Internet Engineering Task Force (IETF) J. Rabadan, Ed.

Internet Engineering Task Force (IETF) J. Rabadan, Ed.
Request for Comments: 9136 W. Henderickx
Category: Standards Track Nokia
ISSN: 2070-1721 J. Drake
W. Lin
Juniper
A. Sajassi
Cisco
October 2021

IP Prefix Advertisement in Ethernet VPN (EVPN)

Abstract

The BGP MPLS-based Ethernet VPN (EVPN) (RFC 7432) mechanism provides
a flexible control plane that allows intra-subnet connectivity in an
MPLS and/or Network Virtualization Overlay (NVO) (RFC 7365) network.
In some networks, there is also a need for dynamic and efficient
inter-subnet connectivity across Tenant Systems and end devices that
can be physical or virtual and do not necessarily participate in
dynamic routing protocols. This document defines a new EVPN route
type for the advertisement of IP prefixes and explains some use-case
examples where this new route type is used.

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/rfc9136.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

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.

Table of Contents

1. Introduction
1.1. Terminology
2. Problem Statement
2.1. Inter-Subnet Connectivity Requirements in Data Centers
2.2. The Need for the EVPN IP Prefix Route
3. The BGP EVPN IP Prefix Route
3.1. IP Prefix Route Encoding
3.2. Overlay Indexes and Recursive Lookup Resolution
4. Overlay Index Use Cases
4.1. TS IP Address Overlay Index Use Case
4.2. Floating IP Overlay Index Use Case
4.3. Bump-in-the-Wire Use Case
4.4. IP-VRF-to-IP-VRF Model
4.4.1. Interface-less IP-VRF-to-IP-VRF Model
4.4.2. Interface-ful IP-VRF-to-IP-VRF with SBD IRB
4.4.3. Interface-ful IP-VRF-to-IP-VRF with Unnumbered SBD IRB
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

[RFC7365] provides a framework for Data Center (DC) Network
Virtualization over Layer 3 and specifies that the Network
Virtualization Edge (NVE) devices must provide Layer 2 and Layer 3
virtualized network services in multi-tenant DCs. [RFC8365]
discusses the use of EVPN as the technology of choice to provide
Layer 2 or intra-subnet services in these DCs. This document, along
with [RFC9135], specifies the use of EVPN for Layer 3 or inter-subnet
connectivity services.

[RFC9135] defines some fairly common inter-subnet forwarding
scenarios where Tenant Systems (TSs) can exchange packets with TSs
located in remote subnets. In order to achieve this, [RFC9135]
describes how Media Access Control (MAC) and IPs encoded in TS RT-2
routes are not only used to populate MAC Virtual Routing and
Forwarding (MAC-VRF) and overlay Address Resolution Protocol (ARP)
tables but also IP-VRF tables with the encoded TS host routes (/32 or
/128). In some cases, EVPN may advertise IP prefixes and therefore
provide aggregation in the IP-VRF tables, as opposed to propagating
individual host routes. This document complements the scenarios
described in [RFC9135] and defines how EVPN may be used to advertise
IP prefixes. Interoperability between EVPN and Layer 3 Virtual
Private Network (VPN) [RFC4364] IP Prefix routes is out of the scope
of this document.

Section 2.1 describes the inter-subnet connectivity requirements in
DCs. Section 2.2 explains why a new EVPN route type is required for
IP prefix advertisements. Sections 3, 4, and 5 will describe this
route type and how it is used in some specific use cases.

1.1. 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.

AC: Attachment Circuit

ARP: Address Resolution Protocol

BD: Broadcast Domain. As per [RFC7432], an EVI consists of a
single BD or multiple BDs. In case of VLAN-bundle and
VLAN-based service models (see [RFC7432]), a BD is
equivalent to an EVI. In case of a VLAN-aware bundle
service model, an EVI contains multiple BDs. Also, in this
document, "BD" and "subnet" are equivalent terms.

BD Route Target: Refers to the broadcast-domain-assigned Route
Target [RFC4364]. In case of a VLAN-aware bundle service
model, all the BD instances in the MAC-VRF share the same
Route Target.

