Internet-Draft Multi-Site EVPN November 2023
Krattiger, et al. Expires 7 May 2024 [Page]
BESS Working Group
Intended Status:
L. Krattiger, Ed.
Cisco Systems
A. Banerjee, Ed.
Cisco Systems
A. Sajassi
Cisco Systems
K. Ananthamurthy
Cisco Systems
R. Sharma
Cisco Systems

Multi-Site Solution for Ethernet VPN (EVPN) Overlay


This document describes the procedures for interconnecting two or more Network Virtualization Overlays (NVOs) with EVPN via NVO over IP-only network. The solution interconnects Ethernet VPN network by using NVO with Ethernet VPN (EVPN) to facilitate the interconnect in a scalable fashion. The motivation is to support extension of Layer-2 and Layer-3, Unicast & Multicast, VPNs without having to rely on typical Data Center Interconnect (DCI) technologies like MPLS/VPLS. The requirements for the interconnect are similar to the ones specified in [RFC7209], "Requirements for Ethernet VPN (EVPN)". In particular, this document describes the difference of the Gateways (GWs) procedure and combined functionality from [RFC9014], "Interconnect Solution for Ethernet VPN (EVPN) Overlay Networks" and and [I-D.ietf-bess-evpn-ipvpn-interworking], "EVPN Interworking with IPVPN", which this solution is interoperable to. This document updates and replaces all previous version of Multi-site EVPN based VXLAN using Border Gateways (draft- sharma-multi-site-evpn).

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 7 May 2024.

Table of Contents

1. Introduction

Ethernet VPNs (EVPNs) are being used to support various VPN topologies with the motivation and requirements being discussed in [RFC7209]. EVPN has been used as the control plane to provide a Network Virtualization Overly (NVO) solution with a variety of tunnel encapsulation options, as per [RFC8365]. The Layer-2 Data center interconnect (DCI) procedures for IP and MPLS hand-off at domain boundaries are additionally discussed in [RFC9014], which is complemented by [I-D.ietf-bess-evpn-ipvpn-interworking] for Layer-3 DCI. The Multi-Site Solution combines Layer-2 and Layer-3 DCI for Ethernet VPN (EVPN) Overlay.

In current EVPN deployments, there is a need to segment the EVPN domains within a Data Center (DC), primarily due to the service architecture and the scaling requirements around it. The number of routes, tunnel end-points (TEPs), and next-hops needed within a DC domain are sometimes larger than the capability of the hardware elements that are being deployed. Network operators would like to interconnect these domains without using traditional DCI technologies. In essence, they want smaller EVPN domains with an IP- based backbone to interconnect. Additionally, they seek a simple and scalable redundancy model for the interconnect gateway with IP-based ECMP load distribution that does not incur additional protocol requirements to any of the surrounding TEPs. Using Anycast for the gateway redundancy provides minimal state sharing and it can scale out widely. A number of gateways participate in a Anycast set, which is represented by a single Anycast IP Address often also referred to as Virtual IP address or VIP. A group of gateways shares the same VIP and together represents the entry and exit of a given DC domain. The many TEPs within a DC domain are masqueraded behind a single Anycast TEP, which represents the gateway between the DC internal and DC external domain. Also, the Anycast gateway approach alleviates the hardware of performing multi-path for overlay reachability and respectively reduces control plane paths.

Network operators today are using the Virtual Network Identifier (VNI) to designate a service. They would like to have this service available to a smaller set of nodes within the DC for administrative reasons; in essence they want to break up the EVPN domain to multiple smaller administrative domains. An advantage of having a smaller footprint for these EVPN sites results in fault isolation domains being constrained. It also allows for flexible VNI allocation across sites, which subsequent can be stitched together for end-to-end communication.

In this document we focus on the Layer-2 and Layer-3 DCI with VXLAN encapsulation for EVPN deployments with the underlay providing only IP connectivity. We describe in detail the IP/VXLAN gateway procedure using the Anycast mode to interconnect smaller sites within the data center itself, and refer to this deployment model as multi-site EVPN (MS-EVPN). The procedures described here goes into substantial details regarding interconnecting Layer-2 (L2) and Layer-3 (L3) networks, for unicast and multicast domains across MS-EVPNs using the Anycast gateway model. In this specification, we are based on the [RFC9014] definitions for Layer-2 DCI with addition for operating with an Anycast gateway approach. The Anycast gateway mode as describe within this document can be extended to interop with a DC domain that interconnects with a [RFC9014] gateway, referred to as multi-path gateway.

2. Conventions and Terminology

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.


