RFC 8365: A Network Virtualization Overlay Solution Using Ethernet VPN (EVPN)

Text version

Internet Engineering Task Force (IETF)
Request for Comments: 8365
Category: Standards Track
ISSN: 2070-1721

A. Sajassi, Ed.
Cisco
J. Drake, Ed.
Juniper
N. Bitar
Nokia
R. Shekhar
Juniper
J. Uttaro
AT&T
W. Henderickx
Nokia
March 2018

A Network Virtualization Overlay Solution Using Ethernet VPN (EVPN)

Abstract

This document specifies how Ethernet VPN (EVPN) can be used as a Network Virtualization Overlay (NVO) solution and explores the various tunnel encapsulation options over IP and their impact on the EVPN control plane and procedures. In particular, the following encapsulation options are analyzed: Virtual Extensible LAN (VXLAN), Network Virtualization using Generic Routing Encapsulation (NVGRE), and MPLS over GRE. This specification is also applicable to Generic Network Virtualization Encapsulation (GENEVE); however, some incremental work is required, which will be covered in a separate document. This document also specifies new multihoming procedures for split-horizon filtering and mass withdrawal. It also specifies EVPN route constructions for VXLAN/NVGRE encapsulations and Autonomous System Border Router (ASBR) procedures for multihoming of Network Virtualization Edge (NVE) devices.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8365.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

   1. Introduction ....................................................4
   2. Requirements Notation and Conventions ...........................5
   3. Terminology .....................................................5
   4. EVPN Features ...................................................7
   5. Encapsulation Options for EVPN Overlays .........................8
      5.1. VXLAN/NVGRE Encapsulation ..................................8
           5.1.1. Virtual Identifiers Scope ...........................9
           5.1.2. Virtual Identifiers to EVI Mapping .................11
           5.1.3. Constructing EVPN BGP Routes .......................13
      5.2. MPLS over GRE .............................................15
   6. EVPN with Multiple Data-Plane Encapsulations ...................15
   7. Single-Homing NVEs - NVE Residing in Hypervisor ................16
      7.1. Impact on EVPN BGP Routes & Attributes for VXLAN/NVGRE ....16
      7.2. Impact on EVPN Procedures for VXLAN/NVGRE Encapsulations ..17
   8. Multihoming NVEs - NVE Residing in ToR Switch ..................18
      8.1. EVPN Multihoming Features .................................18
           8.1.1. Multihomed ES Auto-Discovery .......................18
           8.1.2. Fast Convergence and Mass Withdrawal ...............18
           8.1.3. Split-Horizon ......................................19
           8.1.4. Aliasing and Backup Path ...........................19
           8.1.5. DF Election ........................................20
      8.2. Impact on EVPN BGP Routes and Attributes ..................20
      8.3. Impact on EVPN Procedures .................................20
           8.3.1. Split Horizon ......................................21
           8.3.2. Aliasing and Backup Path ...........................22
           8.3.3. Unknown Unicast Traffic Designation ................22
   9. Support for Multicast ..........................................23
   10. Data-Center Interconnections (DCIs) ...........................24
      10.1. DCI Using GWs ............................................24
      10.2. DCI Using ASBRs ..........................................24
           10.2.1. ASBR Functionality with Single-Homing NVEs ........25
           10.2.2. ASBR Functionality with Multihoming NVEs ..........26
   11. Security Considerations .......................................28
   12. IANA Considerations ...........................................29
   13. References ....................................................29
      13.1. Normative References .....................................29
      13.2. Informative References ...................................30
   Acknowledgements ..................................................32
   Contributors ......................................................32
   Authors' Addresses ................................................33

1. Introduction

This document specifies how Ethernet VPN (EVPN) [RFC7432] can be used as a Network Virtualization Overlay (NVO) solution and explores the various tunnel encapsulation options over IP and their impact on the EVPN control plane and procedures. In particular, the following encapsulation options are analyzed: Virtual Extensible LAN (VXLAN) [RFC7348], Network Virtualization using Generic Routing Encapsulation (NVGRE) [RFC7637], and MPLS over Generic Routing Encapsulation (GRE) [RFC4023]. This specification is also applicable to Generic Network Virtualization Encapsulation (GENEVE) [GENEVE]; however, some incremental work is required, which will be covered in a separate document [EVPN-GENEVE]. This document also specifies new multihoming procedures for split-horizon filtering and mass withdrawal. It also specifies EVPN route constructions for VXLAN/NVGRE encapsulations and Autonomous System Border Router (ASBR) procedures for multihoming of Network Virtualization Edge (NVE) devices.

In the context of this document, an NVO is a solution to address the requirements of a multi-tenant data center, especially one with virtualized hosts, e.g., Virtual Machines (VMs) or virtual workloads. The key requirements of such a solution, as described in [RFC7364], are the following:

Isolation of network traffic per tenant

Support for a large number of tenants (tens or hundreds of thousands)

Extension of Layer 2 (L2) connectivity among different VMs belonging to a given tenant segment (subnet) across different Points of Delivery (PoDs) within a data center or between different data centers

Allowing a given VM to move between different physical points of attachment within a given L2 segment

The underlay network for NVO solutions is assumed to provide IP connectivity between NVO endpoints.

This document describes how EVPN can be used as an NVO solution and explores applicability of EVPN functions and procedures. In particular, it describes the various tunnel encapsulation options for EVPN over IP and their impact on the EVPN control plane as well as procedures for two main scenarios:

(a) single-homing NVEs - when an NVE resides in the hypervisor, and

(b) multihoming NVEs - when an NVE resides in a Top-of-Rack (ToR): device.

The possible encapsulation options for EVPN overlays that are analyzed in this document are:

VXLAN and NVGRE

MPLS over GRE

Before getting into the description of the different encapsulation options for EVPN over IP, it is important to highlight the EVPN solution's main features, how those features are currently supported, and any impact that the encapsulation has on those features.

2. Requirements Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Terminology

Most of the terminology used in this documents comes from [RFC7432] and [RFC7365].

   VXLAN:  Virtual Extensible LAN
   
   GRE:  Generic Routing Encapsulation
   
   NVGRE:  Network Virtualization using Generic Routing Encapsulation
   
   GENEVE:  Generic Network Virtualization Encapsulation
   
   PoD:  Point of Delivery
   
   NV:  Network Virtualization
   NVO:  Network Virtualization Overlay
   
   NVE:  Network Virtualization Edge
   
   VNI:  VXLAN Network Identifier
   
   VSID:  Virtual Subnet Identifier (for NVGRE)
   
   I-SID:  Service Instance Identifier
   
   EVPN:  Ethernet VPN
   
   EVI:  EVPN Instance.  An EVPN instance spanning the Provider Edge
      (PE) devices participating in that EVPN
   
   MAC-VRF:  A Virtual Routing and Forwarding table for Media Access
      Control (MAC) addresses on a PE
   
   IP-VRF:  A Virtual Routing and Forwarding table for Internet Protocol
      (IP) addresses on a PE
   
   ES:  Ethernet Segment.  When a customer site (device or network) is
      connected to one or more PEs via a set of Ethernet links, then
      that set of links is referred to as an 'Ethernet segment'.
   
   Ethernet Segment Identifier (ESI):  A unique non-zero identifier that
      identifies an Ethernet segment is called an '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.
   
