 42
18.1. Transit Link and Node Failures between PEs . . . . . . . . 42
18.2. PE Failures . . . . . . . . . . . . . . . . . . . . . . . 43
18.2. PE to CE Network Failures . . . . . . . . . . . . . . . . 43
19. Frame Ordering . . . . . . . . . . . . . . . . . . . . . . . . 43
20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 44
21. Security Considerations . . . . . . . . . . . . . . . . . . . 44
22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
23. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
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23.1 Normative References . . . . . . . . . . . . . . . . . . . 45
23.2 Informative References . . . . . . . . . . . . . . . . . . 45
24. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 45
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1. Specification of requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Terminology
Bridge Domain:
Broadcast Domain:
CE: Customer Edge device e.g., host or router or switch
EVI: An EVPN instance spanning across the PEs participating in that
VPN
MAC-VRF: A Virtual Routing and Forwarding table for MAC addresses on
a PE for an EVI
Ethernet Segment Identifier (ESI): If a CE is multi-homed to two or
more PEs, the set of Ethernet links that attaches the CE to the PEs
is an 'Ethernet segment'. Ethernet segments MUST have a unique non-
zero identifier, the 'Ethernet Segment Identifier'.
Ethernet Tag: An Ethernet Tag identifies a particular broadcast
domain, e.g., a VLAN. An EVPN instance consists of one or more
broadcast domains. Ethernet tag(s) are assigned to the broadcast
domains of a given EVPN instance by the provider of that EVPN, and
each PE in that EVPN instance performs a mapping between broadcast
domain identifier(s) understood by each of its attached CEs and the
corresponding Ethernet tag.
LACP: Link Aggregation Control Protocol
MP2MP: Multipoint to Multipoint
P2MP: Point to Multipoint
P2P: Point to Point
Single-Active Mode: When a device or a network is multi-homed to two
or more PEs and when only a single PE in such redundancy group can
forward traffic to/from the multi-homed device or network for a given
VLAN, then such multi-homing or redundancy is referred to as "Single-
Active".
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All-Active Mode: When a device is multi-homed to two or more PEs and
when all PEs in such redundancy group can forward traffic to/from the
multi-homed device for a given VLAN, then such multi-homing or
redundancy is referred to as "All-Active".
3. Introduction
This document describes procedures for BGP MPLS based Ethernet VPNs
(EVPN). The procedures described here are intended to meet the
requirements specified in [EVPN-REQ]. Please refer to [EVPN-REQ] for
the detailed requirements and motivation. EVPN requires extensions to
existing IP/MPLS protocols as described in this document. In addition
to these extensions EVPN uses several building blocks from existing
MPLS technologies.
4. Contributors
In addition to the authors listed above, the following individuals
also contributed to this document:
Quaizar Vohra
Kireeti Kompella
Apurva Mehta
Nadeem Mohammad
Juniper Networks
Clarence Filsfils
Dennis Cai
Cisco
5. BGP MPLS Based EVPN Overview
This section provides an overview of EVPN. An EVPN instance comprises
CEs that are connected to PEs that form the edge of the MPLS
infrastructure. A CE may be a host, a router or a switch. The PEs
provide virtual Layer 2 bridged connectivity between the CEs. There
may be multiple EVPN instances in the provider's network.
The PEs may be connected by an MPLS LSP infrastructure which provides
the benefits of MPLS technology such as fast-reroute, resiliency,
etc. The PEs may also be connected by an IP infrastructure in which
case IP/GRE tunneling or other IP tunneling can be used between the
PEs. The detailed procedures in this version of this document are
specified only for MPLS LSPs as the tunneling technology. However
these procedures are designed to be extensible to IP tunneling as the
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PSN tunneling technology.
In an EVPN, MAC learning between PEs occurs not in the data plane (as
happens with traditional bridging) but in the control plane. Control
plane learning offers greater control over the MAC learning process,
such as restricting who learns what, and the ability to apply
policies. Furthermore, the control plane chosen for advertising MAC
reachability information is multi-protocol (MP) BGP (similar to IP
VPNs (RFC 4364)). This provides greater scalability and the ability
to preserve the "virtualization" or isolation of groups of
interacting agents (hosts, servers, virtual machines) from each
other. In EVPN, PEs advertise the MAC addresses learned from the CEs
that are connected to them, along with an MPLS label, to other PEs in
the control plane using MP-BGP. Control plane learning enables load
balancing of traffic to and from CEs that are multi-homed to multiple
PEs. This is in addition to load balancing across the MPLS core via
multiple LSPs between the same pair of PEs. In other words it allows
CEs to connect to multiple active points of attachment. It also
improves convergence times in the event of certain network failures.
However, learning between PEs and CEs is done by the method best
suited to the CE: data plane learning, IEEE 802.1x, LLDP, 802.1aq,
ARP, management plane or other protocols.
It is a local decision as to whether the Layer 2 forwarding table on
an PE is populated with all the MAC destination addresses known to
the control plane, or whether the PE implements a cache based scheme.
For instance the MAC forwarding table may be populated only with the
MAC destinations of the active flows transiting a specific PE.
The policy attributes of EVPN are very similar to those of IP-VPN. A
EVPN instance requires a Route-Distinguisher (RD) which is unique per
PE and one or more globally unique Route-Targets (RTs). A CE attaches
to a MAC-VRF on an PE, on an Ethernet interface which may be
configured for one or more Ethernet Tags, e.g., VLAN IDs. Some
deployment scenarios guarantee uniqueness of VLAN IDs across EVPN
instances: all points of attachment for a given EVPN instance use the
same VLAN ID, and no other EVPN instance uses this VLAN ID. This
document refers to this case as a "Unique VLAN EVPN" and describes
simplified procedures to optimize for it.
6. Ethernet Segment
If a CE is multi-homed to two or more PEs, the set of Ethernet links
constitutes an "Ethernet Segment". An Ethernet segment may appear to
the CE as a Link Aggregation Group (LAG). Ethernet segments have an
identifier, called the "Ethernet Segment Identifier" (ESI) which is
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encoded as a ten octets integer. The following two ESI values are
reserved:
- ESI 0 denotes a single-homed CE.
- ESI {0xFF} (repeated 10 times) is known as MAX-ESI and is reserved.
In general, an Ethernet segment MUST have a non-reserved ESI that is
unique network wide (e.g., across all EVPN instances on all the PEs).
If the CE(s) constituting an Ethernet Segment is (are) managed by the
network operator, then ESI uniqueness should be guaranteed; however,
if the CE(s) is (are) not managed, then the operator MUST configure a
network-wide unique ESI for that Ethernet Segment. This is required
to enable auto-discovery of Ethernet Segments and DF election. The
ESI can be assigned using various mechanisms:
1. If IEEE 802.1AX LACP is used between the PEs and CEs, then
the ESI is determined from LACP by concatenating the following
parameters:
+ CE LACP System Identifier comprised of two octets of System
Priority and six octets of System MAC address, where the
System Priority is encoded in the most significant two octets.
The CE LACP identifier MUST be encoded in the high order eight
octets of the ESI.
+ CE LACP two octets Port Key. The CE LACP port key MUST be
encoded in the low order two octets of the ESI.
As far as the CE is concerned, it would treat the multiple PEs
that it is connected to as the same switch. This allows the CE
to aggregate links that are attached to different PEs in the
same bundle.
This mechanism could be used only if it produces ESIs that satisfy
the uniqueness requirement specified above.
2. In the case of indirectly connected hosts via a bridged LAN
between the CEs and the PEs, the ESI is determined based on the
Layer 2 bridge protocol as follows: If MST is used in the bridged
LAN then the value of the ESI is derived by listening to BPDUs on
the Ethernet segment. To achieve this the PE is not required to
run MST. However the PE must learn the Root Bridge MAC address
and Bridge Priority of the root of the Internal Spanning Tree
(IST) by listening to the BPDUs. The ESI is constructed as
follows:
{Bridge Priority (16 bits) , Root Bridge MAC Address (48 bits)}
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This mechanism could be used only if it produces ESIs that satisfy
the uniqueness requirement specified above.
3. The ESI may be configured.
7. Ethernet Tag
An Ethernet Tag identifies a particular broadcast domain, e.g. a
VLAN, in an EVPN Instance. An EVPN Instance consists of one or more
broadcast domains (one or more VLANs). VLANs are assigned to a given
EVPN Instance by the provider of the EVPN service. A given VLAN can
itself be represented by multiple VLAN IDs (VIDs). In such cases, the
PEs participating in that VLAN for a given EVPN instance are
responsible for performing VLAN ID translation to/from locally
attached CE devices.
If a VLAN is represented by a single VID across all PE devices
participating in that VLAN for that EVPN instance, then there is no
need for VID translation at the PEs. Furthermore, some deployment
scenarios guarantee uniqueness of VIDs across all EVPN instances;
all points of attachment for a given EVPN instance use the same VID
and no other EVPN instances use that VID. This allows the RT(s) for
each EVPN instance to be derived automatically from the corresponding
VID, as described in section 9.4.1.1.1 "Auto-Derivation from the
Ethernet Tag ID".
The following subsections discuss the relationship between broadcast
domains (e.g., VLANs), Ethernet Tags (e.g., VIDs), and MAC-VRFs as
well as the setting of the Ethernet Tag Identifier, in the various
EVPN BGP routes (defined in section 8), for the different types of
service interfaces described in [EVPN-REQ].
7.1 VLAN Based Service Interface
With this service interface, an EVPN instance consists of only a
single broadcast domain (e.g., a single VLAN). Therefore, there is a
one to one mapping between a VID on this interface and a MAC-VRF.
Since a MAC-VRF corresponds to a single VLAN, it consists of a single
bridge domain corresponding to that VLAN. If the VLAN is represented
by different VIDs on different PEs, then each PE needs to perform VID
translation for frames destined to its attached CEs. In such
scenarios, the Ethernet frames transported over MPLS/IP network
SHOULD remain tagged with the originating VID and a VID translation
MUST be supported in the data path and MUST be performed on the
disposition PE. The Ethernet Tag Identifier in all EVPN routes MUST
be set to 0.