Data Center


Data Center Interconnect


Designated Forwarder


EVPN Instance


Ethernet Virtual Private Network, as in [RFC7432]

Border Gateway (BGW): This is the gateway node that is located between the DC/site internal and DC/site external domain. It is responsible for functionality related to traffic entering and exiting a site.

Anycast Border Gateway (A-BGW): A virtual set of BGWs sharing the same Anycast IP address (Virtual IP / VIP) acting as common entry/exit points for a single site.

Multipath Border Gateway: A virtual set of unique gateways, as described in [RFC9014], acting as a multiple individual entry/exit points for a single site.


Ethernet Segment


Ethernet Segment Identifier


Gateway or Data Center Gateway

I-ES and I-ESI: Interconnect Ethernet Segment and Interconnect Ethernet Segment Identifier. An I-ES is defined on the GWs for multihoming to/from the WAN.

RT-X: Route Type X as defined for various EVPN route types.

VNI: refers to VXLAN virtual identifiers

VXLAN: Virtual eXtensible LAN

3. Multi-Site EVPN Overview

In this section we describe the motivation, requirements, and framework for the Multi-Site EVPN (MS-EVPN) functionality. To introduce the Multi-Site solution, we compare [RFC9014] with the Multi-Site solution of this I-D.

|              | DCI EVPN-Overlay   | Multi-Site                    |
| Interconnect | Integrated (1-Box) | Integrated (1-Box)            |
|              | Decoupled (2-Box)  |                               |
| DCI Encap    | VPLS, PBB-VPLS,    | VXLAN                         |
|              |  EVPN-NPLS,        |                               |
|              |  PBB-EVPN, VXLAN   |                               |
| Gateway Mode | Multipath PIP      | Anycast VIP   | Multipath PIP |
| ECMP         | Underlay and       | Underlay      | Underlay and  |
|              |  Overlay           |               |  Overlay      |
| RT-1 on GW   | Consumed           | None          | Consumed      |
|              |  and Generated     |               |  and Generated|
|              |              (PIP) |               |       and PIP |
| RT-2 on GW   | Re-Originated by   | Re-Originated | Re-Originated |
|              | GW with I-ESI (PIP)|  with ESI 0   |  with I-ESI   |
|              |                    |       and VIP |       and PIP |
| RT-3 on GW   | Consumed and       | Consumed and  | Consumed and  |
|              |  Generated (PIP)   |Generated (PIP)|Generated (PIP)|
| RT-4 on GW   | Consumed and       | Consumed and  | Consumed and  |
|              |  Generated (PIP)   |Generated (PIP)|Generated (PIP)|
| RT-5 on GW   | [EVPN-IPVPN]       | Re-Originated | Re-Originated |
|              |       next-hop PIP |      with VIP |      with PIP |
| Route        | Separate RD for    | Separate RD for VIP and PIP   |
| Distinguisher|  Intra and Inter DC|                               |
| Route Target | Separate RT for    | Same RT for Intra and Inter   |
|              |  Intra and Inter DC|    DC with option to separate |
| VNI          | Global and         | Global and Downstream         |
|  Allocation  |  Downstream        |                               |
|  Stitching at| Gateway            | Gateway                       |
| DF Election  | Based on RT-4      | Based on RT-4                 |
|  Identifier  | I-ESI              | I-ESI (Site-ID)               |
|  Split       | Local Bias         | Local Bias                    |
|   Horizon    |                    |                               |
|  ESI-Type    | Type 0             | Type 5 (AS Based) or          |
|              |  (Operator Managed)|  Type 3 (MAC based)           |
| BUM Tree #   | 2, GW stitched     | 2, GW stitched                |
|              | (Intra & Inter DC) |  (Intra & Inter DC)           |

3.1. MS-EVPN Interconnect Requirements

a. Scalability: Multi-Site EVPN (MS-EVPN) should be able to interconnect multiple sites, allowing for addition/deletion of new sites or modifying capacity of existing ones seamlessly.

b. Multi-Destination traffic over unicast-only backbone: MS-EVPN mechanisms should provide an efficient forwarding mechanism for multi-destination frames by using existing network elements as-is. A large flat fabric rules out the option of ingress replication, as the number of replications becomes practically unachievable due to the internal hardware bandwidth needed.

c. Maintain Site-specific Administrative control: MS-EVPN should be able to interconnect fabrics from different Administrative domains. The solution should allow for different sites to have different VLAN- VNI mappings, use different underlay routing protocols, and/or have different PIM-SM group ranges.

d. Isolate fault domains: MS-EVPN technology hand-off should have capability to isolate traffic across site boundaries and prevent defects to percolate from one site to another. As an example, a broadcast storm in a site should not propagate to other sites.