BT: Bridge Table. The instantiation of a BD in a MAC-VRF, as
per [RFC7432].

CE: Customer Edge

DA: Destination Address

DGW: Data Center Gateway

Ethernet A-D Route: Ethernet Auto-Discovery (A-D) route, as per
[RFC7432].

Ethernet NVO Tunnel: Refers to Network Virtualization Overlay
tunnels with Ethernet payload. Examples of this type of
tunnel are VXLAN or GENEVE.

EVI: EVPN Instance spanning the NVE/PE devices that are
participating on that EVPN, as per [RFC7432].

EVPN: Ethernet VPN, as per [RFC7432].

GENEVE: Generic Network Virtualization Encapsulation, as per
[RFC8926].

GRE: Generic Routing Encapsulation

GW IP: Gateway IP address

IPL: IP Prefix Length

IP NVO Tunnel: Refers to Network Virtualization Overlay tunnels with
IP payload (no MAC header in the payload).

IP-VRF: A Virtual Routing and Forwarding table for IP routes on an
NVE/PE. The IP routes could be populated by EVPN and IP-
VPN address families. An IP-VRF is also an instantiation
of a Layer 3 VPN in an NVE/PE.

IRB: Integrated Routing and Bridging interface. It connects an
IP-VRF to a BD (or subnet).

MAC: Media Access Control

MAC-VRF: A Virtual Routing and Forwarding table for MAC addresses on
an NVE/PE, as per [RFC7432]. A MAC-VRF is also an
instantiation of an EVI in an NVE/PE.

ML: MAC Address Length

ND: Neighbor Discovery

NVE: Network Virtualization Edge

NVO: Network Virtualization Overlay

PE: Provider Edge

RT-2: EVPN Route Type 2, i.e., MAC/IP Advertisement route, as
defined in [RFC7432].

RT-5: EVPN Route Type 5, i.e., IP Prefix route, as defined in
Section 3.

SBD: Supplementary Broadcast Domain. A BD that does not have
any ACs, only IRB interfaces, and is used to provide
connectivity among all the IP-VRFs of the tenant. The SBD
is only required in IP-VRF-to-IP-VRF use cases (see
Section 4.4).

SN: Subnet

TS: Tenant System

VA: Virtual Appliance

VM: Virtual Machine

VNI: Virtual Network Identifier. As in [RFC8365], the term is
used as a representation of a 24-bit NVO instance
identifier, with the understanding that "VNI" will refer to
a VXLAN Network Identifier in VXLAN, or a Virtual Network
Identifier in GENEVE, etc., unless it is stated otherwise.

VSID: Virtual Subnet Identifier

VTEP: VXLAN Termination End Point, as per [RFC7348].

VXLAN: Virtual eXtensible Local Area Network, as per [RFC7348].

This document also assumes familiarity with the terminology of
[RFC7365], [RFC7432], and [RFC8365].

2. Problem Statement

This section describes the inter-subnet connectivity requirements in
DCs and why a specific route type to advertise IP prefixes is needed.

2.1. Inter-Subnet Connectivity Requirements in Data Centers

[RFC7432] is used as the control plane for an NVO solution in DCs,
where NVE devices can be located in hypervisors or Top-of-Rack (ToR)
switches, as described in [RFC8365].

The following considerations apply to TSs that are physical or
virtual systems identified by MAC (and possibly IP addresses) and are
connected to BDs by Attachment Circuits:

* The Tenant Systems may be VMs that generate traffic from their own
MAC and IP.

* The Tenant Systems may be VA entities that forward traffic to/from
IP addresses of different end devices sitting behind them.

- These VAs can be firewalls, load balancers, NAT devices, other
appliances, or virtual gateways with virtual routing instances.

- These VAs do not necessarily participate in dynamic routing
protocols and hence rely on the EVPN NVEs to advertise the
routes on their behalf.

- In all these cases, the VA will forward traffic to other TSs
using its own source MAC, but the source IP will be the one
associated with the end device sitting behind the VA or a
translated IP address (part of a public NAT pool) if the VA is
performing NAT.