7.2 VLAN Bundle Service Interface
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With this service interface, an EVPN instance corresponds to several
broadcast domains (e.g., several VLANs); however, only a single
bridge domain is maintained per MAC-VRF which means multiple VLANs
share the same bridge domain. This implies MAC addresses MUST be
unique across different VLANs for this service to work. In other
words, there is a many-to-one mapping between VLANs and a MAC-VRF,
and the MAC-VRF consists of a single bridge domain. Furthermore, a
single VLAN must be represented by a single VID - e.g., no VID
translation is allowed for this service interface type. The MPLS
encapsulated frames MUST remain tagged with the originating VID. Tag
translation is NOT permitted. The Ethernet Tag Identifier in all EVPN
routes MUST be set to 0.
7.2.1 Port Based Service Interface
This service interface is a special case of the VLAN Bundle service
interface, where all of the VLANs on the port are part of the same
service and map to the same bundle. The procedures are identical to
those described in section 7.2.
7.3 VLAN Aware Bundle Service Interface
With this service interface, an EVPN instance consists of several
broadcast domains (e.g., several VLANs) with each VLAN having its own
bridge domain - e.g., multiple bridge domains (one per VLAN) is
maintained by a single MAC-VRF corresponding to the EVPN instance. In
the case where a single VLAN is represented by different VIDs on
different CEs and thus tag (VID) translation is required, a
normalized Ethernet Tag (VID) MUST be carried in the MPLS
encapsulated frames and a tag translation function MUST be supported
in the data path. This translation MUST be performed in data path on
both the imposition as well as the disposition PEs (translating to
normalized tag on imposition PE and translating to local tag on
disposition PE). The Ethernet Tag Identifier in all EVPN routes MUST
be set to the normalized Ethernet Tag assigned by the EVPN provider.
7.3.1 Port Based VLAN Aware Service Interface
This service interface is a special case of the VLAN Aware Bundle
service interface, where all of the VLANs on the port are part of the
same service and map to the same bundle. The procedures are identical
to those described in section 7.3.
8. BGP EVPN NLRI
This document defines a new BGP NLRI, called the EVPN NLRI.
Following is the format of the EVPN NLRI:
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+-----------------------------------+
| Route Type (1 octet) |
+-----------------------------------+
| Length (1 octet) |
+-----------------------------------+
| Route Type specific (variable) |
+-----------------------------------+
The Route Type field defines encoding of the rest of the EVPN NLRI
(Route Type specific EVPN NLRI).
The Length field indicates the length in octets of the Route Type
specific field of EVPN NLRI.
This document defines the following Route Types:
+ 1 - Ethernet Auto-Discovery (A-D) route
+ 2 - MAC advertisement route
+ 3 - Inclusive Multicast Route
+ 4 - Ethernet Segment Route
The detailed encoding and procedures for these route types are
described in subsequent sections.
The EVPN NLRI is carried in BGP [RFC4271] using BGP Multiprotocol
Extensions [RFC4760] with an AFI of 25 (L2VPN) and a SAFI of 70
(EVPN). The NLRI field in the MP_REACH_NLRI/MP_UNREACH_NLRI attribute
contains the EVPN NLRI (encoded as specified above).
In order for two BGP speakers to exchange labeled EVPN NLRI, they
must use BGP Capabilities Advertisement to ensure that they both are
capable of properly processing such NLRI. This is done as specified
in [RFC4760], by using capability code 1 (multiprotocol BGP) with an
AFI of 25 (L2VPN) and a SAFI of 70 (EVPN).
8.1. Ethernet Auto-Discovery Route
A Ethernet A-D route type specific EVPN NLRI consists of the
following:
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+---------------------------------------+
| RD (8 octets) |
+---------------------------------------+
|Ethernet Segment Identifier (10 octets)|
+---------------------------------------+
| Ethernet Tag ID (4 octets) |
+---------------------------------------+
| MPLS Label (3 octets) |
+---------------------------------------+
For procedures and usage of this route please see section 9.2 "Fast
Convergence" and section 9.4 "Aliasing".
8.2. MAC Advertisement Route
A MAC advertisement route type specific EVPN NLRI consists of the
following:
+---------------------------------------+
| RD (8 octets) |
+---------------------------------------+
|Ethernet Segment Identifier (10 octets)|
+---------------------------------------+
| Ethernet Tag ID (4 octets) |
+---------------------------------------+
| MAC Address Length (1 octet) |
+---------------------------------------+
| MAC Address (6 octets) |
+---------------------------------------+
| IP Address Length (1 octet) |
+---------------------------------------+
| IP Address (4 or 16 octets) |
+---------------------------------------+
| MPLS Label (3 octets) |
+---------------------------------------+
For the purpose of BGP route key processing, only the Ethernet Tag
ID, MAC Address Length, MAC Address, IP Address Length, and IP
Address Address fields are considered to be part of the prefix in the
NLRI. The Ethernet Segment Identifier and MPLS Label fields are to be
treated as route attributes as opposed to being part of the "route".
For procedures and usage of this route please see section 10
"Determining Reachability to Unicast MAC Addresses" and section 15
"Load Balancing of Unicast Packets".
8.3. Inclusive Multicast Ethernet Tag Route
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An Inclusive Multicast Ethernet Tag route type specific EVPN NLRI
consists of the following:
+---------------------------------------+
| RD (8 octets) |
+---------------------------------------+
| Ethernet Tag ID (4 octets) |
+---------------------------------------+
| IP Address Length (1 octet) |
+---------------------------------------+
| Originating Router's IP Addr |
| (4 or 16 octets) |
+---------------------------------------+
For procedures and usage of this route please see section 12
"Handling of Multi-Destination Traffic", section 13 "Processing of
Unknown Unicast Traffic" and section 17 "Multicast".
8.4 Ethernet Segment Route
The Ethernet Segment Route is encoded in the EVPN NLRI using the
Route Type value of 4. The Route Type Specific field of the NLRI is
formatted as follows:
+---------------------------------------+
| RD (8 octets) |
+---------------------------------------+
|Ethernet Segment Identifier (10 octets)|
+---------------------------------------+
| IP Address Length (1 octet) |
+---------------------------------------+
| Originating Router's IP Addr |
| (4 or 16 octets) |
+---------------------------------------+
For procedures and usage of this route please see section 9.5
"Designated Forwarder Election".
8.5 ESI Label Extended Community
This extended community is a new transitive extended community with
the Type field is 0x06, and the Sub-Type of 0x01. It may be
advertised along with Ethernet Auto-Discovery routes and it enables
split-horizon procedures for multi-homed sites as described in
section 9.3 "Split Horizon".
Each ESI Label Extended Community is encoded as a 8-octet value as
follows:
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x06 | Sub-Type=0x01 | Flags (One Octet) |Reserved=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved = 0| ESI Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The low order bit of the flags octet is defined as the "Active-
Standby" bit and may be set to 1. A value of 0 means that the multi-
homed site is operating in All-Active mode; whereas, a value of 1
means that the multi-homed site is operating in Single-Active mode.
The second low order bit of the flags octet is defined as the "Root-
Leaf". A value of 0 means that this label is associated with a Root
site; whereas, a value of 1 means that this label is associate with a
Leaf site. The other bits must be set to 0.
8.6 ES-Import Route Target
This is a new transitive Route Target extended community carried with
the Ethernet Segment route. When used, it enables all the PEs
connected to the same multi-homed site to import the Ethernet Segment
routes. The value is derived automatically from the ESI by encoding
the 6-byte MAC address portion of the ESI in the ES-Import Route
Target. The format of this extended community is as follows:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x06 | Sub-Type=0x02 | ES-Import |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ES-Import Cont'd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This document expands the definition of the Route Target extended
community to allow the value of high order octet (Type field) to be
0x06 (in addition to the values specified in rfc4360). The value of
low order octet (Sub-Type field) of 0x02 indicates that this extended
community is of type "Route Target". The new value for Type field of
0x06 indicates that the structure of this RT is a six bytes value
(e.g., a MAC address). A BGP speaker that implements RT-Constrain
(RFC4684) MUST apply the RT-Constrain procedures to the ES-import RT
as-well.
For procedures and usage of this attribute, please see section 9.1
"Redundancy Group Discovery".
8.7 MAC Mobility Extended Community
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This extended community is a new transitive extended community with
the Type field of 0x06 and the Sub-Type of 0x00. It may be advertised
along with MAC Advertisement routes. The procedures for using this
Extended Community are described in section 16 "MAC Mobility".
The MAC Mobility Extended Community is encoded as a 8-octet value as
follows:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x06 | Sub-Type=0x00 |Flags(1 octet)| Reserved=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The low order bit of the flags octet is defined as the
"Sticky/static" flag and may be set to 1. A value of 1 means that the
MAC address is static and cannot move.
8.8 Default Gateway Extended Community
The Default Gateway community is an Extended Community of an Opaque
Type (see 3.3 of rfc4360). It is a transitive community, which means
that the first octet is 0x03. The value of the second octet (Sub-
Type) is 0x030d (Default Gateway) as defined by IANA. The Value field
of this community is reserved (set to 0 by the senders, ignored by
the receivers).
9. Multi-homing Functions
This section discusses the functions, procedures and associated BGP
routes used to support multi-homing in EVPN. This covers both multi-
homed device (MHD) as well as multi-homed network (MHN) scenarios.
9.1 Multi-homed Ethernet Segment Auto-Discovery
PEs connected to the same Ethernet segment can automatically discover
each other with minimal to no configuration through the exchange of
the Ethernet Segment route.
9.1.1 Constructing the Ethernet Segment Route
The Route-Distinguisher (RD) MUST be a Type 1 RD [RFC4364]. The value
field comprises an IP address of the MES (typically, the loopback
address) followed by 0's.
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The Ethernet Segment Identifier MUST be set to the ten octet ESI
identifier described in section 6.
The BGP advertisement that advertises the Ethernet Segment route MUST
also carry an ES-Import extended community attribute, as defined in
section 8.6.
The Ethernet Segment Route filtering MUST be done such that the
Ethernet Segment Route is imported only by the PEs that are multi-
homed to the same Ethernet Segment. To that end, each PE that is
connected to a particular Ethernet segment constructs an import
filtering rule to import a route that carries the ES-Import extended
community, constructed from the ESI.
9.2 Fast Convergence
In EVPN, MAC address reachability is learnt via the BGP control-plane
over the MPLS network. As such, in the absence of any fast protection
mechanism, the network convergence time is a function of the number
of MAC Advertisement routes that must be withdrawn by the PE
encountering a failure. For highly scaled environments, this scheme
yields slow convergence.