   PE:  Provider Edge
   
   Single-Active Redundancy Mode:  When only a single PE, among all the
      PEs attached to an ES, is allowed to forward traffic to/from that
      ES for a given VLAN, then the Ethernet segment is defined to be
      operating in Single-Active redundancy mode.
   
   All-Active Redundancy Mode:  When all PEs attached to an Ethernet
      segment are allowed to forward known unicast traffic to/from that
      ES for a given VLAN, then the ES is defined to be operating in
      All-Active redundancy mode.
   
   PIM-SM:  Protocol Independent Multicast - Sparse-Mode
   PIM-SSM:  Protocol Independent Multicast - Source-Specific Multicast
   
   BIDIR-PIM:  Bidirectional PIM

4. EVPN Features

EVPN [RFC7432] was originally designed to support the requirements detailed in [RFC7209] and therefore has the following attributes which directly address control-plane scaling and ease of deployment issues.

   1.   Control-plane information is distributed with BGP and broadcast
        and multicast traffic is sent using a shared multicast tree or
        with ingress replication.
   
   2.   Control-plane learning is used for MAC (and IP) addresses
        instead of data-plane learning.  The latter requires the
        flooding of unknown unicast and Address Resolution Protocol
        (ARP) frames; whereas, the former does not require any flooding.
   
   3.   Route Reflector (RR) is used to reduce a full mesh of BGP
        sessions among PE devices to a single BGP session between a PE
        and the RR.  Furthermore, RR hierarchy can be leveraged to scale
        the number of BGP routes on the RR.
   
   4.   Auto-discovery via BGP is used to discover PE devices
        participating in a given VPN, PE devices participating in a
        given redundancy group, tunnel encapsulation types, multicast
        tunnel types, multicast members, etc.
   
   5.   All-Active multihoming is used.  This allows a given Customer
        Edge (CE) device to have multiple links to multiple PEs, and
        traffic to/from that CE fully utilizes all of these links.
   
   6.   When a link between a CE and a PE fails, the PEs for that EVI
        are notified of the failure via the withdrawal of a single EVPN
        route.  This allows those PEs to remove the withdrawing PE as a
        next hop for every MAC address associated with the failed link.
        This is termed "mass withdrawal".
   
   7.   BGP route filtering and constrained route distribution are
        leveraged to ensure that the control-plane traffic for a given
        EVI is only distributed to the PEs in that EVI.
   
   8.   When an IEEE 802.1Q [IEEE.802.1Q] interface is used between a CE
        and a PE, each of the VLAN IDs (VIDs) on that interface can be
        mapped onto a bridge table (for up to 4094 such bridge tables).
        All these bridge tables may be mapped onto a single MAC-VRF (in
        case of VLAN-aware bundle service).
   
   9.   VM Mobility mechanisms ensure that all PEs in a given EVI know
        the ES with which a given VM, as identified by its MAC and IP
        addresses, is currently associated.

RTs are used to allow the operator (or customer) to define a spectrum of logical network topologies including mesh, hub and spoke, and extranets (e.g., a VPN whose sites are owned by different enterprises), without the need for proprietary software or the aid of other virtual or physical devices.

Because the design goal for NVO is millions of instances per common physical infrastructure, the scaling properties of the control plane for NVO are extremely important. EVPN and the extensions described herein, are designed with this level of scalability in mind.

5. Encapsulation Options for EVPN Overlays

5.1. VXLAN/NVGRE Encapsulation

Both VXLAN and NVGRE are examples of technologies that provide a data plane encapsulation which is used to transport a packet over the common physical IP infrastructure between Network Virtualization Edges (NVEs) - e.g., VXLAN Tunnel End Points (VTEPs) in VXLAN network. Both of these technologies include the identifier of the specific NVO instance, VNI in VXLAN and VSID in NVGRE, in each packet. In the remainder of this document we use VNI as the representation for NVO instance with the understanding that VSID can equally be used if the encapsulation is NVGRE unless it is stated otherwise.

Note that a PE is equivalent to an NVE/VTEP.

VXLAN encapsulation is based on UDP, with an 8-byte header following the UDP header. VXLAN provides a 24-bit VNI, which typically provides a one-to-one mapping to the tenant VID, as described in [RFC7348]. In this scenario, the ingress VTEP does not include an inner VLAN tag on the encapsulated frame, and the egress VTEP discards the frames with an inner VLAN tag. This mode of operation in [RFC7348] maps to VLAN-Based Service in [RFC7432], where a tenant VID gets mapped to an EVI.

VXLAN also provides an option of including an inner VLAN tag in the encapsulated frame, if explicitly configured at the VTEP. This mode of operation can map to VLAN Bundle Service in [RFC7432] because all the tenant's tagged frames map to a single bridge table / MAC-VRF, and the inner VLAN tag is not used for lookup by the disposition PE when performing VXLAN decapsulation as described in Section 6 of [RFC7348].

[RFC7637] encapsulation is based on GRE encapsulation, and it mandates the inclusion of the optional GRE Key field, which carries the VSID. There is a one-to-one mapping between the VSID and the tenant VID, as described in [RFC7637]. The inclusion of an inner VLAN tag is prohibited. This mode of operation in [RFC7637] maps to VLAN Based Service in [RFC7432].

As described in the next section, there is no change to the encoding of EVPN routes to support VXLAN or NVGRE encapsulation, except for the use of the BGP Encapsulation Extended Community to indicate the encapsulation type (e.g., VXLAN or NVGRE). However, there is potential impact to the EVPN procedures depending on where the NVE is located (i.e., in hypervisor or ToR) and whether multihoming capabilities are required.

5.1.1. Virtual Identifiers Scope

Although VNIs are defined as 24-bit globally unique values, there are scenarios in which it is desirable to use a locally significant value for the VNI, especially in the context of a data-center interconnect.

5.1.1.1. Data-Center Interconnect with Gateway

In the case where NVEs in different data centers need to be interconnected, and the NVEs need to use VNIs as globally unique identifiers within a data center, then a Gateway (GW) needs to be employed at the edge of the data-center network (DCN). This is because the Gateway will provide the functionality of translating the VNI when crossing network boundaries, which may align with operator span-of-control boundaries. As an example, consider the network of Figure 1. Assume there are three network operators: one for each of the DC1, DC2, and WAN networks. The Gateways at the edge of the data centers are responsible for translating the VNIs between the values used in each of the DCNs and the values used in the WAN.

                             +--------------+
                             |              |
           +---------+       |     WAN      |       +---------+
   +----+  |        +---+  +----+        +----+  +---+        |  +----+
   |NVE1|--|        |   |  |WAN |        |WAN |  |   |        |--|NVE3|
   +----+  |IP      |GW |--|Edge|        |Edge|--|GW | IP     |  +----+
   +----+  |Fabric  +---+  +----+        +----+  +---+ Fabric |  +----+
   |NVE2|--|         |       |              |       |         |--|NVE4|
   +----+  +---------+       +--------------+       +---------+  +----+
   
   |<------ DC 1 ------>                          <------ DC2  ------>|

Figure 1: Data-Center Interconnect with Gateway

5.1.1.2. Data-Center Interconnect without Gateway

In the case where NVEs in different data centers need to be interconnected, and the NVEs need to use locally assigned VNIs (e.g., similar to MPLS labels), there may be no need to employ Gateways at the edge of the DCN. More specifically, the VNI value that is used by the transmitting NVE is allocated by the NVE that is receiving the traffic (in other words, this is similar to a "downstream-assigned" MPLS label). This allows the VNI space to be decoupled between different DCNs without the need for a dedicated Gateway at the edge of the data centers. This topic is covered in Section 10.2.