- Note that the same IP address and endpoint could exist behind
two of these TSs. One example of this would be certain
appliance resiliency mechanisms, where a virtual IP or floating
IP can be owned by one of the two VAs running the resiliency
protocol (the Master VA). The Virtual Router Redundancy
Protocol (VRRP) [RFC5798] is one particular example of this.
Another example is multihomed subnets, i.e., the same subnet is
connected to two VAs.

- Although these VAs provide IP connectivity to VMs and the
subnets behind them, they do not always have their own IP
interface connected to the EVPN NVE; Layer 2 firewalls are
examples of VAs not supporting IP interfaces.

Figure 1 illustrates some of the examples described above.

NVE1
+-----------+
TS1(VM)--| (BD-10) |-----+
M1/IP1 +-----------+ | DGW1
+---------+ +-------------+
| |----| (BD-10) |
SN1---+ NVE2 | | | IRB1\ |
| +-----------+ | | | (IP-VRF)|---+
SN2---TS2(VA)--| (BD-10) |-| | +-------------+ _|_
| M2/IP2 +-----------+ | VXLAN/ | ( )
IP4---+ <-+ | GENEVE | DGW2 ( WAN )
| | | +-------------+ (___)
vIP23 (floating) | |----| (BD-10) | |
| +---------+ | IRB2\ | |
SN1---+ <-+ NVE3 | | | | (IP-VRF)|---+
| M3/IP3 +-----------+ | | | +-------------+
SN3---TS3(VA)--| (BD-10) |---+ | |
| +-----------+ | |
IP5---+ | |
| |
NVE4 | | NVE5 +--SN5
+---------------------+ | | +-----------+ |
IP6------| (BD-1) | | +-| (BD-10) |--TS4(VA)--SN6
| \ | | +-----------+ |
| (IP-VRF) |--+ ESI4 +--SN7
| / \IRB3 |
|---| (BD-2) (BD-10) |
SN4| +---------------------+

Note:
ESI4 = Ethernet Segment Identifier 4

Figure 1: DC Inter-subnet Use Cases

Where:

NVE1, NVE2, NVE3, NVE4, NVE5, DGW1, and DGW2 share the same BD for a
particular tenant. BD-10 is comprised of the collection of BD
instances defined in all the NVEs. All the hosts connected to BD-10
belong to the same IP subnet. The hosts connected to BD-10 are
listed below:

* TS1 is a VM that generates/receives traffic to/from IP1, where IP1
belongs to the BD-10 subnet.

* TS2 and TS3 are VAs that send/receive traffic to/from the subnets
and hosts sitting behind them (SN1, SN2, SN3, IP4, and IP5).
Their IP addresses (IP2 and IP3) belong to the BD-10 subnet, and
they can also generate/receive traffic. When these VAs receive
packets destined to their own MAC addresses (M2 and M3), they will
route the packets to the proper subnet or host. These VAs do not
support routing protocols to advertise the subnets connected to
them and can move to a different server and NVE when the cloud
management system decides to do so. These VAs may also support
redundancy mechanisms for some subnets, similar to VRRP, where a
floating IP is owned by the Master VA and only the Master VA
forwards traffic to a given subnet. For example, vIP23 in
Figure 1 is a floating IP that can be owned by TS2 or TS3
depending on which system is the Master. Only the Master will
forward traffic to SN1.

* Integrated Routing and Bridging interfaces IRB1, IRB2, and IRB3
have their own IP addresses that belong to the BD-10 subnet too.
These IRB interfaces connect the BD-10 subnet to Virtual Routing
and Forwarding (IP-VRF) instances that can route the traffic to
other subnets for the same tenant (within the DC or at the other
end of the WAN).

* TS4 is a Layer 2 VA that provides connectivity to subnets SN5,
SN6, and SN7 but does not have an IP address itself in the BD-10.
TS4 is connected to a port on NVE5 that is assigned to Ethernet
Segment Identifier 4 (ESI4).