To alleviate this, EVPN defines a mechanism to efficiently and
quickly signal, to remote PE nodes, the need to update their
forwarding tables upon the occurrence of a failure in connectivity to
an Ethernet segment. This is done by having each PE advertise an
Ethernet A-D Route per Ethernet segment for each locally attached
segment (refer to section 9.2.1 below for details on how this route
is constructed). Upon a failure in connectivity to the attached
segment, the PE withdraws the corresponding Ethernet A-D route. This
triggers all PEs that receive the withdrawal to update their next-hop
adjacencies for all MAC addresses associated with the Ethernet
segment in question. If no other PE had advertised an Ethernet A-D
route for the same segment, then the PE that received the withdrawal
simply invalidates the MAC entries for that segment. Otherwise, the
PE updates the next-hop adjacencies to point to the backup PE(s).
9.2.1 Constructing the Ethernet A-D Route per Ethernet Segment
This section describes procedures to construct the Ethernet A-D route
when a single such route is advertised by an PE for a given Ethernet
Segment. This flavor of the Ethernet A-D route is used for fast
convergence (as discussed above) as well as for advertising the ESI
label used for split-horizon filtering (as discussed in section 9.3).
Support of this route flavor is MANDATORY.
Route-Distinguisher (RD) MUST be a Type 1 RD [RFC4364]. The value
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field comprises an IP address of the PE (typically, the loopback
address) followed by 0.
The Ethernet Segment Identifier MUST be a ten octet entity as
described in section "Ethernet Segment". This document does not
specify the use of the Ethernet A-D route when the Segment Identifier
is set to 0.
The Ethernet Tag ID MUST be set to 0.
The MPLS label in the NLRI MUST be set to 0.
The "ESI Label Extended Community" MUST be included in the route. If
all-Active multi-homing is desired, then the "Active-Standby" bit in
the flags of the ESI Label Extended Community MUST be set to 0 and
the MPLS label in that extended community MUST be set to a valid MPLS
label value. The MPLS label in this Extended Community is referred to
as an "ESI label". This label MUST be a downstream assigned MPLS
label if the advertising PE is using ingress replication for
receiving multicast, broadcast or unknown unicast traffic from other
PEs. If the advertising PE is using P2MP MPLS LSPs for sending
multicast, broadcast or unknown unicast traffic, then this label MUST
be an upstream assigned MPLS label. The usage of this label is
described in section 9.3.
If the Ethernet Segment is connected to more than one PE and Single-
Active multi-homing is desired, then the "Active-Standby" bit in the
flags of the ESI Label Extended Community MUST be set to 1 and ESI
label MUST be set to zero.
9.2.1.1. Ethernet A-D Route Targets
The Ethernet A-D route MUST carry one or more Route Target (RT)
attributes. These RTs MUST be the set of RTs associated with all the
EVPN instances to which the Ethernet Segment, corresponding to the
Ethernet A-D route, belongs.
9.3 Split Horizon
Consider a CE that is multi-homed to two or more PEs on an Ethernet
segment ES1 operating in All-Active mode. If the CE sends a
broadcast, unknown unicast, or multicast (BUM) packet to one of the
non-DF (Designated Forwarder) PEs, say PE1, then PE1 will forward
that packet to all or subset of the other PEs in that EVPN instance
including the DF PE for that Ethernet segment. In this case the DF PE
that the CE is multi-homed to MUST drop the packet and not forward
back to the CE. This filtering is referred to as "split horizon"
filtering in this document.
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In order to achieve this split horizon function, every BUM packet
originating from a non-DF PE is encapsulated with an MPLS label that
identifies the Ethernet segment of origin (i.e. the segment from
which the frame entered the EVPN network). This label is referred to
as the ESI label, and MUST be distributed by all PEs when operating
in All-Active multi-homing mode using the "Ethernet A-D route per
Ethernet Segment" as per the procedures in section 9.2.1 above. This
route is imported by the PEs connected to the Ethernet Segment and
also by the PEs that have at least one EVPN instance in common with
the Ethernet Segment in the route. As described in section 9.1.1, the
route MUST carry an ESI Label Extended Community with a valid ESI
label. The disposition DF PE rely on the value of the ESI label to
determine whether or not a BUM frame is allowed to egress a specific
Ethernet segment. It should be noted that if the BUM frame is
originated from the DF PE operating in All-Active multi-homing mode,
then the DF PE MAY not encapsulate the frame with the ESI label.
Furthermore, if the multi-homed PEs operate in active/standby mode,
then the packet MUST NOT be encapsulated with the ESI label and the
label value MUST be set to zero in ESI Label Extended Community per
section 9.2.1 above.
9.3.1 ESI Label Assignment
The following subsections describe the assignment procedures for the
ESI label, which differ depending on the type of tunnels being used
to deliver multi-destination packets in the EVPN network.
9.3.1.1 Ingress Replication
All PEs operating in an All-Active multi-homing mode that rely on
ingress replication for the reception of BUM traffic, distribute to
other PEs, that belong to the Ethernet segment, a downstream assigned
"ESI label" in the Ethernet A-D route per ESI. This label MUST be
programmed in the platform label space by the advertising PE. Further
the forwarding entry for this label must result in NOT forwarding
packets received with this label onto the Ethernet segment that the
label was distributed for.
Consider PE1 and PE2 that are multi-homed to CE1 on ES1 and operating
in All-Active multi-homing mode. Further consider that PE1 is using
P2P or MP2P LSPs to send packets to PE2. Consider that PE1 is the
non-DF for VLAN1 and PE2 is the DF for VLAN1, and PE1 receives a BUM
packet from CE1 on VLAN1 on ES1. In this scenario, PE2 distributes an
Inclusive Multicast Ethernet Tag route for VLAN1 corresponding to an
EVPN instance. So, when PE1 sends a BUM packet, that it receives from
CE1, it MUST first push onto the MPLS label stack the ESI label that
PE2 has distributed for ES1. It MUST then push on the MPLS label
distributed by PE2 in the Inclusive Multicast Ethernet Tag route for
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VLAN1. The resulting packet is further encapsulated in the P2P or
MP2P LSP label stack required to transmit the packet to PE2. When
PE2 receives this packet, it determines the set of ESIs to replicate
the packet to from the top MPLS label, after any P2P or MP2P LSP
labels have been removed. If the next label is the ESI label assigned
by PE2 for ES1, then PE2 MUST NOT forward the packet onto ES1. If the
next label is an ESI label which has not been assigned by PE2, then
PE2 MUST drop the packet. It should be noted that in this scenario,
if PE2 receives a BUM traffic for VLAN1 from CE1, then it doesn't
need to encapsulate the packet with an ESI label when sending it to
the PE1 since PE1 can use its DF logic to filter the BUM packets and
thus doesn't need to use split-horizon filtering for ES1.
9.3.1.2. P2MP MPLS LSPs
The non-DF PEs operating in an All-Active multi-homing mode that is
using P2MP LSPs for sending BUM traffic, distribute to other PEs,
that belong to the Ethernet segment or have an EVPN instance in
common with the Ethernet Segment, an upstream assigned "ESI label" in
the Ethernet A-D route. This label is upstream assigned by the PE
that advertises the route. This label MUST be programmed by the other
PEs, that are connected to the ESI advertised in the route, in the
context label space for the advertising PE. Further the forwarding
entry for this label must result in NOT forwarding packets received
with this label onto the Ethernet segment that the label was
distributed for. This label MUST also be programmed by the other PEs,
that import the route but are not connected to the ESI advertised in
the route, in the context label space for the advertising PE. Further
the forwarding entry for this label must be a POP with no other
associated action.
Consider PE1 and PE2 that are multi-homed to CE1 on ES1 and operating
in All-Active multi-homing mode. Also consider PE3 belongs to one of
the EVPN instances of ES1. Further, assume that PE1 which is the
non-DF, using P2MP MPLS LSPs to send BUM packets. When PE1 sends a
BUM packet, that it receives from CE1, it MUST first push onto the
MPLS label stack the ESI label that it has assigned for the ESI that
the packet was received on. The resulting packet is further
encapsulated in the P2MP MPLS label stack necessary to transmit the
packet to the other PEs. Penultimate hop popping MUST be disabled on
the P2MP LSPs used in the MPLS transport infrastructure for EVPN.
When PE2 receives this packet, it de-capsulates the top MPLS label
and forwards the packet using the context label space determined by
the top label. If the next label is the ESI label assigned by PE1 to
ES1, then PE2 MUST NOT forward the packet onto ES1. When PE3 receives
this packet, it de-capsulates the top MPLS label and forwards the
packet using the context label space determined by the top label. If
the next label is the ESI label assigned by PE1 to ES1 and PE3 is not
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connected to ES1, then PE3 MUST pop the label and flood the packet
over all local ESIs in that EVPN instance. It should be noted that
when PE2 sends a BUM frame over a P2MP LSP, it does not need to
encapsulate the frame with an ESI label because it is the DF for that
VLAN.
9.4 Aliasing and Backup-Path
In the case where a CE is multi-homed to multiple PE nodes, using a
LAG with All-Active redundancy, it is possible that only a single PE
learns a set of the MAC addresses associated with traffic transmitted
by the CE. This leads to a situation where remote PE nodes receive
MAC advertisement routes, for these addresses, from a single PE even
though multiple PEs are connected to the multi-homed segment. As a
result, the remote PEs are not able to effectively load-balance
traffic among the PE nodes connected to the multi-homed Ethernet
segment. This could be the case, for e.g. when the PEs perform data-
path learning on the access, and the load-balancing function on the
CE hashes traffic from a given source MAC address to a single PE.
Another scenario where this occurs is when the PEs rely on control
plane learning on the access (e.g. using ARP), since ARP traffic will
be hashed to a single link in the LAG.
To alleviate this issue, EVPN introduces the concept of 'Aliasing'.
Aliasing refers to the ability of a PE to signal that it has
reachability to a given locally attached Ethernet segment, even when
it has learnt no MAC addresses from that segment. The Ethernet A-D
route per EVI is used to that end. Remote PEs which receive MAC
advertisement routes with non-reserved ESI SHOULD consider the
advertised MAC address as reachable via all PEs which have advertised
reachability to the relevant Segment using: (1) Ethernet A-D routes
per EVI with the same ESI (and Ethernet Tag if applicable) AND
(2)Ethernet A-D routes per ESI with the same ESI and with the
Active/Standby bit set to 0 in the ESI Label Extended Community.