                              +--------------+
                              |              |
              +---------+     |     WAN      |    +---------+
      +----+  |         |   +----+        +----+  |         |  +----+
      |NVE1|--|         |   |ASBR|        |ASBR|  |         |--|NVE3|
      +----+  |IP Fabric|---|    |        |    |--|IP Fabric|  +----+
      +----+  |         |   +----+        +----+  |         |  +----+
      |NVE2|--|         |     |              |    |         |--|NVE4|
      +----+  +---------+     +--------------+    +---------+  +----+
      
      |<------ DC 1 ----->                        <---- DC2  ------>|

Figure 2: Data-Center Interconnect with ASBR

5.1.2. Virtual Identifiers to EVI Mapping

Just like in [RFC7432], where two options existed for mapping broadcast domains (represented by VLAN IDs) to an EVI, when the EVPN control plane is used in conjunction with VXLAN (or NVGRE encapsulation), there are also two options for mapping broadcast domains represented by VXLAN VNIs (or NVGRE VSIDs) to an EVI:

Option 1: A Single Broadcast Domain per EVI

In this option, a single Ethernet broadcast domain (e.g., subnet) represented by a VNI is mapped to a unique EVI. This corresponds to the VLAN-Based Service in [RFC7432], where a tenant-facing interface, logical interface (e.g., represented by a VID), or physical interface gets mapped to an EVI. As such, a BGP Route Distinguisher (RD) and Route Target (RT) are needed per VNI on every NVE. The advantage of this model is that it allows the BGP RT constraint mechanisms to be used in order to limit the propagation and import of routes to only the NVEs that are interested in a given VNI. The disadvantage of this model may be the provisioning overhead if the RD and RT are not derived automatically from the VNI.

In this option, the MAC-VRF table is identified by the RT in the control plane and by the VNI in the data plane. In this option, the specific MAC-VRF table corresponds to only a single bridge table.

Option 2: Multiple Broadcast Domains per EVI

In this option, multiple subnets, each represented by a unique VNI, are mapped to a single EVI. For example, if a tenant has multiple segments/subnets each represented by a VNI, then all the VNIs for that tenant are mapped to a single EVI; for example, the EVI in this case represents the tenant and not a subnet. This corresponds to the VLAN-aware bundle service in [RFC7432]. The advantage of this model is that it doesn't require the provisioning of an RD/RT per VNI. However, this is a moot point when compared to Option 1 where auto- derivation is used. The disadvantage of this model is that routes would be imported by NVEs that may not be interested in a given VNI.

In this option, the MAC-VRF table is identified by the RT in the control plane; a specific bridge table for that MAC-VRF is identified by the <RT, Ethernet Tag ID> in the control plane. In this option, the VNI in the data plane is sufficient to identify a specific bridge table.

5.1.2.1. Auto-Derivation of RT

In order to simplify configuration, when the option of a single VNI per EVI is used, the RT used for EVPN can be auto-derived. RD can be auto-generated as described in [RFC7432], and RT can be auto-derived as described next.

Since a Gateway PE as depicted in Figure 1 participates in both the DCN and WAN BGP sessions, it is important that, when RT values are auto-derived from VNIs, there be no conflict in RT spaces between DCNs and WANs, assuming that both are operating within the same Autonomous System (AS). Also, there can be scenarios where both VXLAN and NVGRE encapsulations may be needed within the same DCN, and their corresponding VNIs are administered independently, which means VNI spaces can overlap. In order to avoid conflict in RT spaces, the 6-byte RT values with 2-octet AS number for DCNs can be auto-derived as follow:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Global Administrator      |    Local Administrator        |
   +-----------------------------------------------+---------------+
   | Local Administrator (Cont.)   |
   +-------------------------------+
   
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Global Administrator      |A| TYPE| D-ID  | Service ID    |
   +-----------------------------------------------+---------------+
   |       Service ID (Cont.)      |
   +-------------------------------+

The 6-octet RT field consists of two sub-fields:

   -  Global Administrator sub-field: 2 octets.  This sub-field contains
      an AS number assigned by IANA <https://www.iana.org/assignments/
      as-numbers/>.
   
   -  Local Administrator sub-field: 4 octets

A: A single-bit field indicating if this RT is auto-derived

            0: auto-derived
            1: manually derived

Type: A 3-bit field that identifies the space in which the other 3 bytes are defined. The following spaces are defined:

            0 : VID (802.1Q VLAN ID)
            1 : VXLAN
            2 : NVGRE
            3 : I-SID
            4 : EVI
            5 : dual-VID (QinQ VLAN ID)
      
      *  D-ID: A 4-bit field that identifies domain-id.  The default
         value of domain-id is zero, indicating that only a single
         numbering space exist for a given technology.  However, if more
         than one number space exists for a given technology (e.g.,
         overlapping VXLAN spaces), then each of the number spaces need
         to be identified by its corresponding domain-id starting from
         1.
      
      *  Service ID: This 3-octet field is set to VNI, VSID, I-SID, or
         VID.

It should be noted that RT auto-derivation is applicable for 2-octet AS numbers. For 4-octet AS numbers, the RT needs to be manually configured because 3-octet VNI fields cannot be fit within the 2-octet local administrator field.

5.1.3. Constructing EVPN BGP Routes

In EVPN, an MPLS label, for instance, identifying the forwarding table is distributed by the egress PE via the EVPN control plane and is placed in the MPLS header of a given packet by the ingress PE. This label is used upon receipt of that packet by the egress PE for disposition of that packet. This is very similar to the use of the VNI by the egress NVE, with the difference being that an MPLS label has local significance while a VNI typically has global significance. Accordingly, and specifically to support the option of locally assigned VNIs, the MPLS Label1 field in the MAC/IP Advertisement route, the MPLS label field in the Ethernet A-D per EVI route, and the MPLS label field in the P-Multicast Service Interface (PMSI) Tunnel attribute of the Inclusive Multicast Ethernet Tag (IMET) route are used to carry the VNI. For the balance of this memo, the above MPLS label fields will be referred to as the VNI field. The VNI field is used for both local and global VNIs; for either case, the entire 24-bit field is used to encode the VNI value.

For the VLAN-Based Service (a single VNI per MAC-VRF), the Ethernet Tag field in the MAC/IP Advertisement, Ethernet A-D per EVI, and IMET route MUST be set to zero just as in the VLAN-Based Service in [RFC7432].

For the VLAN-Aware Bundle Service (multiple VNIs per MAC-VRF with each VNI associated with its own bridge table), the Ethernet Tag field in the MAC Advertisement, Ethernet A-D per EVI, and IMET route MUST identify a bridge table within a MAC-VRF; the set of Ethernet Tags for that EVI needs to be configured consistently on all PEs within that EVI. For locally assigned VNIs, the value advertised in the Ethernet Tag field MUST be set to a VID just as in the VLAN-aware bundle service in [RFC7432]. Such setting must be done consistently on all PE devices participating in that EVI within a given domain. For global VNIs, the value advertised in the Ethernet Tag field SHOULD be set to a VNI as long as it matches the existing semantics of the Ethernet Tag, i.e., it identifies a bridge table within a MAC-VRF and the set of VNIs are configured consistently on each PE in that EVI.