For a BD to which an ingress NVE is attached, "Overlay Index" is
defined as an identifier that the ingress EVPN NVE requires in order
to forward packets to a subnet or host in a remote subnet. As an
example, vIP23 (Figure 1) is an Overlay Index that any NVE attached
to BD-10 needs to know in order to forward packets to SN1. The IRB3
IP address is an Overlay Index required to get to SN4, and ESI4 is an
Overlay Index needed to forward traffic to SN5. In other words, the
Overlay Index is a next hop in the overlay address space that can be
an IP address, a MAC address, or an ESI. When advertised along with
an IP prefix, the Overlay Index requires a recursive resolution to
find out the egress NVE to which the EVPN packets need to be sent.

All the DC use cases in Figure 1 require inter-subnet forwarding;
therefore, the individual host routes and subnets:

a) must be advertised from the NVEs (since VAs and VMs do not
participate in dynamic routing protocols) and

b) may be associated with an Overlay Index that can be a VA IP
address, a floating IP address, a MAC address, or an ESI. The
Overlay Index is further discussed in Section 3.2.

2.2. The Need for the EVPN IP Prefix Route

[RFC7432] defines a MAC/IP Advertisement route (also referred to as
"RT-2") where a MAC address can be advertised together with an IP
address length and IP address (IP). While a variable IP address
length might have been used to indicate the presence of an IP prefix
in a route type 2, there are several specific use cases in which
using this route type to deliver IP prefixes is not suitable.

One example of such use cases is the "floating IP" example described
in Section 2.1. In this example, it is necessary to decouple the
advertisement of the prefixes from the advertisement of a MAC address
of either M2 or M3; otherwise, the solution gets highly inefficient
and does not scale.

For example, if 1,000 prefixes are advertised from M2 (using RT-2)
and the floating IP owner changes from M2 to M3, 1,000 routes would
be withdrawn by M2 and readvertised by M3. However, if a separate
route type is used, 1,000 routes can be advertised as associated with
the floating IP address (vIP23), and only one RT-2 can be used for
advertising the ownership of the floating IP, i.e., vIP23 and M2 in
the route type 2. When the floating IP owner changes from M2 to M3,
a single RT-2 withdrawal/update is required to indicate the change.
The remote DGW will not change any of the 1,000 prefixes associated
with vIP23 but will only update the ARP resolution entry for vIP23
(now pointing at M3).

An EVPN route (type 5) for the advertisement of IP prefixes is
described in this document. This new route type has a differentiated
role from the RT-2 route and addresses the inter-subnet connectivity
scenarios for DCs (or NVO-based networks in general) described in
this document. Using this new RT-5, an IP prefix may be advertised
along with an Overlay Index, which can be a GW IP address, a MAC, or
an ESI. The IP prefix may also be advertised without an Overlay
Index, in which case the BGP next hop will point at the egress NVE,
Area Border Router (ABR), or ASBR, and the MAC in the EVPN Router's
MAC Extended Community will provide the inner MAC destination address
to be used. As discussed throughout the document, the EVPN RT-2 does
not meet the requirements for all the DC use cases; therefore, this
EVPN route type 5 is required.

The EVPN route type 5 decouples the IP prefix advertisements from the
MAC/IP Advertisement routes in EVPN. Hence:

a) The clean and clear advertisements of IPv4 or IPv6 prefixes in a
Network Layer Reachability Information (NLRI) message without MAC
addresses are allowed.

b) Since the route type is different from the MAC/IP Advertisement
route, the current procedures described in [RFC7432] do not need
to be modified.

c) A flexible implementation is allowed where the prefix can be
linked to different types of Overlay/Underlay Indexes: overlay IP
addresses, overlay MAC addresses, overlay ESIs, underlay BGP next
hops, etc.

d) An EVPN implementation not requiring IP prefixes can simply
discard them by looking at the route type value.

The following sections describe how EVPN is extended with a route
type for the advertisement of IP prefixes and how this route is used
to address the inter-subnet connectivity requirements existing in the
DC.