This flavor of Ethernet A-D route per EVI, associated with aliasing,
can arrive at target PEs asynchronously relative to the flavor of
Ethernet A-D route associated with split-horizon and mass-withdraw
(i.e. per ESI). Therefore, if the Ethernet A-D route per EVI arrives
ahead of the Ethernet A-D route per ESI, then the former must NOT be
used for traffic forwarding till the latter arrives. This will take
care of corner cases and race conditions where the Ethernet A-D route
associated with mass-withdraw is withdrawn but a PE still receives
the route associated with aliasing.
Backup-Path is a closely related function, albeit it applies to the
case where the redundancy mode is Active/Standby. In this case, the
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PE advertises that it has reachability to a given locally attached
Ethernet Segment using the Ethernet A-D route as well. Remote PEs
which receive the MAC advertisement routes, with non-reserved ESI,
MUST consider the MAC address as reachable via the advertising PE.
Furthermore, the remote PEs SHOULD install a Backup-Path, for said
MAC, to the PE which had advertised reachability to the relevant
Segment using (1) an Ethernet A-D routes per EVI with the same ESI
(and Ethernet Tag if applicable) AND (2) Ethernet A-D routes per ESI
with the same ESI and with the Active/Standby bit set to 1 in the ESI
Label Extended Community.
9.4.1 Constructing the Ethernet A-D Route per EVI
This section describes procedures to construct the Ethernet A-D route
when one or more such routes are advertised by an PE for a given EVI.
This flavor of the Ethernet A-D route is used for aliasing, and
support of this route flavor is OPTIONAL.
Route-Distinguisher (RD) MUST be set to the RD of the EVI that is
advertising the NLRI. An RD MUST be assigned for a given EVI on an
PE. This RD MUST be unique across all EVIs on an PE. It is
RECOMMENDED to use the Type 1 RD [RFC4364]. The value field comprises
an IP address of the PE (typically, the loopback address) followed by
a number unique to the PE. This number may be generated by the PE.
Or in the Unique VLAN EVPN case, the low order 12 bits may be the 12
bit VLAN ID, with the remaining high order 4 bits set to 0.
The Ethernet Segment Identifier MUST be a ten octet entity as
described in section "Ethernet Segment Identifier". This document
does not specify the use of the Ethernet A-D route when the Segment
Identifier is set to 0.
The Ethernet Tag ID is the identifier of an Ethernet Tag on the
Ethernet segment. This value may be a 12 bit VLAN ID, in which case
the low order 12 bits are set to the VLAN ID and the high order 20
bits are set to 0. Or it may be another Ethernet Tag used by the
EVPN. It MAY be set to the default Ethernet Tag on the Ethernet
segment or to the value 0.
Note that the above allows the Ethernet A-D route to be advertised
with one of the following granularities:
+ One Ethernet A-D route for a given <ESI, Ethernet Tag ID> tuple
per EVI. This is applicable when the PE uses MPLS-based
disposition.
+ One Ethernet A-D route per <ESI, EVI> (where the Ethernet
Tag ID is set to 0). This is applicable when the PE uses
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MAC-based disposition, or when the PE uses MPLS-based
disposition when no VLAN translation is required.
The usage of the MPLS label is described in the section on "Load
Balancing of Unicast Packets".
The Next Hop field of the MP_REACH_NLRI attribute of the route MUST
be set to the IPv4 or IPv6 address of the advertising PE.
9.4.1.1 Ethernet A-D Route Targets
The Ethernet A-D route MUST carry one or more Route Target (RT)
attributes. RTs may be configured (as in IP VPNs), or may be derived
automatically.
If an PE uses Route Target Constrain [RT-CONSTRAIN], the PE SHOULD
advertise all such RTs using Route Target Constrains. The use of RT
Constrains allows each Ethernet A-D route to reach only those PEs
that are configured to import at least one RT from the set of RTs
carried in the Ethernet A-D route.
9.4.1.1.1 Auto-Derivation from the Ethernet Tag ID
The following is the procedure for deriving the RT attribute
automatically from the Ethernet Tag ID associated with the
advertisement:
+ The Global Administrator field of the RT MUST
be set to the Autonomous System (AS) number that the PE
belongs to.
+ The Local Administrator field of the RT contains a 4
octets long number that encodes the Ethernet Tag-ID. If the
Ethernet Tag-ID is a two octet VLAN ID then it MUST be
encoded in the lower two octets of the Local Administrator
field and the higher two octets MUST be set to zero.
For the "Unique VLAN EVPN" this results in auto-deriving the RT from
the Ethernet Tag, e.g., VLAN ID for that EVPN.
9.5 Designated Forwarder Election
Consider a CE that is a host or a router that is multi-homed directly
to more than one PE in an EVPN instance on a given Ethernet segment.
One or more Ethernet Tags may be configured on the Ethernet segment.
In this scenario only one of the PEs, referred to as the Designated
Forwarder (DF), is responsible for certain actions:
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- Sending multicast and broadcast traffic, on a given Ethernet
Tag on a particular Ethernet segment, to the CE.
- Flooding unknown unicast traffic (i.e. traffic for
which an PE does not know the destination MAC address),
on a given Ethernet Tag on a particular Ethernet segment
to the CE, if the environment requires flooding of
unknown unicast traffic.
Note that this behavior, which allows selecting a DF at the
granularity of <ESI, EVI> for multicast, broadcast and unknown
unicast traffic, is the default behavior in this specification.
Note that a CE always sends packets belonging to a specific flow
using a single link towards an PE. For instance, if the CE is a host
then, as mentioned earlier, the host treats the multiple links that
it uses to reach the PEs as a Link Aggregation Group (LAG). The CE
employs a local hashing function to map traffic flows onto links in
the LAG.
If a bridged network is multi-homed to more than one PE in an EVPN
network via switches, then the support of All-Active points of
attachments, as described in this specification, requires the bridge
network to be connected to two or more PEs using a LAG. In this case
the reasons for doing DF election are the same as those described
above when a CE is a host or a router.
If a bridged network does not connect to the PEs using LAG, then only
one of the links between the switched bridged network and the PEs
must be the active link for a given Ethernet Tag. In this case, the
Ethernet A-D route per Ethernet segment MUST be advertised with the
"Active-Standby" flag set to one. Procedures for supporting All-
Active points of attachments, when a bridge network connects to the
PEs using LAG, are for further study.
The default procedure for DF election at the granularity of <ESI,
EVI> is referred to as "service carving". With service carving, it is
possible to elect multiple DFs per Ethernet Segment (one per EVI) in
order to perform load-balancing of multi-destination traffic destined
to a given Segment. The load-balancing procedures carve up the EVI
space among the PE nodes evenly, in such a way that every PE is the
DF for a disjoint set of EVIs. The procedure for service carving is
as follows:
1. When a PE discovers the ESI of the attached Ethernet Segment, it
advertises an Ethernet Segment route with the associated ES-Import
extended community attribute.
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2. The PE then starts a timer (default value = 3 seconds) to allow
the reception of Ethernet Segment routes from other PE nodes
connected to the same Ethernet Segment. This timer value MUST be same
across all PEs connected to the same Ethernet Segment.
3. When the timer expires, each PE builds an ordered list of the IP
addresses of all the PE nodes connected to the Ethernet Segment
(including itself), in increasing numeric value. Each IP address in
this list is extracted from the "Originator Router's IP address"
field of the advertised Ethernet Segment route. Every PE is then
given an ordinal indicating its position in the ordered list,
starting with 0 as the ordinal for the PE with the numerically lowest
IP address. The ordinals are used to determine which PE node will be
the DF for a given EVPN instance on the Ethernet Segment using the
following rule: Assuming a redundancy group of N PE nodes, the PE
with ordinal i is the DF for an EVPN instance with an associated
Ethernet Tag value V when (V mod N) = i. In the case where multiple
Ethernet Tags are associated with a single EVPN instance, then the
numerically lowest Ethernet Tag value in that EVPN instance MUST be
used in the modulo function.
It should be noted that using "Originator Router's IP address" field
in the Ethernet Segment route to get the PE IP address needed for the
ordered list, allows for a CE to be multi-homed across different ASes
if such need every arises.
4. The PE that is elected as a DF for a given EVPN instance will
unblock traffic for the Ethernet Tags associated with that EVPN
instance. Note that the DF PE unblocks multi-destination traffic in
the egress direction towards the Segment. All non-DF PEs continue to
drop multi-destination traffic (for the associated EVPN instances) in
the egress direction towards the Segment.
In the case of link or port failure, the affected PE withdraws its
Ethernet Segment route. This will re-trigger the service carving
procedures on all the PEs in the RG. For PE node failure, or upon PE
commissioning or decommissioning, the PEs re-trigger the service
carving. In case of a Single-Active multi-homing, when a service
moves from one PE in the RG to another PE as a result of re-carving,
the PE, which ends up being the elected DF for the service, must
trigger a MAC address flush notification towards the associated
Ethernet Segment. This can be done, for e.g. using IEEE 802.1ak MVRP
'new' declaration.
9.6. Interoperability with Single-homing PEs
Let's refer to PEs that only support single-homed CE devices as
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single-homing PEs. For single-homing PEs, all the above multi-homing
procedures can be omitted; however, to allow for single-homing PEs to
fully inter-operate with multi-homing PEs, some of the multi-homing
procedures described above SHOULD be supported even by single-homing
PEs:
- procedures related to processing Ethernet A-D route for the purpose
of Fast Convergence (9.2 Fast Convergence), to let single-homing PEs
benefit from fast convergence
- procedures related to processing Ethernet A-D route for the purpose
of Aliasing (9.4 Aliasing and Backup-path), to let single-homing PEs
benefit from load balancing
- procedures related to processing Ethernet A-D route for the purpose
of Backup-path (9.4 Aliasing and Backup-path), to let single-homing
PEs to benefit from the corresponding convergence improvement
10. Determining Reachability to Unicast MAC Addresses
PEs forward packets that they receive based on the destination MAC
address. This implies that PEs must be able to learn how to reach a
given destination unicast MAC address.
There are two components to MAC address learning, "local learning"
and "remote learning":
10.1. Local Learning
A particular PE must be able to learn the MAC addresses from the CEs
that are connected to it. This is referred to as local learning.
The PEs in a particular EVPN instance MUST support local data plane
learning using standard IEEE Ethernet learning procedures. An PE must
be capable of learning MAC addresses in the data plane when it
receives packets such as the following from the CE network:
- DHCP requests
- ARP request for its own MAC.
- ARP request for a peer.
Alternatively PEs MAY learn the MAC addresses of the CEs in the
control plane or via management plane integration between the PEs and
the CEs.