3.2. MS-EVPN Interconnect concept and framework

MS-EVPN is conceptualized as multiple EVPN control plane and NVO forwarding domains, interconnected via a single common EVPN control and NVO forwarding domain. A set of gateway node are identified with a unique identifier, which then represent a site. A site is a EVPN domain, consisting of multiple EVPN nodes frontended by a set of gateways.

Border Gateways (BGWs) are explicitly part of one site-specific EVPN domain, and implicitly part of a common interconnect EVPN domain wit BGWs from other sites. Although a BGW has only a single explicit site-id (that of the site it is a member of, see Section X.X), it can be considered to also have a second implicit site-id, that of the interconnect-domain which has membership of all the BGWs from all sites that are being interconnected. BGWs act implicitly given they are the BGP next-hop from an entry/exit perspective; they perform both, the control and forwarding plane gateway functionally. This facilitates site internal nodes to visualize all other sites to be reachable only via its BGWs

We describe the MS-EVPN deployment model using the topology as shown in Figure 1. In the topology there are 3 sites, Site A, Site B, and Site C that are inter-connected using a IP backbone. This entire topology is deemed to be part of the same Data Center. In most deployments these sites can be thought of as pods, which may span a rack, a row, or multiple rows in the data center, depending on the size of domain desired for scale and fault and/or administrative isolation. Nothing prevents MS-EVPN to perform long distance or geographically dispersed Data center interconnect service.

In this topology, site internal nodes are connected to each other by iBGP EVPN peering and BGWs are connected by eBGP Muti-hop EVPN peering towards remote site BGW. We explicitly spell this out to ensure that we can re-use BGP semantics of route announcement between and across the sites. Other BGP mechanisms to instantiate this will be discussed in a separate document. This implies that each domain/site has its own AS number. In the topology, only 2 border gateway per site are shown; this is more for ease of illustration and explanation. The technology poses no such limitation. As mentioned earlier, site internal EVPN domain consists of only nodes within a site. A BGW is logically partitioned into site internal EVPN domain towards the site and into common EVPN domain towards other sites (external). This facilitates them to act as control and forwarding plane gateway for forwarding traffic across sites.

EVPN nodes within a site will discover each other via regular EVPN procedures and build site internal bidirectional VXLAN tunnels and multi-destination trees from leaves to BGWs. Similarly BGWs will discover each other by regular EVPN procedure and build site external bi-directional VXLAN tunnels and multi-destination trees between them. We thus build an end-to-end bidirectional forwarding path across all sites by stitching (and not by stretching end-to-end) site internal VXLAN tunnels with site external VXLAN tunnels. In essence, a MS-EVPN fabric is built in complete downstream and modular fashion.

    +----+    +----+        +----+    +----+          ___
    |    |    |    |        |    |    |    |           |
    |NVE1|    |NVE2|        |NVE3|    |NVE4|           |
    |    |    |    |        |    |    |    |           |
    +----+    +----+        +----+    +----+           |
      |         |             |         |            EVPN
  +------------------+    +------------------+        Ovl*
  |                  |    |                  |         |
  |     Site A       |    |      Site B      |         |
  | +----+    +----+ |    | +----+    +----+ |         |
  +-|    |----|    |-+    +-|    |----|    |-+         |
    |BGW1|    |BGW2|        |BGW3|    |BGW4|          ---
+---|    |----|    |--------|    |----|    |---+       |
|   +----+    +----+        +----+    +----+   |       |
|                                              |       |
|                 IP Backbone                  |      EVPN
|                                              |      Ovl*
|              +----+     +----+               |       |
+--------------|    |-----|    |---------------+       |
               |BGW5|     |BGW6|                      ---
           +---|    |-----|    |---+                   |
           |   +----+     +----+   |                   |
           |         Site C        |                   |
           |                       |                   |
           +-----------------------+                   |
                |          |                         EVPN
              +----+    +----+                       Ovl*
              |    |    |    |                         |
              |NVE5|    |NVE6|                         |
              |    |    |    |                         |
              +----+    +----+                        ---

* EVPN-Ovl stands for EVPN-Overlay (and it's an interconnect option).
Figure 1

Intra site tenant domains (for example, bridging, flood, routing, and multicast) are interconnected only via BGWs with site external tenant domains (bridging, flood, routing, and multicast respectively) from remote sites. It stitches such tenant domains (bridging, flood, routing, and multicast) in complete downstream fashion using EVPN route advertisements. Such interconnects do not assume uniform mappings of mac-vrf (or IP-VRF) to VNI across sites.