3. The BGP EVPN IP Prefix Route

The BGP EVPN NLRI as defined in [RFC7432] is shown below:

+-----------------------------------+
| Route Type (1 octet) |
+-----------------------------------+
| Length (1 octet) |
+-----------------------------------+
| Route Type specific (variable) |
+-----------------------------------+

Figure 2: BGP EVPN NLRI

This document defines an additional route type (RT-5) in the IANA
"EVPN Route Types" registry [EVPNRouteTypes] to be used for the
advertisement of EVPN routes using IP prefixes:

Value: 5
Description: IP Prefix

According to Section 5.4 of [RFC7606], a node that doesn't recognize
the route type 5 (RT-5) will ignore it. Therefore, an NVE following
this document can still be attached to a BD where an NVE ignoring RT-
5s is attached. Regular procedures described in [RFC7432] would
apply in that case for both NVEs. In case two or more NVEs are
attached to different BDs of the same tenant, they MUST support the
RT-5 for the proper inter-subnet forwarding operation of the tenant.

The detailed encoding of this route and associated procedures are
described in the following sections.

3.1. IP Prefix Route Encoding

An IP Prefix route type for IPv4 has the Length field set to 34 and
consists of the following fields:

+---------------------------------------+
| RD (8 octets) |
+---------------------------------------+
|Ethernet Segment Identifier (10 octets)|
+---------------------------------------+
| Ethernet Tag ID (4 octets) |
+---------------------------------------+
| IP Prefix Length (1 octet, 0 to 32) |
+---------------------------------------+
| IP Prefix (4 octets) |
+---------------------------------------+
| GW IP Address (4 octets) |
+---------------------------------------+
| MPLS Label (3 octets) |
+---------------------------------------+

Figure 3: EVPN IP Prefix Route NLRI for IPv4

An IP Prefix route type for IPv6 has the Length field set to 58 and
consists of the following fields:

+---------------------------------------+
| RD (8 octets) |
+---------------------------------------+
|Ethernet Segment Identifier (10 octets)|
+---------------------------------------+
| Ethernet Tag ID (4 octets) |
+---------------------------------------+
| IP Prefix Length (1 octet, 0 to 128) |
+---------------------------------------+
| IP Prefix (16 octets) |
+---------------------------------------+
| GW IP Address (16 octets) |
+---------------------------------------+
| MPLS Label (3 octets) |
+---------------------------------------+

Figure 4: EVPN IP Prefix Route NLRI for IPv6

Where:

* The Length field of the BGP EVPN NLRI for an EVPN IP Prefix route
MUST be either 34 (if IPv4 addresses are carried) or 58 (if IPv6
addresses are carried). The IP prefix and gateway IP address MUST
be from the same IP address family.

* The Route Distinguisher (RD) and Ethernet Tag ID MUST be used as
defined in [RFC7432] and [RFC8365]. In particular, the RD is
unique per MAC-VRF (or IP-VRF). The MPLS Label field is set to
either an MPLS label or a VNI, as described in [RFC8365] for other
EVPN route types.

* The Ethernet Segment Identifier MUST be a non-zero 10-octet
identifier if the ESI is used as an Overlay Index (see the
definition of "Overlay Index" in Section 3.2). It MUST be all
bytes zero otherwise. The ESI format is described in [RFC7432].

* The IP prefix length can be set to a value between 0 and 32 (bits)
for IPv4 and between 0 and 128 for IPv6, and it specifies the
number of bits in the prefix. The value MUST NOT be greater than
128.

* The IP prefix is a 4- or 16-octet field (IPv4 or IPv6).

* The GW IP Address field is a 4- or 16-octet field (IPv4 or IPv6)
and will encode a valid IP address as an Overlay Index for the IP
prefixes. The GW IP field MUST be all bytes zero if it is not
used as an Overlay Index. Refer to Section 3.2 for the definition
and use of the Overlay Index.

* The MPLS Label field is encoded as 3 octets, where the high-order
20 bits contain the label value, as per [RFC7432]. When sending,
the label value SHOULD be zero if a recursive resolution based on
an Overlay Index is used. If the received MPLS label value is
zero, the route MUST contain an Overlay Index, and the ingress
NVE/PE MUST perform a recursive resolution to find the egress NVE/
PE. If the received label is zero and the route does not contain
an Overlay Index, it MUST be "treat as withdraw" [RFC7606].