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There are applications where a MAC address that is reachable via a
given PE on a locally attached Segment (e.g. with ESI X) may move
such that it becomes reachable via another PE on another Segment
(e.g. with ESI Y). This is referred to as a "MAC Mobility".
Procedures to support this are described in section "MAC Mobility".
10.2. Remote learning
A particular PE must be able to determine how to send traffic to MAC
addresses that belong to or are behind CEs connected to other PEs
i.e. to remote CEs or hosts behind remote CEs. We call such MAC
addresses as "remote" MAC addresses.
This document requires an PE to learn remote MAC addresses in the
control plane. In order to achieve this, each PE advertises the MAC
addresses it learns from its locally attached CEs in the control
plane, to all the other PEs in that EVPN instance, using MP-BGP and
specifically the MAC Advertisement route.
10.2.1. Constructing the BGP EVPN MAC Address Advertisement
BGP is extended to advertise these MAC addresses using the MAC
Advertisement route type in the EVPN NLRI.
The RD MUST be the RD of the EVI that is advertising the NLRI. The
procedures for setting the RD for a given EVI are described in
section 9.4.1.
The Ethernet Segment Identifier is set to the ten octet ESI described
in section "Ethernet Segment".
The Ethernet Tag ID may be zero or may represent a valid Ethernet Tag
ID. This field may be non-zero when there are multiple bridge
domains in the MAC-VRF (e.g., the PE needs to perform qualified
learning for the VLANs in that MAC-VRF).
When the the Ethernet Tag ID in the NLRI is set to a non-zero value,
for a particular bridge domain, then this Ethernet Tag may either be
the Ethernet tag value associated with the CE, e.g., VLAN ID, or it
may be the Ethernet Tag Identifier, e.g., VLAN ID assigned by the
EVPN provider and mapped to the CE's Ethernet tag. The latter would
be the case if the CE Ethernet tags, e.g., VLAN ID, for a particular
bridge domain are different on different CEs.
The MAC address length field is in bits and it is typically set to
48. However this specification enables specifying the MAC address as
a prefix; in which case, the MAC address length field is set to the
length of the prefix. This provides the ability to aggregate MAC
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addresses if the deployment environment supports that. The encoding
of a MAC address MUST be the 6-octet MAC address specified by
[802.1D-ORIG] [802.1D-REV]. If the MAC address is advertised as a
prefix then the trailing bits of the prefix MUST be set to 0 to
ensure that the entire prefix is encoded as 6 octets.
The IP Address field is optional. By default, the IP Address Length
field is set to 0 and the IP address field is omitted from the route.
When a valid IP address or address prefix needs to be advertised
(e.g., for ARP suppression purposes or for inter-subnet switching),
it is then encoded in this route.
The IP Address Length field is in bits and it is the length of the IP
prefix. This provides the ability to advertise IP address prefixes
when the deployment environment supports that. The encoding of an IP
address MUST be either 4 octets for IPv4 or 16 octets for IPv6. When
the IP address is advertised as a prefix, then the trailing bits of
the prefix MUST be set to 0 to ensure that the entire prefix is
encoded as either 4 or 16 octets. The length field of EVPN NLRI
(which is in octets and is described in section 8) is sufficient to
determine whether an IP address/prefix is encoded in this route and
if so, whether the encoded IP address/prefix is IPV4 or IPv6.
The MPLS label field carries a single label and it is encoded as 3
octets, where the high-order 20 bits contain the label value. The
MPLS label MUST be the downstream assigned that is used by the PE to
forward MPLS-encapsulated Ethernet frames, where the destination MAC
address in the Ethernet frame is the MAC address advertised in the
above NLRI. The forwarding procedures are specified in section
"Forwarding Unicast Packets" and "Load Balancing of Unicast Packets".
An PE may advertise the same single EVPN label for all MAC addresses
in a given EVI. This label assignment methodology is referred to as a
per EVI label assignment. Alternatively, an PE may advertise a unique
EVPN label per <ESI, Ethernet Tag> combination. This label assignment
methodology is referred to as a per <ESI, Ethernet Tag> label
assignment. As a third option, an PE may advertise a unique EVPN
label per MAC address. All of these methodologies have their
tradeoffs. The choice of a particular label assignment methodology is
purely local to the PE that originates the route.
Per EVI label assignment requires the least number of EVPN labels,
but requires a MAC lookup in addition to an MPLS lookup on an egress
PE for forwarding. On the other hand, a unique label per <ESI,
Ethernet Tag> or a unique label per MAC allows an egress PE to
forward a packet that it receives from another PE, to the connected
CE, after looking up only the MPLS labels without having to perform a
MAC lookup. This includes the capability to perform appropriate VLAN
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ID translation on egress to the CE.
The Next Hop field of the MP_REACH_NLRI attribute of the route MUST
be set to the IPv4 or IPv6 address of the advertising PE.
The BGP advertisement for the MAC advertisement route MUST also carry
one or more Route Target (RT) attributes. RTs may be configured (as
in IP VPNs), or may be derived automatically from the Ethernet Tag
ID, in the Unique VLAN case, as described in section "Ethernet A-D
Route per EVPN".
It is to be noted that this document does not require PEs to create
forwarding state for remote MACs when they are learnt in the control
plane. When this forwarding state is actually created is a local
implementation matter.
10.2.2 Route Resolution
If the Ethernet Segment Identifier field in a received MAC
Advertisement route is set to the reserved ESI value of 0 or MAX-ESI,
then the receiving PE MUST install forwarding state for the
associated MAC Address based on the MAC Advertisement route alone.
If the Ethernet Segment Identifier field in a received MAC
Advertisement route is set to a non-reserved ESI, and the receiving
PE is locally attached to the same ESI, then the PE does not alter
its forwarding state based on the received route. This ensures that
local routes are preferred to remote routes.
If the Ethernet Segment Identifier field in a received MAC
Advertisement route is set to a non-reserved ESI, then the receiving
PE MUST install forwarding state for a given MAC address only when
both the MAC Advertisement route AND the associated Ethernet A-D
route per ESI have been received.
To illustrate this with an example, consider two PEs (PE1 and PE2)
connected to a multi-homed Ethernet Segment ES1. All-Active
redundancy mode is assumed. A given MAC address M1 is learnt by PE1
but not PE2. On PE3, the following states may arise:
T1- When the MAC Advertisement Route from PE1 and the Ethernet A-D
routes per ESI from PE1 and PE2 are received, PE3 can forward traffic
destined to M1 to both PE1 and PE2.
T2- If after T1, PE1 withdraws its Ethernet A-D route per ESI, then
PE3 forwards traffic destined to M1 to PE2 only.
T3- If after T1, PE2 withdraws its Ethernet A-D route per ESI, then
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PE3 forwards traffic destined to M1 to PE1 only.
T4- If after T1, PE1 withdraws its MAC Advertisement route, then PE3
treats traffic to M1 as unknown unicast. Note, here, that had PE2
also advertised a MAC route for M1 before PE1 withdraws its MAC
route, then PE3 would have continued forwarding traffic destined to
M1 to PE2.
11. ARP and ND
The IP address field in the MAC advertisement route may optionally
carry one of the IP addresses associated with the MAC address. This
provides an option which can be used to minimize the flooding of ARP
or Neighbor Discovery (ND) messages over the MPLS network and to
remote CEs. This option also minimizes ARP (or ND) message processing
on end-stations/hosts connected to the EVPN network. An PE may learn
the IP address associated with a MAC address in the control or
management plane between the CE and the PE. Or, it may learn this
binding by snooping certain messages to or from a CE. When an PE
learns the IP address associated with a MAC address, of a locally
connected CE, it may advertise this address to other PEs by including
it in the MAC Advertisement route. The IP Address may be an IPv4
address encoded using four octets, or an IPv6 address encoded using
sixteen octets. The IP Address length field MUST be set to 32 for an
IPv4 address or to 128 for an IPv6 address.
If there are multiple IP addresses associated with a MAC address,
then multiple MAC advertisement routes MUST be generated, one for
each IP address. For instance, this may be the case when there are
both an IPv4 and an IPv6 address associated with the MAC address.
When the IP address is dissociated with the MAC address, then the MAC
advertisement route with that particular IP address MUST be
withdrawn.
When an PE receives an ARP request for an IP address from a CE, and
if the PE has the MAC address binding for that IP address, the PE
SHOULD perform ARP proxy by responding to the ARP request.
11.1 Default Gateway
When a PE needs to perform inter-subnet forwarding where each subnet
is represented by a different broadcast domain (e.g., different VLAN)
the inter-subnet forwarding is performed at layer 3 and the PE that
performs such function is called the default gateway. In this case
when the PE receives an ARP Request for the IP address of the default
gateway, the PE originates an ARP Reply.
Each PE that acts as a default gateway for a given EVPN instance MAY
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advertise in the EVPN control plane its default gateway MAC address
using the MAC advertisement route, and indicates that such route is
associated with the default gateway. This is accomplished by
requiring the route to carry the Default Gateway extended community
defined in [Section 8.8 Default Gateway Extended Community]. The IP
address field (4 octets for IPv4, 16 octets for IPv6) is set to zero
when advertising the MAC route with the Default Gateway extended
community. Both ESI and Ethernet Tag fields are also set to zero for
this advertisement.
Unless it is known a priori (by means outside of this document) that
all PEs of a given EVPN instance act as a default gateway for that
EVPN instance, the MPLS label MUST be set to a valid downstream
assigned label.
Furthermore, even if all PEs of a given EVPN instance do act as a
default gateway for that EVPN instance, but only some, but not all,
of these PEs have sufficient (routing) information to provide inter-
subnet routing for all the inter-subnet traffic originated within the
subnet associated with the EVPN instance, then when such PE
advertises in the EVPN control plane its default gateway MAC address
using the MAC advertisement route, and indicates that such route is
associated with the default gateway, the route MUST carry a valid
downstream assigned label.
If all PEs of a given EVPN instance act as a default gateway for that
EVPN instance, and the same default gateway MAC address is used
across all gateway devices, then no such advertisement is needed.
However, if each default gateway uses a different MAC address, then
each default gateway needs to be aware of other gateways' MAC
addresses and thus the need for such advertisement. This is called
MAC address aliasing since a single default GW can be represented by
multiple MAC addresses.
Each PE that receives this route and imports it as per procedures
specified in this document follows the procedures in this section
when replying to ARP Requests that it receives if such Requests are
for the IP address in the received EVPN route.
