BESS Working Group L. Krattiger, Ed.
INTERNET-DRAFT A. Banerjee, Ed.
Intended Status: Informational A. Sajassi
Expires: June 22, 2023 R. Sharma
R. Sivaramu
Cisco Systems
December 20, 2022
Multi-Site Solution for Ethernet VPN (EVPN) Overlay
draft-sharma-bess-multi-site-evpn-03
Abstract
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 [EVPN-IPVPN], "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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. Multi-Site EVPN Overview . . . . . . . . . . . . . . . . . . . 5
3.1. MS-EVPN Interconnect Requirements . . . . . . . . . . . . 6
3.2. MS-EVPN Interconnect concept and framework . . . . . . . . 7
4. Multi-site EVPN Interconnect Procedures . . . . . . . . . . . 9
4.1. Border Gateway Discovery . . . . . . . . . . . . . . . . . 9
4.2. Border Gateway Provisioning . . . . . . . . . . . . . . . 11
4.2.1. Border Gateway Designated Forwarder Election . . . . . 11
4.2.2. Anycast Border Gateway . . . . . . . . . . . . . . . . 12
4.2.3. Multi-path Border Gateway . . . . . . . . . . . . . . 13
4.3. EVPN route processing at Border Gateway . . . . . . . . . 13
4.4. Multi-Destination tree between Border Gateways . . . . . . 14
4.5. Inter-site Unicast traffic . . . . . . . . . . . . . . . . 15
4.6. Inter-site Multi-destination traffic . . . . . . . . . . . 15
4.7. Host Mobility . . . . . . . . . . . . . . . . . . . . . . 15
5. Convergence . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Fabric to Border Gateway Failure . . . . . . . . . . . . . 16
5.2. Border Gateway to Border Gateway Failures . . . . . . . . 16
6. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 16
7. Isolation of Fault Domains . . . . . . . . . . . . . . . . . . 17
8. MVPN with Multi-site EVPN . . . . . . . . . . . . . . . . . . 17
8.1. Inter-Site MI-PMSI . . . . . . . . . . . . . . . . . . . . 17
8.2. Stitching of customer multicast trees across sites . . . . 18
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8.3. RP placement across sites . . . . . . . . . . . . . . . . 18
8.4. Inter-Site S-PMSI . . . . . . . . . . . . . . . . . . . . 18
9. Secure Data and Control Plane . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 19
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . 19
13.2. Informative References . . . . . . . . . . . . . . . . . 20
14. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 21
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 [EVPN-IPVPN] 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.
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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.
DC: Data Center
DCI: Data Center Interconnect
DF: Designated Forwarder
EVI: EVPN Instance
EVPN: 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.
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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.
ES: Ethernet Segment
ESI: Ethernet Segment Identifier
GW: 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 |
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| | 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.
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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
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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| ---
+---| |-----| |---+ |
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| +----+ +----+ | |
| 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
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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
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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
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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
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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 [EVPN-IPVPN], 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 [EVPN-
IPVPN].
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.
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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.
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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
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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
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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
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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 [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
[SECURE-EVPN] can provide EVPN domains with secure communications
between them.
10. Acknowledgements
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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
[EVPN-IPVPN].
11. Security Considerations
TBD.
12. IANA Considerations
TBD.
13. References
13.1. Normative References
[SHARMA-MULTI-SITE] Sharma et. al., "Multi-site EVPN based
VXLAN using Border Gateways", 2017, <draft-sharma-multi-
site-evpn>.
[RFC9014] Rabadan et al., "Interconnect Solution for Ethernet VPN
(EVPN) Overlay Networks", RFC 9014, DOI 10.17487, May
2021, <https://www.rfc-editor.org/info/rfc9014>.
[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>.
[RFC9136] Rabadan et al., "IP Prefix Advertisement in Ethernet VPN
(EVPN)", RFC 9136, DOI 10.17487, October 2021,
<https://www.rfc-editor.org/info/rfc913>.
[EVPN-IPVPN] J. Rabadan et. al., "EVPN Interworking with IPVPN",
2022, <https://tools.ietf.org/html/draft-ietf-bess-evpn-
ipvpn-interworking-07>.
[SECURE-EVPN] A. Sajassi et. al., "Secure EVPN", 2021,
<https://tools.ietf.org/html/draft-sajassi-bess-secure-
evpn-05>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
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10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[RFC1776] Crocker, S., "The Address is the Message", RFC 1776, DOI
10.17487/RFC1776, April 1 1995, <http://www.rfc-
editor.org/info/rfc1776>.
[TRUTHS] Callon, R., "The Twelve Networking Truths", RFC 1925, DOI
10.17487/RFC1925, April 1 1996, <http://www.rfc-
editor.org/info/rfc1925>.
13.2. Informative References
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/BGP
IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February 2012,
<https://www.rfc-editor.org/info/rfc6513>.
[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>.
[RFC7209] Sajassi, A., Ed., Aggarwal, R., Uttaro, J., Bitar, N.,
Henderickx, W., and Isaac, A., "Requirements for Ethernet
VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
<https://www.rfc-editor.org/info/rfc7209>.
[RFC8365] 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, March 2018, <https://www.rfc-
editor.org/info/rfc8365>.
[EVILBIT] Bellovin, S., "The Security Flag in the IPv4 Header",
RFC 3514, DOI 10.17487/RFC3514, April 1 2003,
<http://www.rfc-editor.org/info/rfc3514>.
[RFC5513] Farrel, A., "IANA Considerations for Three Letter
Acronyms", RFC 5513, DOI 10.17487/RFC5513, April 1 2009,
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<http://www.rfc-editor.org/info/rfc5513>.
[RFC5514] Vyncke, E., "IPv6 over Social Networks", RFC 5514, DOI
10.17487/RFC5514, April 1 2009, <http://www.rfc-
editor.org/info/rfc5514>.
14. Authors' Addresses
Lukas Krattiger (editor)
Cisco Systems
170 W Tasman Drive
San Jose, CA
USA
Email: lkrattig@cisco.com
Ayan Banerjee (editor)
Cisco Systems
170 W Tasman Drive
San Jose, CA
USA
Email: ayabaner@cisco.com
Ali Sajassi
Cisco Systems
170 W Tasman Drive
San Jose, CA
USA
Email: sajassi@cisco.com
Rajesh Sharma
Cisco Systems
170 W Tasman Drive
San Jose, CA
USA
Email: rajshr@cisco.com
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Raghava Sivaramu
Cisco Systems
170 W Tasman Drive
San Jose, CA
USA
Email: raghavas@cisco.com
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