In order to indicate which type of data-plane encapsulation (i.e., VXLAN, NVGRE, MPLS, or MPLS in GRE) is to be used, the BGP Encapsulation Extended Community defined in [RFC5512] is included with all EVPN routes (i.e., MAC Advertisement, Ethernet A-D per EVI, Ethernet A-D per ESI, IMET, and Ethernet Segment) advertised by an egress PE. Five new values have been assigned by IANA to extend the list of encapsulation types defined in [RFC5512]; they are listed in Section 11.

The MPLS encapsulation tunnel type, listed in Section 11, is needed in order to distinguish between an advertising node that only supports non-MPLS encapsulations and one that supports MPLS and non-MPLS encapsulations. An advertising node that only supports MPLS encapsulation does not need to advertise any encapsulation tunnel types; i.e., if the BGP Encapsulation Extended Community is not present, then either MPLS encapsulation or a statically configured encapsulation is assumed.

The Next Hop field of the MP_REACH_NLRI attribute of the route MUST be set to the IPv4 or IPv6 address of the NVE. The remaining fields in each route are set as per [RFC7432].

Note that the procedure defined here -- to use the MPLS Label field to carry the VNI in the presence of a Tunnel Encapsulation Extended Community specifying the use of a VNI -- is aligned with the procedures described in Section 8.2.2.2 of [TUNNEL-ENCAP] ("When a Valid VNI has not been Signaled").

5.2. MPLS over GRE

The EVPN data plane is modeled as an EVPN MPLS client layer sitting over an MPLS PSN tunnel server layer. Some of the EVPN functions (split-horizon, Aliasing, and Backup Path) are tied to the MPLS client layer. If MPLS over GRE encapsulation is used, then the EVPN MPLS client layer can be carried over an IP PSN tunnel transparently. Therefore, there is no impact to the EVPN procedures and associated data-plane operation.

[RFC4023] defines the standard for using MPLS over GRE encapsulation, which can be used for this purpose. However, when MPLS over GRE is used in conjunction with EVPN, it is recommended that the GRE key field be present and be used to provide a 32-bit entropy value only if the P nodes can perform Equal-Cost Multipath (ECMP) hashing based on the GRE key; otherwise, the GRE header SHOULD NOT include the GRE key field. The Checksum and Sequence Number fields MUST NOT be included, and the corresponding C and S bits in the GRE header MUST be set to zero. A PE capable of supporting this encapsulation SHOULD advertise its EVPN routes along with the Tunnel Encapsulation Extended Community indicating MPLS over GRE encapsulation as described in the previous section.

6. EVPN with Multiple Data-Plane Encapsulations

The use of the BGP Encapsulation Extended Community per [RFC5512] allows each NVE in a given EVI to know each of the encapsulations supported by each of the other NVEs in that EVI. That is, each of the NVEs in a given EVI may support multiple data-plane encapsulations. An ingress NVE can send a frame to an egress NVE only if the set of encapsulations advertised by the egress NVE forms a non-empty intersection with the set of encapsulations supported by the ingress NVE; it is at the discretion of the ingress NVE which encapsulation to choose from this intersection. (As noted in Section 5.1.3, if the BGP Encapsulation extended community is not present, then the default MPLS encapsulation or a locally configured encapsulation is assumed.)

When a PE advertises multiple supported encapsulations, it MUST advertise encapsulations that use the same EVPN procedures including procedures associated with split-horizon filtering described in Section 8.3.1. For example, VXLAN and NVGRE (or MPLS and MPLS over GRE) encapsulations use the same EVPN procedures; thus, a PE can advertise both of them and can support either of them or both of them simultaneously. However, a PE MUST NOT advertise VXLAN and MPLS encapsulations together because (a) the MPLS field of EVPN routes is set to either an MPLS label or a VNI, but not both and (b) some EVPN procedures (such as split-horizon filtering) are different for VXLAN/ NVGRE and MPLS encapsulations.

An ingress node that uses shared multicast trees for sending broadcast or multicast frames MAY maintain distinct trees for each different encapsulation type.

It is the responsibility of the operator of a given EVI to ensure that all of the NVEs in that EVI support at least one common encapsulation. If this condition is violated, it could result in service disruption or failure. The use of the BGP Encapsulation Extended Community provides a method to detect when this condition is violated, but the actions to be taken are at the discretion of the operator and are outside the scope of this document.

7. Single-Homing NVEs - NVE Residing in Hypervisor

When an NVE and its hosts/VMs are co-located in the same physical device, e.g., when they reside in a server, the links between them are virtual and they typically share fate. That is, the subject hosts/VMs are typically not multihomed or, if they are multihomed, the multihoming is a purely local matter to the server hosting the VM and the NVEs, and it need not be "visible" to any other NVEs residing on other servers. Thus, it does not require any specific protocol mechanisms. The most common case of this is when the NVE resides on the hypervisor.

In the subsections that follow, we will discuss the impact on EVPN procedures for the case when the NVE resides on the hypervisor and the VXLAN (or NVGRE) encapsulation is used.

7.1. Impact on EVPN BGP Routes & Attributes for VXLAN/NVGRE

Encapsulations

In scenarios where different groups of data centers are under different administrative domains, and these data centers are connected via one or more backbone core providers as described in [RFC7365], the RD must be a unique value per EVI or per NVE as described in [RFC7432]. In other words, whenever there is more than one administrative domain for global VNI, a unique RD must be used; or, whenever the VNI value has local significance, a unique RD must be used. Therefore, it is recommended to use a unique RD as described in [RFC7432] at all times.

When the NVEs reside on the hypervisor, the EVPN BGP routes and attributes associated with multihoming are no longer required. This reduces the required routes and attributes to the following subset of four out of the total of eight listed in Section 7 of [RFC7432]:

MAC/IP Advertisement Route

Inclusive Multicast Ethernet Tag Route

MAC Mobility Extended Community

Default Gateway Extended Community

However, as noted in Section 8.6 of [RFC7432], in order to enable a single-homing ingress NVE to take advantage of fast convergence, Aliasing, and Backup Path when interacting with multihomed egress NVEs attached to a given ES, the single-homing ingress NVE should be able to receive and process routes that are Ethernet A-D per ES and Ethernet A-D per EVI.

7.2. Impact on EVPN Procedures for VXLAN/NVGRE Encapsulations

When the NVEs reside on the hypervisors, the EVPN procedures associated with multihoming are no longer required. This limits the procedures on the NVE to the following subset.

Local learning of MAC addresses received from the VMs per Section 10.1 of [RFC7432].

Advertising locally learned MAC addresses in BGP using the MAC/IP Advertisement routes.

Performing remote learning using BGP per Section 9.2 of [RFC7432].

Discovering other NVEs and constructing the multicast tunnels using the IMET routes.

Handling MAC address mobility events per the procedures of Section 15 in [RFC7432].