Each PE that acts as a default gateway for a given EVPN instance that
receives this route and imports it as per procedures specified in
this document MUST create MAC forwarding state that enables it to
apply IP forwarding to the packets destined to the MAC address
carried in the route.
12. Handling of Multi-Destination Traffic
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Procedures are required for a given PE to send broadcast or multicast
traffic, received from a CE encapsulated in a given Ethernet Tag
(VLAN) in an EVPN instance, to all the other PEs that span that
Ethernet Tag (VLAN) in that EVPN instance. In certain scenarios,
described in section "Processing of Unknown Unicast Packets", a given
PE may also need to flood unknown unicast traffic to other PEs.
The PEs in a particular EVPN instance may use ingress replication,
P2MP LSPs or MP2MP LSPs to send unknown unicast, broadcast or
multicast traffic to other PEs.
Each PE MUST advertise an "Inclusive Multicast Ethernet Tag Route" to
enable the above. The following subsection provides the procedures to
construct the Inclusive Multicast Ethernet Tag route. Subsequent
subsections describe in further detail its usage.
12.1. Construction of the Inclusive Multicast Ethernet Tag Route
The RD MUST be the RD of the EVI that is advertising the NLRI. The
procedures for setting the RD for a given EVPN instance on a PE are
described in section 9.4.1.
The Ethernet Tag ID is the identifier of the Ethernet Tag. It MAY be
set to 0 or to a valid Ethernet Tag value.
The Originating Router's IP address MUST be set to an IP address of
the PE. This address SHOULD be common for all the EVIs on the PE
(e.,g., this address may be PE's loopback address). The IP Address
Length field is in bits.
The Next Hop field of the MP_REACH_NLRI attribute of the route MUST
be set to the same IP address as the one carried in the Originating
Router's IP Address field.
The BGP advertisement for the Inclusive Multicast Ethernet Tag route
MUST also carry one or more Route Target (RT) attributes. The
assignment of RTs described in the section on "Constructing the BGP
EVPN MAC Address Advertisement" MUST be followed.
12.2. P-Tunnel Identification
In order to identify the P-Tunnel used for sending broadcast, unknown
unicast or multicast traffic, the Inclusive Multicast Ethernet Tag
route MUST carry a "PMSI Tunnel Attribute" as specified in [BGP
MVPN].
Depending on the technology used for the P-tunnel for the EVPN
instance on the PE, the PMSI Tunnel attribute of the Inclusive
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Multicast Ethernet Tag route is constructed as follows.
+ If the PE that originates the advertisement uses a
P-Multicast tree for the P-tunnel for EVPN, the PMSI
Tunnel attribute MUST contain the identity of the tree
(note that the PE could create the identity of the
tree prior to the actual instantiation of the tree).
+ An PE that uses a P-Multicast tree for the P-tunnel MAY
aggregate two or more Ethernet Tags in the same or different
EVIs present on the PE onto the same tree. In this case, in
addition to carrying the identity of the tree, the PMSI Tunnel
attribute MUST carry an MPLS upstream assigned label which
the PE has bound uniquely to the Ethernet Tag for the EVI
associated with this update (as determined by its RTs).
If the PE has already advertised Inclusive Multicast
Ethernet Tag routes for two or more Ethernet Tags that it
now desires to aggregate, then the PE MUST re-advertise
those routes. The re-advertised routes MUST be the same
as the original ones, except for the PMSI Tunnel attribute
and the label carried in that attribute.
+ If the PE that originates the advertisement uses ingress
replication for the P-tunnel for EVPN, the route MUST
include the PMSI Tunnel attribute with the Tunnel Type set to
Ingress Replication and Tunnel Identifier set to a routable
address of the PE. The PMSI Tunnel attribute MUST carry a
downstream assigned MPLS label. This label is used to
demultiplex the broadcast, multicast or unknown unicast EVPN
traffic received over a MP2P tunnel by the PE.
+ The Leaf Information Required flag of the PMSI Tunnel
attribute MUST be set to zero, and MUST be ignored on receipt.
13. Processing of Unknown Unicast Packets
The procedures in this document do not require the PEs to flood
unknown unicast traffic to other PEs. If PEs learn CE MAC addresses
via a control plane protocol, the PEs can then distribute MAC
addresses via BGP, and all unicast MAC addresses will be learnt prior
to traffic to those destinations.
However, if a destination MAC address of a received packet is not
known by the PE, the PE may have to flood the packet. When flooding,
one must take into account "split horizon forwarding" as follows: The
principles behind the following procedures are borrowed from the
split horizon forwarding rules in VPLS solutions [RFC4761] and
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[RFC4762]. When an PE capable of flooding (say PEx) receives an
unknown destination MAC address, it floods the frame. If the frame
arrived from an attached CE, PEx must send a copy of the frame to
every other attached CE participating in that EVPN instance, on a
different ESI than the one it received the frame on, as long as the
PE is the DF for the egress ESI. In addition, the PE must flood the
frame to all other PEs participating in that EVPN instance. If, on
the other hand, the frame arrived from another PE (say PEy), PEx must
send a copy of the packet only to attached CEs as long as it is the
DF for the egress ESI. PEx MUST NOT send the frame to other PEs,
since PEy would have already done so. Split horizon forwarding rules
apply to unknown MAC addresses.
Whether or not to flood packets to unknown destination MAC addresses
should be an administrative choice, depending on how learning happens
between CEs and PEs.
The PEs in a particular EVPN instance may use ingress replication
using RSVP-TE P2P LSPs or LDP MP2P LSPs for sending unknown unicast
traffic to other PEs. Or they may use RSVP-TE P2MP or LDP P2MP for
sending such traffic to other PEs.
13.1. Ingress Replication
If ingress replication is in use, the P-Tunnel attribute, carried in
the Inclusive Multicast Ethernet Tag routes for the EVPN instance,
specifies the downstream label that the other PEs can use to send
unknown unicast, multicast or broadcast traffic for that EVPN
instance to this particular PE.
The PE that receives a packet with this particular MPLS label MUST
treat the packet as a broadcast, multicast or unknown unicast packet.
Further if the MAC address is a unicast MAC address, the PE MUST
treat the packet as an unknown unicast packet.
13.2. P2MP MPLS LSPs
The procedures for using P2MP LSPs are very similar to VPLS
procedures [VPLS-MCAST]. The P-Tunnel attribute used by an PE for
sending unknown unicast, broadcast or multicast traffic for a
particular EVPN instance is advertised in the Inclusive Ethernet Tag
Multicast route as described in section "Handling of Multi-
Destination Traffic".
The P-Tunnel attribute specifies the P2MP LSP identifier. This is the
equivalent of an Inclusive tree in [VPLS-MCAST]. Note that multiple
Ethernet Tags, which may be in different EVPN instances, may use the
same P2MP LSP, using upstream labels [VPLS-MCAST]. This is the
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equivalent of an Aggregate Inclusive tree in [VPLS-MCAST]. When P2MP
LSPs are used for flooding unknown unicast traffic, packet re-
ordering is possible.
The PE that receives a packet on the P2MP LSP specified in the PMSI
Tunnel Attribute MUST treat the packet as a broadcast, multicast or
unknown unicast packet. Further if the MAC address is a unicast MAC
address, the PE MUST treat the packet as an unknown unicast packet.
14. Forwarding Unicast Packets
This section describes procedures for forwarding unicast packets by
PEs, where such packets are received from either directly connected
CEs, or from some other PEs.
14.1. Forwarding packets received from a CE
When an PE receives a packet from a CE, on a given Ethernet Tag, it
must first look up the source MAC address of the packet. In certain
environments the source MAC address MAY be used to authenticate the
CE and determine that traffic from the host can be allowed into the
network. Source MAC lookup MAY also be used for local MAC address
learning.
If the PE decides to forward the packet, the destination MAC address
of the packet must be looked up. If the PE has received MAC address
advertisements for this destination MAC address from one or more
other PEs or learned it from locally connected CEs, it is considered
as a known MAC address. Otherwise, the MAC address is considered as
an unknown MAC address.
For known MAC addresses the PE forwards this packet to one of the
remote PEs or to a locally attached CE. When forwarding to a remote
PE, the packet is encapsulated in the EVPN MPLS label advertised by
the remote PE, for that MAC address, and in the MPLS LSP label stack
to reach the remote PE.
If the MAC address is unknown and if the administrative policy on the
PE requires flooding of unknown unicast traffic then:
- The PE MUST flood the packet to other PEs. The PE MUST first
encapsulate the packet in the ESI MPLS label as described in section
9.3. If ingress replication is used, the packet MUST be replicated
one or more times to each remote PE with the outermost label being an
MPLS label determined as follows: This is the MPLS label advertised
by the remote PE in a PMSI Tunnel Attribute in the Inclusive
Multicast Ethernet Tag route for an <EVPN instance, Ethernet Tag>
combination. The Ethernet Tag in the route must be the same as the
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Ethernet Tag associated with the interface on which the ingress PE
receives the packet. If P2MP LSPs are being used the packet MUST be
sent on the P2MP LSP that the PE is the root of for the Ethernet Tag
in the EVPN instance. If the same P2MP LSP is used for all Ethernet
Tags, then all the PEs in the EVPN instance MUST be the leaves of the
P2MP LSP. If a distinct P2MP LSP is used for a given Ethernet Tag in
the EVPN instance, then only the PEs in the Ethernet Tag MUST be the
leaves of the P2MP LSP. The packet MUST be encapsulated in the P2MP
LSP label stack.
If the MAC address is unknown then, if the administrative policy on
the PE does not allow flooding of unknown unicast traffic:
- The PE MUST drop the packet.
14.2. Forwarding packets received from a remote PE
This section described the procedures for forwarding known and
unknown unicast packets received from a remote PE.
14.2.1. Unknown Unicast Forwarding
When an PE receives an MPLS packet from a remote PE then, after
processing the MPLS label stack, if the top MPLS label ends up being
a P2MP LSP label associated with an EVPN instance or in case of
ingress replication the downstream label advertised in the P-Tunnel
attribute, and after performing the split horizon procedures
described in section "Split Horizon":
- If the PE is the designated forwarder of BUM traffic on a
particular set of ESIs for the Ethernet Tag, the default behavior is
for the PE to flood the packet on these ESIs. In other words, the
default behavior is for the PE to assume that for BUM traffic, it is
not required to perform a destination MAC address lookup. As an
option, the PE may perform a destination MAC lookup to flood the
packet to only a subset of the CE interfaces in the Ethernet Tag. For
instance the PE may decide to not flood an BUM packet on certain
Ethernet segments even if it is the DF on the Ethernet segment, based
on administrative policy.