4. Multi-site EVPN Interconnect Procedures

In this section we describe the new functionalities in the Border Gateway (BGW) nodes for interconnecting EVPN sites within the DC.

In a nutshell, BGW discovery will facilitate termination and re- origination of inter-site VXLAN tunnels. Such discovery provides flexibility for intra-site TEP-to-TEP VXLAN tunnels to co-exist with inter-site tunnels terminating on BGWs. Additionally, BGWs need to discover each other such that it is possible to run the Designated Forwarder (DF) election between the border nodes of a site. It also needs to be aware of other remote BGWs such that it can allow for appropriate import/export of routes from other sites.

4.1. Border Gateway Discovery

BGW nodes of the same site MUST be configured or auto-generate the same site-identifier. In addition, the BGW is aware of its site internal and site external connection. Nodes that are part of the same site will build VXLAN tunnels only between members of the same site including the BGW; this is facilitated by site internal EVPN node reachability that stays site internal. BGWs will additionally build VXLAN tunnels between itself and other BGWs that are of a remote site. The remote BGWs are identified by the EVPN peering of type "external".

The site-identifier, used for BGW site participation and DF election, is encoded within a Site ESI label (I-ESI) itself as described below.

In this specification, we reuse the AS-based Ethernet Segment Identifier (ESI) Type 5 (see Section 5 of [RFC7432]) that can be auto-generated or configured by the operator. It is repeated here to illustrate the encoding of the site-identifier.

o Type 5 (T=0x05): The ESI value is constructed with the site-id parameter being embedded as follows.

  • AS number (4 octets). This is an AS number owned by the system and

    MUST be encoded in the high-order 4 octets of the ESI Value field. If
    a 2-octet AS number is used, the high-order extra 2 octets will be 0x0000.
  • Local Discriminator/Site Identifier (4 octets): The Local

    Discriminator is also referred to as the Site Identifier and its
    value MUST be encoded as follows. The high-order 2 octets will be 0x0000, and the low order 2 octets will be set to the site-identifier to which this node belongs. All border gateways MUST announce this value. We need the AS number and the site identifier together to be automatically derivable to less than 6 octets; this enables for auto import and export of routes (see the ES-Import RT definition in [RFC7432]).
  • Reserved (1 octet): The low-order octets of the ESI Value will be

    set to 0 on transmission and will be ignored on receipt.
        0   1   2   3   4   5   6   7   8   9
     | T |          ESI Value                |
Figure 2

The site identifier value must be globally unique within the deployments. Hence all BGWs are able to figure out other BGWs belonging to the same site, and armed with this information is able to run a Designated Forwarder (DF) election for BGWs site and VNI scoped as against the traditional Ethernet segment DF election. This said, the usage of the Type 5 ESI is not absolute, meaning other ESI Types could be leverage, like how [RFC9014] describes the usage. This alternate numbering is sufficient as long as the type and value requirement has ben satisfied globally, as well as for a set of BGW serving a common site. For example, if a implementation chooses to leverage a ESI of Type 0 or Type 3 and encodes the site-identifier respectively, this should not result in any disadvantage to any site internal or site external EVPN node. [RFC9014] for example recommends the usage of ESI Type 0 for the I-ESI. In Figure 1, nodes BGW1, BGW2, BGW3, BGW4, BGW5 and BGW6, will announce the ESI Label and the per- VNI RT Extended Communities. Nodes, BGW1, and BGW2, will perform a DF election for Site-A, whereas, nodes BGW3, and BGW4 will perform one for site-B. Even though, all BGW nodes are able to see all the advertisements, the site identifier scopes the DF election (using RT- 4 ES Routes) to its site members. This specification uses the All- Active Redundancy Mode specially when the Anycast model of route announcements are used for the local routes. It is noteworthy that even with the DF election based on RT-4, the EVPN RT-2, MAC/IP Route, will not leverage any ESI in its NLRI and hence is not required to send a related RT-1 (EAD route). Given the Anycast BGW model, no overlay multi-path is required given the next-hop is always the VIP address.

4.2. Border Gateway Provisioning

Border Gateways manage both the control-plane communications and the data forwarding plane for any traffic between sites.

BGWs are implicitly discovered by any RT-2/RT-5 routes from other sites. Any RT-2/RT-5 route will be terminated and re-originated on such BGWs. RT-2/RT-5 routes carry downstream VNI labels. As BGW discovery is agnostic to symmetric or downstream VNI provisioning, rewriting next-hop attributes before re-advertising these routes from other sites to a given site provides flexibility to keep different mac-VRF or IP-VRF to VNI mapping in different sites and still able to interconnect L3 and L2 domains.