The RD, Ethernet Tag ID, IP prefix length, and IP prefix are part of
the route key used by BGP to compare routes. The rest of the fields
are not part of the route key.

An IP Prefix route MAY be sent along with an EVPN Router's MAC
Extended Community (defined in [RFC9135]) to carry the MAC address
that is used as the Overlay Index. Note that the MAC address may be
that of a TS.

As described in Section 3.2, certain data combinations in a received
route would imply a treat-as-withdraw handling of the route
[RFC7606].

3.2. Overlay Indexes and Recursive Lookup Resolution

RT-5 routes support recursive lookup resolution through the use of
Overlay Indexes as follows:

* An Overlay Index can be an ESI or IP address in the address space
of the tenant or MAC address, and it is used by an NVE as the next
hop for a given IP prefix. An Overlay Index always needs a
recursive route resolution on the NVE/PE that installs the RT-5
into one of its IP-VRFs so that the NVE knows to which egress NVE/
PE it needs to forward the packets. It is important to note that
recursive resolution of the Overlay Index applies upon
installation into an IP-VRF and not upon BGP propagation (for
instance, on an ASBR). Also, as a result of the recursive
resolution, the egress NVE/PE is not necessarily the same NVE that
originated the RT-5.

* The Overlay Index is indicated along with the RT-5 in the ESI
field, GW IP field, or EVPN Router's MAC Extended Community,
depending on whether the IP prefix next hop is an ESI, an IP
address, or a MAC address in the tenant space. The Overlay Index
for a given IP prefix is set by local policy at the NVE that
originates an RT-5 for that IP prefix (typically managed by the
cloud management system).

* In order to enable the recursive lookup resolution at the ingress
NVE, an NVE that is a possible egress NVE for a given Overlay
Index must originate a route advertising itself as the BGP next
hop on the path to the system denoted by the Overlay Index. For
instance:

- If an NVE receives an RT-5 that specifies an Overlay Index, the
NVE cannot use the RT-5 in its IP-VRF unless (or until) it can
recursively resolve the Overlay Index.

- If the RT-5 specifies an ESI as the Overlay Index, a recursive
resolution can only be done if the NVE has received and
installed an RT-1 (auto-discovery per EVI) route specifying
that ESI.

- If the RT-5 specifies a GW IP address as the Overlay Index, a
recursive resolution can only be done if the NVE has received
and installed an RT-2 (MAC/IP Advertisement route) specifying
that IP address in the IP Address field of its NLRI.

- If the RT-5 specifies a MAC address as the Overlay Index, a
recursive resolution can only be done if the NVE has received
and installed an RT-2 (MAC/IP Advertisement route) specifying
that MAC address in the MAC Address field of its NLRI.

Note that the RT-1 or RT-2 routes needed for the recursive
resolution may arrive before or after the given RT-5 route.

* Irrespective of the recursive resolution, if there is no IGP or
BGP route to the BGP next hop of an RT-5, BGP MUST NOT install the
RT-5 even if the Overlay Index can be resolved.

* The ESI and GW IP fields may both be zero at the same time.
However, they MUST NOT both be non-zero at the same time. A route
containing a non-zero GW IP and a non-zero ESI (at the same time)
SHOULD be treat as withdraw [RFC7606].

* If either the ESI or the GW IP are non-zero, then the non-zero one
is the Overlay Index, regardless of whether the EVPN Router's MAC
Extended Community is present or the value of the label. In case
the GW IP is the Overlay Index (hence, ESI is zero), the EVPN
Router's MAC Extended Community is ignored if present.

* A route where ESI, GW IP, MAC, and Label are all zero at the same
time SHOULD be treat as withdraw.

The indirection provided by the Overlay Index and its recursive
lookup resolution is required to achieve fast convergence in case of
a failure of the object represented by the Overlay Index (see the
example described in Section 2.2).