- If the PE is not the designated forwarder on any of the ESIs for
the Ethernet Tag, the default behavior is for it to drop the packet.
14.2.2. Known Unicast Forwarding
If the top MPLS label ends up being an EVPN label that was advertised
in the unicast MAC advertisements, then the PE either forwards the
packet based on CE next-hop forwarding information associated with
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the label or does a destination MAC address lookup to forward the
packet to a CE.
15. Load Balancing of Unicast Frames
This section specifies the load balancing procedures for sending
known unicast frames to a multi-homed CE.
15.1. Load balancing of traffic from an PE to remote CEs
Whenever a remote PE imports a MAC advertisement for a given <ESI,
Ethernet Tag> in an EVI, it MUST examine all imported Ethernet A-D
routes for that ESI in order to determine the load-balancing
characteristics of the Ethernet segment.
15.1.1 Single-Active Redundancy Mode
For a given ESI, if the remote PE has imported an Ethernet A-D route
per Ethernet Segment from at least one PE, where the "Active-Standby"
flag in the ESI Label Extended Community is set, then the remote PE
MUST deduce that the Ethernet segment is operating in Single-Active
redundancy mode. As such, the MAC address will be reachable only via
the PE announcing the associated MAC Advertisement route - this is
referred to as the primary PE. The set of other PE nodes advertising
Ethernet A-D routes per Ethernet Segment for the same ESI serve as
backup paths, in case the active PE encounters a failure. These are
referred to as the backup PEs. It should be noted that the primary PE
for a given <ESI, EVI> is the DF for that <ESI, EVI>.
If the primary PE encounters a failure, it MAY withdraw its Ethernet
A-D route for the affected segment prior to withdrawing the entire
set of MAC Advertisement routes.
In the case where only a single other backup PE in the network had
advertised an Ethernet A-D route for the same ESI, the remote PE can
then use the Ethernet A-D route withdrawal as a trigger to update its
forwarding entries, for the associated MAC addresses, to point
towards the backup PE. As the backup PE starts learning the MAC
addresses over its attached Ethernet segment, it will start sending
MAC Advertisement routes while the failed PE withdraws its own. This
mechanism minimizes the flooding of traffic during fail-over events.
In the case where multiple other backup PE in the network had
advertised an Ethernet A-D route for the same ESI, the remote PE MUST
then use the Ethernet A-D route withdrawal as a trigger to start
flooding traffic destined to the associated MAC addresses (as long as
flooding of unknown unicast is administratively allowed). It is not
possible to select a single backup path in this case.
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15.1.2 All-Active Redundancy Mode
If for the given ESI, none of the Ethernet A-D routes per Ethernet
Segment imported by the remote PE have the "Active-Standby" flag set
in the ESI Label Extended Community, then the remote PE MUST treat
the Ethernet segment as operating in All-Active redundancy mode. The
remote PE would then treat the MAC address as reachable via all of
the PE nodes from which it has received both an Ethernet A-D route
per Ethernet Segment as well as an Ethernet A-D route per EVI for the
ESI in question. The remote PE MUST use the MAC advertisement and
eligible Ethernet A-D routes to construct the set of next-hops that
it can use to send the packet to the destination MAC. Each next-hop
comprises an MPLS label stack that is to be used by the egress PE to
forward the packet. This label stack is determined as follows:
-If the next-hop is constructed as a result of a MAC route then this
label stack MUST be used. However, if the MAC route doesn't exist,
then the next-hop and MPLS label stack is constructed as a result of
the Ethernet A-D routes. Note that the following description applies
to determining the label stack for a particular next-hop to reach a
given PE, from which the remote PE has received and imported Ethernet
A-D routes that have the matching ESI and Ethernet Tag as the one
present in the MAC advertisement. The Ethernet A-D routes mentioned
in the following description refer to the ones imported from this
given PE.
-If an Ethernet A-D route per Ethernet Segment for that ESI exists,
together with an Ethernet A-D route per EVI, then the label from that
latter route must be used.
The following example explains the above.
Consider a CE (CE1) that is dual-homed to two PEs (PE1 and PE2) on a
LAG interface (ES1), and is sending packets with MAC address MAC1 on
VLAN1. A remote PE, say PE3, is able to learn that MAC1 is reachable
via PE1 and PE2. Both PE1 and PE2 may advertise MAC1 in BGP if they
receive packets with MAC1 from CE1. If this is not the case, and if
MAC1 is advertised only by PE1, PE3 still considers MAC1 as reachable
via both PE1 and PE2 as both PE1 and PE2 advertise a Ethernet A-D
route per ESI for ES1 as well as an Ethernet A-D route per EVI for
<ES1, VLAN1>.
The MPLS label stack to send the packets to PE1 is the MPLS LSP stack
to get to PE1 and the EVPN label advertised by PE1 for CE1's MAC.
The MPLS label stack to send packets to PE2 is the MPLS LSP stack to
get to PE2 and the MPLS label in the Ethernet A-D route advertised by
PE2 for <ES1, VLAN1>, if PE2 has not advertised MAC1 in BGP.
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We will refer to these label stacks as MPLS next-hops.
The remote PE (PE3) can now load balance the traffic it receives from
its CEs, destined for CE1, between PE1 and PE2. PE3 may use N-Tuple
flow information to hash traffic into one of the MPLS next-hops for
load balancing of IP traffic. Alternatively PE3 may rely on the
source MAC addresses for load balancing.
Note that once PE3 decides to send a particular packet to PE1 or PE2
it can pick one out of multiple possible paths to reach the
particular remote PE using regular MPLS procedures. For instance, if
the tunneling technology is based on RSVP-TE LSPs, and PE3 decides to
send a particular packet to PE1, then PE3 can choose from multiple
RSVP-TE LSPs that have PE1 as their destination.
When PE1 or PE2 receive the packet destined for CE1 from PE3, if the
packet is a unicast MAC packet it is forwarded to CE1. If it is a
multicast or broadcast MAC packet then only one of PE1 or PE2 must
forward the packet to the CE. Which of PE1 or PE2 forward this packet
to the CE is determined based on which of the two is the DF.
If the connectivity between the multi-homed CE and one of the PEs
that it is attached to fails, the PE MUST withdraw the Ethernet Tag
A-D routes, that had been previously advertised, for the Ethernet
Segment to the CE. When the MAC entry on the PE ages out, the PE MUST
withdraw the MAC address from BGP. Note that to aid convergence, the
Ethernet Tag A-D routes MAY be withdrawn before the MAC routes. This
enables the remote PEs to remove the MPLS next-hop to this particular
PE from the set of MPLS next-hops that can be used to forward traffic
to the CE. For further details and procedures on withdrawal of EVPN
route types in the event of PE to CE failures please section "PE to
CE Network Failures".
15.2. Load balancing of traffic between an PE and a local CE
A CE may be configured with more than one interface connected to
different PEs or the same PE for load balancing, using a technology
such as LAG. The PE(s) and the CE can load balance traffic onto these
interfaces using one of the following mechanisms.
15.2.1. Data plane learning
Consider that the PEs perform data plane learning for local MAC
addresses learned from local CEs. This enables the PE(s) to learn a
particular MAC address and associate it with one or more interfaces,
if the technology between the PE and the CE supports multi-pathing.
The PEs can now load balance traffic destined to that MAC address on
the multiple interfaces.
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Whether the CE can load balance traffic that it generates on the
multiple interfaces is dependent on the CE implementation.
15.2.2. Control plane learning
The CE can be a host that advertises the same MAC address using a
control protocol on both interfaces. This enables the PE(s) to learn
the host's MAC address and associate it with one or more interfaces.
The PEs can now load balance traffic destined to the host on the
multiple interfaces. The host can also load balance the traffic it
generates onto these interfaces and the PE that receives the traffic
employs EVPN forwarding procedures to forward the traffic.
16. MAC Mobility
It is possible for a given host or end-station (as defined by its MAC
address) to move from one Ethernet segment to another; this is
referred to as 'MAC Mobility' or 'MAC move' and it is different from
the multi-homing situation in which a given MAC address is reachable
via multiple PEs for the same Ethernet segment. In a MAC move, there
would be two sets of MAC Advertisement routes, one set with the new
Ethernet segment and one set with the previous Ethernet segment, and
the MAC address would appear to be reachable via each of these
segments.
In order to allow all of the PEs in the EVPN instance to correctly
determine the current location of the MAC address, all advertisements
of it being reachable via the previous Ethernet segment MUST be
withdrawn by the PEs, for the previous Ethernet segment, that had
advertised it.
If local learning is performed using the data plane, these PEs will
not be able to detect that the MAC address has moved to another
Ethernet segment and the receipt of MAC Advertisement routes, with
the MAC Mobility extended community attribute, from other PEs serves
as the trigger for these PEs to withdraw their advertisements. If
local learning is performed using the control or management planes,
these interactions serve as the trigger for these PEs to withdraw
their advertisements.
In a situation where there are multiple moves of a given MAC,
possibly between the same two Ethernet segments, there may be
multiple withdrawals and re-advertisements. In order to ensure that
all PEs in the EVPN instance receive all of these correctly through
the intervening BGP infrastructure, it is necessary to introduce a
sequence number into the MAC Mobility extended community attribute.
An implementation MUST handle the scenarios where the sequence number
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wraps around to process mobility event correctly.
Every MAC mobility event for a given MAC address will contain a
sequence number that is set using the following rules:
- A PE advertising a MAC address for the first time advertises it
with no MAC Mobility extended community attribute.
- A PE detecting a locally attached MAC address for which it had
previously received a MAC Advertisement route with a different
Ethernet segment identifier advertises the MAC address in a MAC
Advertisement route tagged with a MAC Mobility extended community
attribute with a sequence number one greater than the sequence number
in the MAC mobility attribute of the received MAC Advertisement
route. In the case of the first mobility event for a given MAC
address, where the received MAC Advertisement route does not carry a
MAC Mobility attribute, the value of the sequence number in the
received route is assumed to be 0 for purpose of this processing.