RT-1, RT-3, and RT-4 from other sites will be terminated at the BGWs. As has been defined in the specifications, RT-3 routes carry downstream VNI labels and will be used to pre-build VXLAN tunnels in the common EVPN domain for L2, L3, and Multi-Destination traffic.

4.2.1. Border Gateway Designated Forwarder Election

In the presence of more than one BGW node in a site, forwarding of multi-destination L2 or L3 traffic both into the site and out of the site needs to be carried out by a single node. This node is termed as a designated forwarder and elected per-VNI as per rules defined in Section 8.5 of [RFC7432]. RT-4 Ethernet Segment routes are used for the DF election. In the multi-site deployment, the RT-4 Ethernet Segment routes carry a ES-Import RT Extended Community attribute with it. We need to enforce that these are imported to only the local site members when the ES-Import value matches with its own value. The 6- byte values are generated using a concatenation of the 4-byte AS number the member belongs, with the 2-bytes of site-identifier. As a result, only local site-members will match to form the candidate list. All the BGWs are able to extract the site-identifier from this attribute and the list of nodes where this election is run is now constrained to the BGWs between same site members.

In both modes (Anycast and Multipath), RT-3 routes will be generated
locally and advertised by each Border Gateway with unique gateway IP. This will facilitate building fast converging flood domain
connectivity inter-site and intra-site and on same time avoiding
duplicate traffic by electing DF winner to forward multi-destination inter-site traffic.

Failure events which lead to a BGW losing all of its connectivity to the IP interconnect backbone should trigger the BGW to withdraw its Border RT-4 Ethernet Segment route(s), to indicate to other BGW's of the same site that it is no longer a candidate BGW.

4.2.2. Anycast Border Gateway

In this mode all BGWs share same gateway IP (VIP) and rewrite EVPN next-hop attributes with a shared logical next-hop entity. However, these BGWs will maintain unique gateway IP (PIP) to facilitate building IR trees from site-local nodes to forward Multi-Destination traffic. EVPN RT-2, RT-5 routes will be advertised to the nodes in the site from all other BGWs and BGW will run DF election per VNI for Multi destination traffic. RT-3 routes will be advertised by each BGW for a given VNI so that only DF will receive and forward inter-site traffic. It is also possible to advertise and draw traffic by all BGWs at a site to improve convergence properties of the network. In case of multi-destination trees built by non-EVPN procedures (say PIM), all BGWs will receive but only DF winner will forward traffic.

It is recommended that BGW be enabled in the Anycast mode wherein the BGW functionality is available to the rest of the network as a single logical entity for inter-site communication. In the absence of Anycast capability the BGW could be enabled as individual gateways. As of now, the Border Gateway system MAC of the other border nodes belonging to the same site is expected to be configured out-of-band.

The Anycast Border Gateway the RT-2 MAC/IP Advertisement route is set to the reserved ESI value of 0. Hence the route resolution is performed based on the MAC/IP Advertisement alone as described in [RFC743]. Similar, RT-5 IP Prefix Advertisement requires no additional resolution as per [I-D.ietf-bess-evpn-ipvpn-interworking], as long as in interface- less per [RFC9136].

4.2.3. Multi-path Border Gateway

In this mode, Border gateways will rewrite EVPN Next-hop attributes with unique next-hop entities. This provides flexibility to apply usual policies and pick per-VRF, per-VNI or per-flow primary/backup border Gateways. Hence, an intra-site node will see each BGW as a next-hop for any external L2 or L3 unicast destination, and would perform an ECMP path selection to load-balance traffic sent to external destinations. In case an intra-site node is not capable of performing ECMP hash based path-selection (possibly some L2 forwarding implementations), the node is expected to choose one of the BGW's as its designated forwarder. EVPN RT-2, RT-5 routes will be advertised to the nodes in the site from all border gateways and Border gateway will run DF election per VNI for Multi destination traffic. RT-3 routes will be advertised by each Border gateway for a given VNI and only DF will receive and forward inter-site traffic. In case of multi-destination trees built by non-EVPN procedures (say PIM), all border gateways will receive but only DF winner will forward traffic. The Multi-path Border Gateway follows the model of the interconnect ESI (I-ESI) as described in [RFC9014]. With this requirement of multi-path, the RT-2 are labeled with the I-ESI and a RT-1 is used for the route resolution as described in [RFC7432] section 9.2.2. RT-5 requires no additional resolution as per [I-D.ietf-bess-evpn-ipvpn-interworking].