Table 1 shows the different RT-5 field combinations allowed by this
specification and what Overlay Index must be used by the receiving
NVE/PE in each case. Cases where there is no Overlay Index are
indicated as "None" in Table 1. If there is no Overlay Index, the
receiving NVE/PE will not perform any recursive resolution, and the
actual next hop is given by the RT-5's BGP next hop.

+==========+==========+==========+============+===============+
| ESI | GW IP | MAC* | Label | Overlay Index |
+==========+==========+==========+============+===============+
| Non-Zero | Zero | Zero | Don't Care | ESI |
+----------+----------+----------+------------+---------------+
| Non-Zero | Zero | Non-Zero | Don't Care | ESI |
+----------+----------+----------+------------+---------------+
| Zero | Non-Zero | Zero | Don't Care | GW IP |
+----------+----------+----------+------------+---------------+
| Zero | Zero | Non-Zero | Zero | MAC |
+----------+----------+----------+------------+---------------+
| Zero | Zero | Non-Zero | Non-Zero | MAC or None** |
+----------+----------+----------+------------+---------------+
| Zero | Zero | Zero | Non-Zero | None*** |
+----------+----------+----------+------------+---------------+

Table 1: RT-5 Fields and Indicated Overlay Index

Table Notes:

* MAC with "Zero" value means no EVPN Router's MAC Extended
Community is present along with the RT-5. "Non-Zero" indicates
that the extended community is present and carries a valid MAC
address. The encoding of a MAC address MUST be the 6-octet MAC
address specified by [IEEE-802.1Q]. Examples of invalid MAC
addresses are broadcast or multicast MAC addresses. The route
MUST be treat as withdraw in case of an invalid MAC address.
The presence of the EVPN Router's MAC Extended Community alone
is not enough to indicate the use of the MAC address as the
Overlay Index since the extended community can be used for
other purposes.

** In this case, the Overlay Index may be the RT-5's MAC address
or "None", depending on the local policy of the receiving NVE/
PE. Note that the advertising NVE/PE that sets the Overlay
Index SHOULD advertise an RT-2 for the MAC Overlay Index if
there are receiving NVE/PEs configured to use the MAC as the
Overlay Index. This case in Table 1 is used in the IP-VRF-to-
IP-VRF implementations described in Sections 4.4.1 and 4.4.3.
The support of a MAC Overlay Index in this model is OPTIONAL.

*** The Overlay Index is "None". This is a special case used for
IP-VRF-to-IP-VRF where the NVE/PEs are connected by IP NVO
tunnels as opposed to Ethernet NVO tunnels.

If the combination of ESI, GW IP, MAC, and Label in the receiving
RT-5 is different than the combinations shown in Table 1, the router
will process the route as per the rules described at the beginning of
this section (Section 3.2).

Table 2 shows the different inter-subnet use cases described in this
document and the corresponding coding of the Overlay Index in the
route type 5 (RT-5).

+=========+=====================+===========================+
| Section | Use Case | Overlay Index in the RT-5 |
+=========+=====================+===========================+
| 4.1 | TS IP address | GW IP |
+---------+---------------------+---------------------------+
| 4.2 | Floating IP address | GW IP |
+---------+---------------------+---------------------------+
| 4.3 | "Bump-in-the-wire" | ESI or MAC |
+---------+---------------------+---------------------------+
| 4.4 | IP-VRF-to-IP-VRF | GW IP, MAC, or None |
+---------+---------------------+---------------------------+

Table 2: Use Cases and Overlay Indexes for Recursive
Resolution

The above use cases are representative of the different Overlay
Indexes supported by the RT-5 (GW IP, ESI, MAC, or None).

4. Overlay Index Use Cases

This section describes some use cases for the Overlay Index types
used with the IP Prefix route. Although the examples use IPv4
prefixes and subnets, the descriptions of the RT-5 are valid for the
same cases with IPv6, except that IP Prefixes, IPL, and GW IP are
replaced by the corresponding IPv6 values.

4.1. TS IP Address Overlay Index Use Case

Figure 5 illustrates an example of inter-subnet forwarding for
subnets sitting behind VAs (on TS2 and TS3).