- A PE detecting a locally attached MAC address for which it had
previously received a MAC Advertisement route with the same non-zero
Ethernet segment identifier advertises it with:
i. no MAC Mobility extended community attribute, if the received
route did not carry said attribute.
ii. a MAC Mobility extended community attribute with the sequence
number equal to the highest of the sequence number(s) in the
received MAC Advertisement route(s), if the received route(s) is
(are) tagged with a MAC Mobility extended community attribute.
- A PE detecting a locally attached MAC address for which it had
previously received a MAC Advertisement route with the same zero
Ethernet segment identifier (single-homed scenarios) advertises it
with MAC mobility extended community attribute with the sequence
number set properly. In case of single-homed scenarios, there is no
need for ESI comparison. The reason ESI comparison is done for multi-
homing, is to prevent false detection of MAC move among the PEs
attached to the same multi-homed site.
A PE receiving a MAC Advertisement route for a MAC address with a
different Ethernet segment identifier and a higher sequence number
than that which it had previously advertised, withdraws its MAC
Advertisement route. If two (or more) PEs advertise the same MAC
address with same sequence number but different Ethernet segment
identifiers, a PE that receives these routes selects the route
advertised by the PE with lowest IP address as the best route.
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16.1. MAC Duplication Issue
A situation may arise where the same MAC address is learned by
different PEs in the same VLAN because of two (or more hosts) being
mis-configured with the same (duplicate) MAC address. In such
situation, the traffic originating from these hosts would trigger
continuous MAC moves among the PEs attached to these hosts. It is
important to recognize such situation and avoid incrementing the
sequence number (in the MAC Mobility attribute) to infinity. In order
to remedy such situation, a PE that detects a MAC mobility event by
way of local learning starts an M-second timer (default value of M =
5) and if it detects N MAC moves before the timer expires (default
value for N = 3), it concludes that a duplicate MAC situation has
occurred. The PE MUST alert the operator and stop sending and
processing any BGP MAC Advertisement routes for that MAC address till
a corrective action is taken by the operator. The values of M and N
MUST be configurable to allow for flexibility in operator control.
Note that the other PEs in the E-VPN instance will forward the
traffic for the duplicate MAC address to one of the PEs advertising
the duplicate MAC address.
16.2. Sticky MAC addresses
There are scenarios in which it is desired to configure some MAC
addresses as static so that they are not subjected to MAC move. In
such scenarios, these MAC addresses are advertised with MAC Mobility
Extended Community where static flag is set to 1 and sequence number
is set to zero. If a PE receives such advertisements and later learns
the same MAC address(es) via local learning, then the PE MUST alert
the operator.
17. Multicast & Broadcast
The PEs in a particular EVPN instance may use ingress replication or
P2MP LSPs to send multicast traffic to other PEs.
17.1. Ingress Replication
The PEs may use ingress replication for flooding BUM traffic as
described in section "Handling of Multi-Destination Traffic". A given
broadcast packet must be sent to all the remote PEs. However a given
multicast packet for a multicast flow may be sent to only a subset of
the PEs. Specifically a given multicast flow may be sent to only
those PEs that have receivers that are interested in the multicast
flow. Determining which of the PEs have receivers for a given
multicast flow is done using explicit tracking described below.
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17.2. P2MP LSPs
An PE may use an "Inclusive" tree for sending an BUM packet. This
terminology is borrowed from [VPLS-MCAST].
A variety of transport technologies may be used in the SP network.
For inclusive P-Multicast trees, these transport technologies include
point-to-multipoint LSPs created by RSVP-TE or mLDP.
17.2.1. Inclusive Trees
An Inclusive Tree allows the use of a single multicast distribution
tree, referred to as an Inclusive P-Multicast tree, in the SP network
to carry all the multicast traffic from a specified set of EVPN
instances on a given PE. A particular P-Multicast tree can be set up
to carry the traffic originated by sites belonging to a single EVPN
instance, or to carry the traffic originated by sites belonging to
different EVPN instances. The ability to carry the traffic of more
than one EVPN instance on the same tree is termed 'Aggregation'. The
tree needs to include every PE that is a member of any of the EVPN
instances that are using the tree. This implies that an PE may
receive multicast traffic for a multicast stream even if it doesn't
have any receivers that are interested in receiving traffic for that
stream.
An Inclusive P-Multicast tree as defined in this document is a P2MP
tree. A P2MP tree is used to carry traffic only for EVPN CEs that
are connected to the PE that is the root of the tree.
The procedures for signaling an Inclusive Tree are the same as those
in [VPLS-MCAST] with the VPLS-AD route replaced with the Inclusive
Multicast Ethernet Tag route. The P-Tunnel attribute [VPLS-MCAST] for
an Inclusive tree is advertised in the Inclusive Multicast route as
described in section "Handling of Multi-Destination Traffic". Note
that an PE can "aggregate" multiple inclusive trees for different
EVPN instances on the same P2MP LSP using upstream labels. The
procedures for aggregation are the same as those described in [VPLS-
MCAST], with VPLS A-D routes replaced by EVPN Inclusive Multicast
routes.
18. Convergence
This section describes failure recovery from different types of
network failures.
18.1. Transit Link and Node Failures between PEs
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The use of existing MPLS Fast-Reroute mechanisms can provide failure
recovery in the order of 50ms, in the event of transit link and node
failures in the infrastructure that connects the PEs.
18.2. PE Failures
Consider a host host1 that is dual homed to PE1 and PE2. If PE1
fails, a remote PE, PE3, can discover this based on the failure of
the BGP session. This failure detection can be in the sub-second
range if BFD is used to detect BGP session failure. PE3 can update
its forwarding state to start sending all traffic for host1 to only
PE2. It is to be noted that this failure recovery is potentially
faster than what would be possible if data plane learning were to be
used. As in that case PE3 would have to rely on re-learning of MAC
addresses via PE2.
18.2. PE to CE Network Failures
When an Ethernet segment connected to an PE fails or when a Ethernet
Tag is decommissioned on an Ethernet segment, then the PE MUST
withdraw the Ethernet A-D route(s) announced for the <ESI, Ethernet
Tags> that are impacted by the failure or decommissioning. In
addition, the PE MUST also withdraw the MAC advertisement routes that
are impacted by the failure or decommissioning.
The Ethernet A-D routes should be used by an implementation to
optimize the withdrawal of MAC advertisement routes. When an PE
receives a withdrawal of a particular Ethernet A-D route from an PE
it SHOULD consider all the MAC advertisement routes, that are learned
from the same <ESI, Ethernet Tag> as in the Ethernet A-D route, from
the advertising PE, as having been withdrawn. This optimizes the
network convergence times in the event of PE to CE failures.
19. Frame Ordering
In a MAC address, bit-1 of the most significant byte is used for
unicast/multicast indication and bit-2 is used for globally unique
versus locally administered MAC address. If the value of the 2nd
nibble (bits 4 thorough 8) of the most significant byte of the
destination MAC address (which follows the last MPLS label) happens
to be 0x4 or 0x6, then the Ethernet frame can be misinterpreted as an
IPv4 or IPv6 packet by intermediate P nodes performing ECMP resulting
in load balancing packets belonging to the same flow on different
ECMP paths, thus subjecting them to different delays. Therefore,
packets belonging to the same flow can arrive at the destination out
of order. This out of order delivery can happen during steady state
in absence of any failures resulting in significant impact to the
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network operation.
In order to avoid any such mis-ordering, the usage of control word
SHALL adhere to the following rules:
- A PE MUST use the control world when sending EVPN encapsulated
packets over a MP2P or a P2P LSP
- A PE MUST NOT use the control world when sending EVPN encapsulated
packets over a P2MP LSP
The control word is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Reserved | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram the first 4 bits MUST be set to 0. The rest of
the first 16 bits are reserved for future use. They MUST be set to 0
when transmitting, and MUST be ignored upon receipt. The next 16 bits
provide a sequence number that MUST also be set to zero by default.
20. Acknowledgements
Special thanks to Yakov Rekhter for reviewing this draft several
times and providing valuable comments and for his very engaging
discussions on several topics of this draft that helped shape this
document. We would also like to thank Pedro Marques, Kaushik Ghosh,
Nischal Sheth, Robert Raszuk, Amit Shukla and Nadeem Mohammed for
discussions that helped shape this document. We would also like to
thank Han Nguyen for his comments and support of this work. We would
also like to thank Steve Kensil and Reshad Rahman for their reviews.
Last but not least, many thanks to Jakob Heitz for his help to
improve several sections of this draft.
21. Security Considerations
22. IANA Considerations
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23. References
23.1 Normative References
[RFC4364] "BGP/MPLS IP VPNs", Rosen, Rekhter, et. al., February 2006
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC4271] Y. Rekhter et. al., "A Border Gateway Protocol 4 (BGP-4)",
RFC 4271, January 2006
[RFC4760] T. Bates et. al., "Multiprotocol Extensions for BGP-4", RFC
4760, January 2007
23.2 Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[EVPN-REQ] A. Sajassi, R. Aggarwal et. al., "Requirements for
Ethernet VPN", draft-ietf-l2vpn-evpn-req-04.txt, July
2013.
[VPLS-MCAST] "Multicast in VPLS". R. Aggarwal et.al., draft-ietf-
l2vpn-vpls-mcast-14.txt, July 2013.
[RT-CONSTRAIN] P. Marques et. al., "Constrained Route Distribution
for Border Gateway Protocol/MultiProtocol Label Switching
(BGP/MPLS) Internet Protocol (IP) Virtual Private Networks
(VPNs)", RFC 4684, November 2006
24. Author's Address
Ali Sajassi
Cisco
Email: sajassi@cisco.com
Rahul Aggarwal
Email: raggarwa_1@yahoo.com
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Wim Henderickx
Alcatel-Lucent
e-mail: wim.henderickx@alcatel-lucent.com
Aldrin Isaac
Bloomberg
Email: aisaac71@bloomberg.net
James Uttaro
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748
USA
Email: uttaro@att.com
Nabil Bitar
Verizon Communications
Email : nabil.n.bitar@verizon.com
Ravi Shekhar
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089 US
Email: rshekhar@juniper.net
Florin Balus
Alcatel-Lucent
e-mail: Florin.Balus@alcatel-lucent.com
Keyur Patel
Cisco
170 West Tasman Drive
San Jose, CA 95134, US
Email: keyupate@cisco.com
Sami Boutros
Cisco
170 West Tasman Drive
San Jose, CA 95134, US
Email: sboutros@cisco.com
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Samer Salam
Cisco
Email: ssalam@cisco.com
John Drake
Juniper Networks
Email: jdrake@juniper.net
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