4.3. EVPN route processing at Border Gateway

BGW functionality in an EVPN site SHOULD be enabled on more than one node in the network for redundancy and high-availability purposes. Any external RT-2/RT-5 routes that are received by the BGWs of a site are advertised to all the intra-site nodes by all the BGWs. For internal RT-2/RT-5 routes received by the BGW's from the intra-site nodes, all the BGWs of a site would advertise them to the remote BGW's, so any L2/L3 known unicast traffic to internal destinations could be sent to any one of the local BGW's by remote sources. For known L2 and L3 unicast traffic, all of the individual BGWs will behave either as single logical forwarding node (Anycast model) or a set of active forwarding nodes.

All control plane and data plane states are interconnected in a complete downstream fashion. For example, BGP import rules for a RT-3 route should be able to extend a flood domain for a VNI and flood traffic destined to advertised EVPN node should carry the VNI which is announced in RT-3 route. Similarly Type 2, Type 5 control and forwarding states should be interconnected in a complete downstream fashion.

o Route Target processing for RT-4 routes: Every IP-VRF and MAC-VRF will generate RT-4 with the format described in section 4.1. Route targets can be auto derived from Ethernet Tag ID (VLAN ID) for that EVPN instance as described in Section 7.10.1 of [RFC7432]. ES import route target extended community as described in Section 7.6 of [RFC7432] is mandatory for RT-4 in this context. The encoding of ES- Import is based on AS number and Site-identifier as described in Section 4.2.1. Such import route target will allow import of RT-4 only to the Border gateways of same sites.

o Route Target processing for RT-2, RT-3, RT-5 routes: These routes will carry either auto-derived route targets (based on Ethernet Tag ID (VLAN ID) for that EVPN instance) or explicit route targets. Border gateways usual import rules will imports these routes and re- advertise these with border gateway next hops. Also the routes which are imported at Border Gateways and re-advertised SHOULD implement a mechanism to avoid looping of updates should they come back at Border Gateways. RT-3 routes will be imported and processed on border gateways from other border gateways but MUST NOT be advertised again.

4.4. Multi-Destination tree between Border Gateways

The procedures described here recommends building an Ingress Replication (IR) tree between Border Gateways. This will facilitate every site to independently build site-specific Multi-destination trees. Multi-destination end-to-end trees between leafs could be PIM (site 1) + IR (between border Gateways) + PIM (site 2) or IR-IR-IR or PIM-IR-IR. However this does not rule out using IR-PIM-IR or end-to- end PIM to build multi-destination trees end-to-end.

Border Gateways will generate RT-3 routes with unique gateway IP and advertise to Border Gateways of other sites. These RT-3 routes will help in building IR trees between border gateways. However, only DF winner per VNI will forward multi-destination traffic across sites.

As Border Gateways are part of both site-specific and inter-site Multi-destination IR trees, split-horizon mechanism will be used to avoid loops. Multi-destination tree with Border gateway as root to other sites (or Border-Gateways) will be in a separate horizon group.

Similarity Multi-destination IR tree with Border Gateway as root to site-local nodes will be in another split horizon group.

If PIM is used to build Multi-Destination trees in site-specific domain, all Border gateway will join such PIM trees and draw multi- destination traffic. However only DF Border Gateway will forward traffic towards other sites.

4.5. Inter-site Unicast traffic

As site internal node will see all site external EVPN routes via Border Gateways, VXLAN tunnels will be built between leafs and site internal Border Gateways and Inter-site VXLAN tunnels will be built between Border gateways in different sites. An end-to-end VXLAN bidirectional forwarding path between inter-site leafs will consist of VXLAN tunnel from leaf (say Site A) to its Border Gateway (BGW1), another VXLAN tunnel from Border Gateway (BGW1) to Border Gateway (BGW3) in another site (say site B) and Border gateway (BGW3) to leaf (in site B). Such an arrangement of a hierarchical tunnel topology is more scalable as a full mesh of VXLAN tunnels across inter-site leafs is substituted by combination of intra-site and inter-site tunnels.

L2 and L3 unicast frames from site internal leafs will reach border gateway using VXLAN encapsulation. At Border gateway, VXLAN header is stripped out and another VXLAN header is pushed to sent frames to destination site Border Gateway. Destination site Border gateway will strip off VXLAN header and push another VXLAN header to send frame to the destination site leaf.