However, as noted in Section 8.6 of [RFC7432], in order to enable a single-homing ingress NVE to take advantage of fast convergence, Aliasing, and Backup Path when interacting with multihomed egress NVEs attached to a given ES, a single-homing ingress NVE should implement the ingress node processing of routes that are Ethernet A-D per ES and Ethernet A-D per EVI as defined in Sections 8.2 ("Fast Convergence") and 8.4 ("Aliasing and Backup Path") of [RFC7432].

8. Multihoming NVEs - NVE Residing in ToR Switch

In this section, we discuss the scenario where the NVEs reside in the ToR switches AND the servers (where VMs are residing) are multihomed to these ToR switches. The multihoming NVE operates in All-Active or Single-Active redundancy mode. If the servers are single-homed to the ToR switches, then the scenario becomes similar to that where the NVE resides on the hypervisor, as discussed in Section 7, as far as the required EVPN functionality is concerned.

[RFC7432] defines a set of BGP routes, attributes, and procedures to support multihoming. We first describe these functions and procedures, then discuss which of these are impacted by the VXLAN (or NVGRE) encapsulation and what modifications are required. As will be seen later in this section, the only EVPN procedure that is impacted by non-MPLS overlay encapsulation (e.g., VXLAN or NVGRE) where it provides space for one ID rather than a stack of labels, is that of split-horizon filtering for multihomed ESs described in Section 8.3.1.

8.1. EVPN Multihoming Features

In this section, we will recap the multihoming features of EVPN to highlight the encapsulation dependencies. The section only describes the features and functions at a high level. For more details, the reader is to refer to [RFC7432].

8.1.1. Multihomed ES Auto-Discovery

EVPN NVEs (or PEs) connected to the same ES (e.g., the same server via Link Aggregation Group (LAG)) can automatically discover each other with minimal to no configuration through the exchange of BGP routes.

8.1.2. Fast Convergence and Mass Withdrawal

EVPN defines a mechanism to efficiently and quickly signal, to remote NVEs, the need to update their forwarding tables upon the occurrence of a failure in connectivity to an ES (e.g., a link or a port failure). This is done by having each NVE advertise an Ethernet A-D route per ES for each locally attached segment. Upon a failure in connectivity to the attached segment, the NVE withdraws the corresponding Ethernet A-D route. This triggers all NVEs that receive the withdrawal to update their next-hop adjacencies for all MAC addresses associated with the ES in question. If no other NVE had advertised an Ethernet A-D route for the same segment, then the

NVE that received the withdrawal simply invalidates the MAC entries for that segment. Otherwise, the NVE updates the next-hop adjacency list accordingly.

8.1.3. Split-Horizon

If a server is multihomed to two or more NVEs (represented by an ES ES1) and operating in an All-Active redundancy mode, sends a BUM (i.e., Broadcast, Unknown unicast, or Multicast) packet to one of these NVEs, then it is important to ensure the packet is not looped back to the server via another NVE connected to this server. The filtering mechanism on the NVE to prevent such loop and packet duplication is called "split-horizon filtering".

8.1.4. Aliasing and Backup Path

In the case where a station is multihomed to multiple NVEs, it is possible that only a single NVE learns a set of the MAC addresses associated with traffic transmitted by the station. This leads to a situation where remote NVEs receive MAC Advertisement routes, for these addresses, from a single NVE even though multiple NVEs are connected to the multihomed station. As a result, the remote NVEs are not able to effectively load-balance traffic among the NVEs connected to the multihomed ES. For example, this could be the case when the NVEs perform data-path learning on the access and the load- balancing function on the station hashes traffic from a given source MAC address to a single NVE. Another scenario where this occurs is when the NVEs 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". This refers to the ability of an NVE to signal that it has reachability to a given locally attached ES, even when it has learned no MAC addresses from that segment. The Ethernet A-D route per EVI is used to that end. Remote NVEs that receive MAC Advertisement routes with non-zero ESIs should consider the MAC address as reachable via all NVEs that advertise reachability to the relevant Segment using Ethernet A-D routes with the same ESI and with the Single-Active flag reset.

Backup Path is a closely related function, albeit one that applies to the case where the redundancy mode is Single-Active. In this case, the NVE signals that it has reachability to a given locally attached ES using the Ethernet A-D route as well. Remote NVEs that receive the MAC Advertisement routes, with non-zero ESI, should consider the MAC address as reachable via the advertising NVE. Furthermore, the remote NVEs should install a Backup Path, for said MAC, to the NVE that had advertised reachability to the relevant segment using an Ethernet A-D route with the same ESI and with the Single-Active flag set.

8.1.5. DF Election

If a host is multihomed to two or more NVEs on an ES operating in All-Active redundancy mode, then, for a given EVI, only one of these NVEs, termed the "Designated Forwarder" (DF) is responsible for sending it broadcast, multicast, and, if configured for that EVI, unknown unicast frames.

This is required in order to prevent duplicate delivery of multi- destination frames to a multihomed host or VM, in case of All-Active redundancy.

In NVEs where frames tagged as IEEE 802.1Q [IEEE.802.1Q] are received from hosts, the DF election should be performed based on host VIDs per Section 8.5 of [RFC7432]. Furthermore, multihoming PEs of a given ES MAY perform DF election using configured IDs such as VNI, EVI, normalized VIDs, and etc., as along the IDs are configured consistently across the multihoming PEs.

In GWs where VXLAN-encapsulated frames are received, the DF election is performed on VNIs. Again, it is assumed that, for a given Ethernet segment, VNIs are unique and consistent (e.g., no duplicate VNIs exist).

8.2. Impact on EVPN BGP Routes and Attributes

Since multihoming is supported in this scenario, the entire set of BGP routes and attributes defined in [RFC7432] is used. The setting of the Ethernet Tag field in the MAC Advertisement, Ethernet A-D per EVI, and IMET) routes follows that of Section 5.1.3. Furthermore, the setting of the VNI field in the MAC Advertisement and Ethernet A-D per EVI routes follows that of Section 5.1.3.

8.3. Impact on EVPN Procedures

Two cases need to be examined here, depending on whether the NVEs are operating in Single-Active or in All-Active redundancy mode.

First, let's consider the case of Single-Active redundancy mode, where the hosts are multihomed to a set of NVEs; however, only a single NVE is active at a given point of time for a given VNI. In this case, the Aliasing is not required, and the split-horizon filtering may not be required, but other functions such as multihomed ES auto-discovery, fast convergence and mass withdrawal, Backup Path, and DF election are required.

Second, let's consider the case of All-Active redundancy mode. In this case, out of all the EVPN multihoming features listed in Section 8.1, the use of the VXLAN or NVGRE encapsulation impacts the split-horizon and Aliasing features, since those two rely on the MPLS client layer. Given that this MPLS client layer is absent with these types of encapsulations, alternative procedures and mechanisms are needed to provide the required functions. Those are discussed in detail next.

8.3.1. Split Horizon

In EVPN, an MPLS label is used for split-horizon filtering to support All-Active multihoming where an ingress NVE adds a label corresponding to the site of origin (aka an ESI label) when encapsulating the packet. The egress NVE checks the ESI label when attempting to forward a multi-destination frame out an interface, and if the label corresponds to the same site identifier (ESI) associated with that interface, the packet gets dropped. This prevents the occurrence of forwarding loops.