4.6. Inter-site Multi-destination traffic

Multi-destination traffic will be forwarded from one site to other site only by DF for that VNI. As frames reach Border Gateway from site internal nodes, VXLAN header will be decapsulated from the payload, and encapsulated with another VXLAN header (derived from downstream RT-3 EVPN routes received from the border gateways of the destination site) to forward the payload to the destination site border gateway. Similarly destination site Border Gateway will strip off VXLAN header and forward the payload after encapsulating with another VXLAN header towards the destination leaf.

As explained in Section 4.4, split horizon mechanism will be used to avoid looping of inter-site multi-destination frames.

4.7. Host Mobility

Host movement handling will be same as defined in [RFC7432]. When host moves, EVPN RT-2 routes with updated sequence number will be propagated to every EVPN node. When a host moves inter-site, only Border gateways may see EVPN updates with both next-hop attributes and sequence number changes and leafs may see updates only with updated sequence numbers; this is as described in [RFC9014] section 4.4.4. However in other cases, both Border gateway and leaves may see next-hop and sequence number changes.

5. Convergence

5.1. Fabric to Border Gateway Failure

If a Border Gateway is lost, Border gateway next-hop will be withdrawn for RT-2/RT-5 routes. Also per-VNI DF election will be triggered to chose new DF. DF new winner will become forwarder of Multi-destination inter-site traffic.

5.2. Border Gateway to Border Gateway Failures

In case where inter-site cloud has link failures, direct forwarding path between border gateways can be lost. In this case, traffic from one site can reach other site via border gateway of an intermediate site. However, this will be addressed like regular underlay failure and traffic terminations end-points will still stay same for inter- site traffic flows.

6. Interoperability

The procedures defined here are only for Border Gateways. Therefore other EVPN nodes require only to be compliant with [RFC7432] and [RFC8365] to operate in such topologies

As the procedures described here are applicable only based on the respective topology configuration or discovery, if other domains are connected which are not capable of such multi-site gateway model, they can work in regular EVPN mode. In the case of remote sites operate in different modes, for example some in Anycast mode, others in Multi-Path or [RFC9014] mode, the Anycast BGW will be able to accommodate either and adjusts the respective mode. The signalization of the respective mode is driven through the presence of ESI in RT-2 and the per-ES EAD RT-1 route. The purpose of RT-3 routes are solely for the case of BUM replication and doesn't provide any neighbor discovery function. This is crucial as in cases where only IP routing is used, with the IP portion of RT-2 and/or RT-5 only, the MP_RACH_NLRI attribute, or the RT-1 in the case of ESI, is sufficient for route resolution.

The procedures here provides flexibility to connect non-EVPN VXLAN sites by provisioning Border Gateways on such sites and inter- connecting such Border Gateways by Border Gateways of other sites. Such Border Gateways in non-EVPN VXLAN sites will play dual role of EVPN gateway towards common EVPN domain and non-EVPN gateway towards non-EVPN VXLAN site.

7. Isolation of Fault Domains

Isolation of network defects requires policies like storm control, security ACLs etc to be implemented at site boundaries. Border gateways should be capable of inspecting inner payload of packets received from VXLAN tunnels and enforce configured policies to prevent defects percolating from one part to rest of the network.

8. MVPN with Multi-site EVPN

BGP based MVPN as defined in [RFC6513] and [RFC6514] will coexist with Multisite-EVPN with out any changes in route types and encodings defined for MVPN route types in these RFCs. Route Distinguisher and VRF route import extended communities will be attached to MVPN routes as defined in the BGP MVPN RFCs. Import and Export Route targets will be attached to MVPN routes either by Auto-generating them from VNI or by explicit configuration per MVPN. Since, BGP MVPN RFC adapts to any VPN address family to provide RPF information to build C-Multicast trees, EVPN route types will be used to provide required RPF information for Multicast sources in MVPNs. In order to follow segmentation model of Multisite-EVPN, following procedures are recommended to build provider and customer multicast trees between sources and receivers across sites.

8.1. Inter-Site MI-PMSI

As defined in above mentioned MVPN RFCs, I-PMSI A-D routes are used to signal a provider tunnel or MI-PMSI per MVPN. Multisite-EVPN recommends EVPN Type-3 routes to build such MI-PMSI provider tunnel per VPN between Border Gateways of different sites. Every MVPN node will use its unique router identifier to build these MI-PMSI provider tunnels. In Anycast Border gateway model also, these MI-PMSI provider tunnels are built using unique router identifier of Border gateways. In similar fashion, these Type-3 routes can be used to build MI-PMSI provider tunnel per MVPN with in sites.