Since VXLAN and NVGRE encapsulations do not include the ESI label, other means of performing the split-horizon filtering function must be devised for these encapsulations. The following approach is recommended for split-horizon filtering when VXLAN (or NVGRE) encapsulation is used.

Every NVE tracks the IP address(es) associated with the other NVE(s) with which it has shared multihomed ESs. When the NVE receives a multi-destination frame from the overlay network, it examines the source IP address in the tunnel header (which corresponds to the ingress NVE) and filters out the frame on all local interfaces connected to ESs that are shared with the ingress NVE. With this approach, it is required that the ingress NVE perform replication locally to all directly attached Ethernet segments (regardless of the DF election state) for all flooded traffic ingress from the access interfaces (i.e., from the hosts). This approach is referred to as "Local Bias", and has the advantage that only a single IP address need be used per NVE for split-horizon filtering, as opposed to requiring an IP address per Ethernet segment per NVE.

In order to allow proper operation of split-horizon filtering among the same group of multihoming PE devices, a mix of PE devices with MPLS over GRE encapsulations running the procedures from [RFC7432] for split-horizon filtering on the one hand and VXLAN/NVGRE encapsulation running local-bias procedures on the other on a given Ethernet segment MUST NOT be configured.

8.3.2. Aliasing and Backup Path

The Aliasing and the Backup Path procedures for VXLAN/NVGRE encapsulation are very similar to the ones for MPLS. In the case of MPLS, Ethernet A-D route per EVI is used for Aliasing when the corresponding ES operates in All-Active multihoming, and the same route is used for Backup Path when the corresponding ES operates in Single-Active multihoming. In the case of VXLAN/NVGRE, the same route is used for the Aliasing and the Backup Path with the difference that the Ethernet Tag and VNI fields in Ethernet A-D per EVI route are set as described in Section 5.1.3.

8.3.3. Unknown Unicast Traffic Designation

In EVPN, when an ingress PE uses ingress replication to flood unknown unicast traffic to egress PEs, the ingress PE uses a different EVPN MPLS label (from the one used for known unicast traffic) to identify such BUM traffic. The egress PEs use this label to identify such BUM traffic and, thus, apply DF filtering for All-Active multihomed sites. In absence of an unknown unicast traffic designation and in the presence of enabling unknown unicast flooding, there can be transient duplicate traffic to All-Active multihomed sites under the following condition: the host MAC address is learned by the egress PE(s) and advertised to the ingress PE; however, the MAC Advertisement has not been received or processed by the ingress PE, resulting in the host MAC address being unknown on the ingress PE but known on the egress PE(s). Therefore, when a packet destined to that host MAC address arrives on the ingress PE, it floods it via ingress replication to all the egress PE(s), and since they are known to the egress PE(s), multiple copies are sent to the All-Active multihomed site. It should be noted that such transient packet duplication only happens when a) the destination host is multihomed via All-Active redundancy mode, b) flooding of unknown unicast is enabled in the network, c) ingress replication is used, and d) traffic for the destination host is arrived on the ingress PE before it learns the host MAC address via BGP EVPN advertisement. If it is desired to avoid occurrence of such transient packet duplication (however low probability that may be), then VXLAN-GPE encapsulation needs to be used between these PEs and the ingress PE needs to set the BUM Traffic Bit (B bit) [VXLAN-GPE] to indicate that this is an ingress- replicated BUM traffic.

9. Support for Multicast

The EVPN IMET route is used to discover the multicast tunnels among the endpoints associated with a given EVI (e.g., given VNI) for VLAN- Based Service and a given <EVI, VLAN> for VLAN-Aware Bundle Service. All fields of this route are set as described in Section 5.1.3. The originating router's IP address field is set to the NVE's IP address. This route is tagged with the PMSI Tunnel attribute, which is used to encode the type of multicast tunnel to be used as well as the multicast tunnel identifier. The tunnel encapsulation is encoded by adding the BGP Encapsulation Extended Community as per Section 5.1.1. For example, the PMSI Tunnel attribute may indicate the multicast tunnel is of type Protocol Independent Multicast - Sparse-Mode (PIM- SM); whereas, the BGP Encapsulation Extended Community may indicate the encapsulation for that tunnel is of type VXLAN. The following tunnel types as defined in [RFC6514] can be used in the PMSI Tunnel attribute for VXLAN/NVGRE:

         + 3 - PIM-SSM Tree
         + 4 - PIM-SM Tree
         + 5 - BIDIR-PIM Tree
         + 6 - Ingress Replication

In case of VXLAN and NVGRE encapsulations with locally assigned VNIs, just as in [RFC7432], each PE MUST advertise an IMET route to other PEs in an EVPN instance for the multicast tunnel type that it uses (i.e., ingress replication, PIM-SM, PIM-SSM, or BIDIR-PIM tunnel). However, for globally assigned VNIs, each PE MUST advertise an IMET route to other PEs in an EVPN instance for ingress replication or a PIM-SSM tunnel, and they MAY advertise an IMET route for a PIM-SM or BIDIR-PIM tunnel. In case of a PIM-SM or BIDIR-PIM tunnel, no information in the IMET route is needed by the PE to set up these tunnels.

In the scenario where the multicast tunnel is a tree, both the Inclusive as well as the Aggregate Inclusive variants may be used. In the former case, a multicast tree is dedicated to a VNI. Whereas, in the latter, a multicast tree is shared among multiple VNIs. For VNI-Based Service, the Aggregate Inclusive mode is accomplished by having the NVEs advertise multiple IMET routes with different RTs (one per VNI) but with the same tunnel identifier encoded in the PMSI Tunnel attribute. For VNI-Aware Bundle Service, the Aggregate Inclusive mode is accomplished by having the NVEs advertise multiple IMET routes with different VNIs encoded in the Ethernet Tag field, but with the same tunnel identifier encoded in the PMSI Tunnel attribute.

10. Data-Center Interconnections (DCIs)

For DCIs, the following two main scenarios are considered when connecting data centers running evpn-overlay (as described here) over an MPLS/IP core network:

Scenario 1: DCI using GWs

Scenario 2: DCI using ASBRs

The following two subsections describe the operations for each of these scenarios.

10.1. DCI Using GWs

This is the typical scenario for interconnecting data centers over WAN. In this scenario, EVPN routes are terminated and processed in each GW and MAC/IP route are always re-advertised from DC to WAN but from WAN to DC, they are not re-advertised if unknown MAC addresses (and default IP address) are utilized in the NVEs. In this scenario, each GW maintains a MAC-VRF (and/or IP-VRF) for each EVI. The main advantage of this approach is that NVEs do not need to maintain MAC and IP addresses from any remote data centers when default IP routes and unknown MAC routes are used; that is, they only need to maintain routes that are local to their own DC. When default IP routes and unknown MAC routes are used, any unknown IP and MAC packets from NVEs are forwarded to the GWs where all the VPN MAC and IP routes are maintained. This approach reduces the size of MAC-VRF and IP-VRF significantly at NVEs. Furthermore, it results in a faster convergence time upon a link or NVE failure in a multihomed network or device redundancy scenario, because the failure-related BGP routes (such as mass withdrawal message) do not need to get propagated all the way to the remote NVEs in the remote DCs. This approach is described in detail in Section 3.4 of [DCI-EVPN-OVERLAY].

10.2. DCI Using ASBRs

This approach can be considered as the opposite of the first approach. It favors simplification at DCI devices over NVEs such that larger MAC-VRF (and IP-VRF) tables need to be maintained on NVEs; whereas DCI devices don't need to maintain any MAC (and IP) forwarding tables. Furthermore, DCI devices do not need to terminate and process routes related to multihoming but rather to relay these messages for the establishment of an end-to-end Label Switched Path (LSP). In other words, DCI devices in this approach operate similar to ASBRs for inter-AS Option B (see Section 10 of [RFC4364]). This requires locally assigned VNIs to be used just like downstream- assigned MPLS VPN labels where, for all practical purposes, the VNIs function like 24-bit VPN labels. This approach is equally applicable to data centers (or Carrier Ethernet networks) with MPLS encapsulation.

In inter-AS Option B, when ASBR receives an EVPN route from its DC over internal BGP (iBGP) and re-advertises it to other ASBRs, it re-advertises the EVPN route by re-writing the BGP next hops to itself, thus losing the identity of the PE that originated the advertisement. This rewrite of BGP next hop impacts the EVPN mass withdrawal route (Ethernet A-D per ES) and its procedure adversely. However, it does not impact the EVPN Aliasing mechanism/procedure because when the Aliasing routes (Ethernet A-D per EVI) are advertised, the receiving PE first resolves a MAC address for a given EVI into its corresponding <ES, EVI>, and, subsequently, it resolves the <ES, EVI> into multiple paths (and their associated next hops) via which the <ES, EVI> is reachable. Since Aliasing and MAC routes are both advertised on a per-EVI-basis and they use the same RD and RT (per EVI), the receiving PE can associate them together on a per-BGP-path basis (e.g., per originating PE). Thus, it can perform recursive route resolution, e.g., a MAC is reachable via an <ES, EVI> which in turn, is reachable via a set of BGP paths; thus, the MAC is reachable via the set of BGP paths. Due to the per-EVI basis, the association of MAC routes and the corresponding Aliasing route is fixed and determined by the same RD and RT; there is no ambiguity when the BGP next hop for these routes is rewritten as these routes pass through ASBRs. That is, the receiving PE may receive multiple Aliasing routes for the same EVI from a single next hop (a single ASBR), and it can still create multiple paths toward that <ES, EVI>.

However, when the BGP next-hop address corresponding to the originating PE is rewritten, the association between the mass withdrawal route (Ethernet A-D per ES) and its corresponding MAC routes cannot be made based on their RDs and RTs because the RD for the mass Withdrawal route is different than the one for the MAC routes. Therefore, the functionality needed at the ASBRs and the receiving PEs depends on whether the Mass Withdrawal route is originated and whether there is a need to handle route resolution ambiguity for this route. The following two subsections describe the functionality needed by the ASBRs and the receiving PEs depending on whether the NVEs reside in a hypervisors or in ToR switches.

10.2.1. ASBR Functionality with Single-Homing NVEs

When NVEs reside in hypervisors as described in Section 7.1, there is no multihoming; thus, there is no need for the originating NVE to send Ethernet A-D per ES or Ethernet A-D per EVI routes. However, as noted in Section 7, in order to enable a single-homing ingress NVE to take advantage of fast convergence, Aliasing, and Backup Path when interacting with multihoming egress NVEs attached to a given ES, the single-homing NVE should be able to receive and process Ethernet A-D per ES and Ethernet A-D per EVI routes. The handling of these routes is described in the next section.

10.2.2. ASBR Functionality with Multihoming NVEs

When NVEs reside in ToR switches and operate in multihoming redundancy mode, there is a need, as described in Section 8, for the originating multihoming NVE to send Ethernet A-D per ES route(s) (used for mass withdrawal) and Ethernet A-D per EVI routes (used for Aliasing). As described above, the rewrite of BGP next hop by ASBRs creates ambiguities when Ethernet A-D per ES routes are received by the remote NVE in a different ASBR because the receiving NVE cannot associate that route with the MAC/IP routes of that ES advertised by the same originating NVE. This ambiguity inhibits the function of mass withdrawal per ES by the receiving NVE in a different AS.

As an example, consider a scenario where a CE is multihomed to PE1 and PE2, where these PEs are connected via ASBR1 and then ASBR2 to the remote PE3. Furthermore, consider that PE1 receives M1 from CE1 but not PE2. Therefore, PE1 advertises Ethernet A-D per ES1, Ethernet A-D per EVI1, and M1; whereas, PE2 only advertises Ethernet A-D per ES1 and Ethernet A-D per EVI1. ASBR1 receives all these five advertisements and passes them to ASBR2 (with itself as the BGP next hop). ASBR2, in turn, passes them to the remote PE3, with itself as the BGP next hop. PE3 receives these five routes where all of them have the same BGP next hop (i.e., ASBR2). Furthermore, the two Ethernet A-D per ES routes received by PE3 have the same information, i.e., same ESI and the same BGP next hop. Although both of these routes are maintained by the BGP process in PE3 (because they have different RDs and, thus, are treated as different BGP routes), information from only one of them is used in the L2 routing table (L2 RIB).

                      PE1
                     /   \
                    CE     ASBR1---ASBR2---PE3
                     \   /
                      PE2

Figure 3: Inter-AS Option B

Now, when the AC between the PE2 and the CE fails and PE2 sends Network Layer Reachability Information (NLRI) withdrawal for Ethernet A-D per ES route, and this withdrawal gets propagated and received by the PE3, the BGP process in PE3 removes the corresponding BGP route; however, it doesn't remove the associated information (namely ESI and

BGP next hop) from the L2 routing table (L2 RIB) because it still has the other Ethernet A-D per ES route (originated from PE1) with the same information. That is why the mass withdrawal mechanism does not work when doing DCI with inter-AS Option B. However, as described previously, the Aliasing function works and so does "mass withdrawal per EVI" (which is associated with withdrawing the EVPN route associated with Aliasing, i.e., Ethernet A-D per EVI route).

In the above example, the PE3 receives two Aliasing routes with the same BGP next hop (ASBR2) but different RDs. One of the Aliasing route has the same RD as the advertised MAC route (M1). PE3 follows the route resolution procedure specified in [RFC7432] upon receiving the two Aliasing routes; that is, it resolves M1 to <ES, EVI1>, and, subsequently, it resolves <ES, EVI1> to a BGP path list with two paths along with the corresponding VNIs/MPLS labels (one associated with PE1 and the other associated with PE2). It should be noted that even though both paths are advertised by the same BGP next hop (ASRB2), the receiving PE3 can handle them properly. Therefore, M1 is reachable via two paths. This creates two end-to-end LSPs, from PE3 to PE1 and from PE3 to PE2, for M1 such that when PE3 wants to forward traffic destined to M1, it can load-balance between the two LSPs. Although route resolution for Aliasing routes with the same BGP next hop is not explicitly mentioned in [RFC7432], this is the expected operation; thus, it is elaborated here.