8.2. Stitching of customer multicast trees across sites

All Border Gateways will rewrite next-hop and re-originate MVPN routes received from other sites to local site and from local site to other sites. Therefore customer Multicast trees will be logically built end-to-end across sites by stitching these trees via Border gateways. A C-multicast join route (say Type 7 MVPN) will follow EVPN RPF path to build C-multicast tree from leaf in a site to its Border gateway and to destination site leafs via destination site Border Gateways. Similarly Source-Active A-D MVPN route (Type 5 MVPN) will be rewritten with next-hop and re-originated via Border gateways so that source C-Multicast trees will be stitched via Border gateways.

8.3. RP placement across sites

Multisite-EVPN recommends only Source C-Multicast trees across sites. Therefore Customer RP placement per MVPN should be restricted with in sites. Source-Active A-D MVPN route type (Type 5) will be used to signal C-Multicast sources across sites.

8.4. Inter-Site S-PMSI

As defined in BGP MVPN RFCs, S-PMSI A-D routes (Type 3 MVPN) will be used to signal selective PMSI trees for high bandwidth C-Multicast streams. These S-PMSI A-D routes will be signaled across sites via Border gateways rewriting next-hop and re-originating them to other sites. PMSI tunnel attribute in re-originated S-PMSI routes will be adjusted to the provide tunnel types between Border gateways across sites.

9. Secure Data and Control Plane

In [I-D.sajassi-bess-secure-evpn] the use case is centered around providing inter-site and WAN connectivity over public Internet in a secured manner with same level of privacy, integrity, and authentication for tenant's traffic as IPsec tunneling using IKEv2. The multi-site enhancements in this draft in conjunction with the definitions specified in [I-D.sajassi-bess-secure-evpn] can provide EVPN domains with secure communications between them.

10. Acknowledgements

These authors would like to thank Max Ardica, Murali Garimella, Swaraj Kumar Chikyala, Anuj,Mittal, Lilian Quan, Veera Ravinutala, Tarun Wadhwa for their review and comments. Special thanks to Jorge Rabadan for his contribution and feedback to align with [RFC9014] and [I-D.ietf-bess-evpn-ipvpn-interworking].

11. Security Considerations


12. IANA Considerations


13. References

13.1. Normative References

Sharma, R., Banerjee, A., Sivaramu, R., and A. Sajassi, "Multi-site EVPN based VXLAN using Border Gateways", Work in Progress, Internet-Draft, draft-sharma-multi-site-evpn-04, , <>.
Rabadan, J., Ed., Sathappan, S., Henderickx, W., Sajassi, A., and J. Drake, "Interconnect Solution for Ethernet VPN (EVPN) Overlay Networks", RFC 9014, DOI 10.17487/RFC9014, , <>.
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, , <>.
Rabadan, J., Ed., Henderickx, W., Drake, J., Lin, W., and A. Sajassi, "IP Prefix Advertisement in Ethernet VPN (EVPN)", RFC 9136, DOI 10.17487/RFC9136, , <>.
Rabadan, J., Sajassi, A., Rosen, E. C., Drake, J., Lin, W., Uttaro, J., and A. Simpson, "EVPN Interworking with IPVPN", Work in Progress, Internet-Draft, draft-ietf-bess-evpn-ipvpn-interworking-09, , <>.
Sajassi, A., Banerjee, A., Thoria, S., Carrel, D., Weis, B., and J. Drake, "Secure EVPN", Work in Progress, Internet-Draft, draft-sajassi-bess-secure-evpn-06, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Crocker, S., "The Address is the Message", RFC 1776, DOI 10.17487/RFC1776, , <>.
Callon, R., "The Twelve Networking Truths", RFC 1925, DOI 10, <>.

13.2. Informative References

Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, , <>.
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, , <>.
Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N., Henderickx, W., and A. Isaac, "Requirements for Ethernet VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, , <>.
Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R., Uttaro, J., and W. Henderickx, "A Network Virtualization Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365, DOI 10.17487/RFC8365, , <>.
Bellovin, S., "The Security Flag in the IPv4 Header", RFC 3514, DOI 10, <>.
Farrel, A., "IANA Considerations for Three Letter Acronyms", RFC 5513, DOI 10.17487/RFC5513, , <>.
Vyncke, E., "IPv6 over Social Networks", RFC 5514, DOI 10.17487/RFC5514, , <>.

Appendix A. Authors' Addresses

Authors' Addresses

Lukas Krattiger (editor)
Cisco Systems
Ayan Banerjee (editor)
Cisco Systems
Ali Sajassi
Cisco Systems
K. Ananthamurthy
Cisco Systems
R. Sharma
Cisco Systems