When the AC between the PE2 and the CE fails and PE2 sends NLRI withdrawal for Ethernet A-D per EVI routes, and these withdrawals get propagated and received by the PE3, the PE3 removes the Aliasing route and updates the path list; that is, it removes the path corresponding to the PE2. Therefore, all the corresponding MAC routes for that <ES, EVI> that point to that path list will now have the updated path list with a single path associated with PE1. This action can be considered to be the mass withdrawal at the per-EVI level. The mass withdrawal at the per-EVI level has a longer convergence time than the mass withdrawal at the per-ES level; however, it is much faster than the convergence time when the withdrawal is done on a per-MAC basis.

If a PE becomes detached from a given ES, then, in addition to withdrawing its previously advertised Ethernet A-D per ES routes, it MUST also withdraw its previously advertised Ethernet A-D per EVI routes for that ES. For a remote PE that is separated from the withdrawing PE by one or more EVPN inter-AS Option B ASBRs, the withdrawal of the Ethernet A-D per ES routes is not actionable. However, a remote PE is able to correlate a previously advertised Ethernet A-D per EVI route with any MAC/IP Advertisement routes also advertised by the withdrawing PE for that <ES, EVI, BD>. Hence, when it receives the withdrawal of an Ethernet A-D per EVI route, it SHOULD remove the withdrawing PE as a next hop for all MAC addresses associated with that <ES, EVI, BD>.

In the previous example, when the AC between PE2 and the CE fails, PE2 will withdraw its Ethernet A-D per ES and per EVI routes. When PE3 receives the withdrawal of an Ethernet A-D per EVI route, it removes PE2 as a valid next hop for all MAC addresses associated with the corresponding <ES, EVI, BD>. Therefore, all the MAC next hops for that <ES, EVI, BD> will now have a single next hop, viz. the LSP to PE1.

In summary, it can be seen that Aliasing (and Backup Path) functionality should work as is for inter-AS Option B without requiring any additional functionality in ASBRs or PEs. However, the mass withdrawal functionality falls back from per-ES mode to per-EVI mode for inter-AS Option B. That is, PEs receiving a mass withdrawal route from the same AS take action on Ethernet A-D per ES route; whereas, PEs receiving mass withdrawal routes from different ASes take action on the Ethernet A-D per EVI route.

11. Security Considerations

This document uses IP-based tunnel technologies to support data-plane transport. Consequently, the security considerations of those tunnel technologies apply. This document defines support for VXLAN [RFC7348] and NVGRE encapsulations [RFC7637]. The security considerations from those RFCs apply to the data-plane aspects of this document.

As with [RFC5512], any modification of the information that is used to form encapsulation headers, to choose a tunnel type, or to choose a particular tunnel for a particular payload type may lead to user data packets getting misrouted, misdelivered, and/or dropped.

More broadly, the security considerations for the transport of IP reachability information using BGP are discussed in [RFC4271] and [RFC4272] and are equally applicable for the extensions described in this document.

12. IANA Considerations

This document registers the following in the "BGP Tunnel Encapsulation Attribute Tunnel Types" registry.

   Value    Name
   -----    ------------------------
   8        VXLAN Encapsulation
   9        NVGRE Encapsulation
   10       MPLS Encapsulation
   11       MPLS in GRE Encapsulation
   12       VXLAN GPE Encapsulation

13. References

13.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
   
   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.
   
   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <https://www.rfc-editor.org/info/rfc7348>.
   
   [RFC5512]  Mohapatra, P. and E. Rosen, "The BGP Encapsulation
              Subsequent Address Family Identifier (SAFI) and the BGP
              Tunnel Encapsulation Attribute", RFC 5512,
              DOI 10.17487/RFC5512, April 2009,
              <https://www.rfc-editor.org/info/rfc5512>.
   
   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <https://www.rfc-editor.org/info/rfc4023>.
   
   [RFC7637]  Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,
              <https://www.rfc-editor.org/info/rfc7637>.

13.2. Informative References

   [RFC7209]  Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
              Henderickx, W., and A. Isaac, "Requirements for Ethernet
              VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
              <https://www.rfc-editor.org/info/rfc7209>.
   
   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.
   
   [RFC7364]  Narten, T., Ed., Gray, E., Ed., Black, D., Fang, L.,
              Kreeger, L., and M. Napierala, "Problem Statement:
              Overlays for Network Virtualization", RFC 7364,
              DOI 10.17487/RFC7364, October 2014,
              <https://www.rfc-editor.org/info/rfc7364>.
   
   [RFC7365]  Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
              Rekhter, "Framework for Data Center (DC) Network
              Virtualization", RFC 7365, DOI 10.17487/RFC7365, October
              2014, <https://www.rfc-editor.org/info/rfc7365>.
   
   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <https://www.rfc-editor.org/info/rfc6514>.
   
   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.
   
   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

[TUNNEL-ENCAP]

              Rosen, E., Ed., Patel, K., and G. Velde, "The BGP Tunnel
              Encapsulation Attribute", Work in Progress draft-ietf-idr-
              tunnel-encaps-09, February 2018.

[DCI-EVPN-OVERLAY]

              Rabadan, J., Ed., Sathappan, S., Henderickx, W., Sajassi,
              A., and J. Drake, "Interconnect Solution for EVPN Overlay
              networks", Work in Progress, draft-ietf-bess-dci-evpn-
              overlay-10, March 2018.

[EVPN-GENEVE]

Boutros, S., Sajassi, A., Drake, J., and J. Rabadan, "EVPN

control plane for Geneve", Work in Progress,

draft-boutros-bess-evpn-geneve-02, March 2018.

[VXLAN-GPE]

Maino, F., Kreeger, L., Ed., and U. Elzur, Ed., "Generic

Protocol Extension for VXLAN", Work in Progress,

draft-ietf-nvo3-vxlan-gpe-05, October 2017.

   [GENEVE]   Gross, J., Ed., Ganga, I., Ed., and T. Sridhar, Ed.,
              "Geneve: Generic Network Virtualization Encapsulation",
              Work in Progress, draft-ietf-nvo3-geneve-06, March 2018.

[IEEE.802.1Q]

IEEE, "IEEE Standard for Local and metropolitan area networks - Bridges and Bridged Networks - Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks", IEEE Std 802.1Q.

Acknowledgements

The authors would like to thank Aldrin Isaac, David Smith, John Mullooly, Thomas Nadeau, Samir Thoria, and Jorge Rabadan for their valuable comments and feedback. The authors would also like to thank Jakob Heitz for his contribution on Section 10.2.

Contributors

   S. Salam
   K. Patel
   D. Rao
   S. Thoria
   D. Cai
   Cisco
   
   Y. Rekhter
   A. Issac
   W. Lin
   N. Sheth
   Juniper

L. Yong
Huawei

Authors' Addresses

Ali Sajassi (editor)
Cisco
United States of America

Email:

          sajassi@cisco.com

John Drake (editor)
Juniper Networks
United States of America

Email:

          jdrake@juniper.net

Nabil Bitar
Nokia
United States of America

   Email: nabil.bitar@nokia.com

R. Shekhar
Juniper
United States of America

Email:

          rshekhar@juniper.net

James Uttaro
AT&T
United States of America

Email:

          uttaro@att.com

Wim Henderickx
Nokia
Copernicuslaan 50
2018 Antwerp
Belgium

   Email: wim.henderickx@nokia.com