IP4---+ NVE2 DGW1
| +-----------+ +---------+ +-------------+
SN2---TS2(VA)--| (BD-10) |-| |----| (BD-10) |
| M2/IP2 +-----------+ | | | IRB1\ |
-+---+ | | | (IP-VRF)|---+
| | | +-------------+ _|_
SN1 | VXLAN/ | ( )
| | GENEVE | DGW2 ( WAN )
-+---+ NVE3 | | +-------------+ (___)
| M3/IP3 +-----------+ | |----| (BD-10) | |
SN3---TS3(VA)--| (BD-10) |-| | | IRB2\ | |
| +-----------+ +---------+ | (IP-VRF)|---+
IP5---+ +-------------+

Figure 5: TS IP Address Use Case

An example of inter-subnet forwarding between subnet SN1, which uses
a 24-bit IP prefix (written as SN1/24 in the future), and a subnet
sitting in the WAN is described below. NVE2, NVE3, DGW1, and DGW2
are running BGP EVPN. TS2 and TS3 do not participate in dynamic
routing protocols, and they only have a static route to forward the
traffic to the WAN. SN1/24 is dual-homed to NVE2 and NVE3.

In this case, a GW IP is used as an Overlay Index. Although a
different Overlay Index type could have been used, this use case
assumes that the operator knows the VA's IP addresses beforehand,
whereas the VA's MAC address is unknown and the VA's ESI is zero.
Because of this, the GW IP is the suitable Overlay Index to be used
with the RT-5s. The NVEs know the GW IP to be used for a given
prefix by policy.

(1) NVE2 advertises the following BGP routes on behalf of TS2:

* Route type 2 (MAC/IP Advertisement route) containing: ML = 48
(MAC address length), M = M2 (MAC address), IPL = 32 (IP
prefix length), IP = IP2, and BGP Encapsulation Extended
Community [RFC9012] with the corresponding tunnel type. The
MAC and IP addresses may be learned via ARP snooping.

* Route type 5 (IP Prefix route) containing: IPL = 24, IP =
SN1, ESI = 0, and GW IP address = IP2. The prefix and GW IP
are learned by policy.

(2) Similarly, NVE3 advertises the following BGP routes on behalf of
TS3:

* Route type 2 (MAC/IP Advertisement route) containing: ML =
48, M = M3, IPL = 32, IP = IP3 (and BGP Encapsulation
Extended Community).

* Route type 5 (IP Prefix route) containing: IPL = 24, IP =
SN1, ESI = 0, and GW IP address = IP3.

(3) DGW1 and DGW2 import both received routes based on the Route
Targets:

* Based on the BD-10 Route Target in DGW1 and DGW2, the MAC/IP
Advertisement route is imported, and M2 is added to the BD-10
along with its corresponding tunnel information. For
instance, if VXLAN is used, the VTEP will be derived from the
MAC/IP Advertisement route BGP next hop and VNI from the MPLS
Label1 field. M2/IP2 is added to the ARP table. Similarly,
M3 is added to BD-10, and M3/IP3 is added to the ARP table.

* Based on the BD-10 Route Target in DGW1 and DGW2, the IP
Prefix route is also imported, and SN1/24 is added to the IP-
VRF with Overlay Index IP2 pointing at the local BD-10. In
this example, it is assumed that the RT-5 from NVE2 is
preferred over the RT-5 from NVE3. If both routes were
equally preferable and ECMP enabled, SN1/24 would also be
added to the routing table with Overlay Index IP3.

(4) When DGW1 receives a packet from the WAN with destination IPx,
where IPx belongs to SN1/24:

* A destination IP lookup is performed on the DGW1 IP-VRF
table, and Overlay Index = IP2 is found. Since IP2 is an
Overlay Index, a recursive route resolution is required for
IP2.

* IP2 is resolved to M2 in the ARP table, and M2 is resolved to
the tunnel information given by the BD FIB (e.g., remote VTEP
and VNI for the VXLAN case).

* The IP packet destined to IPx is encapsulated with:

- Inner source MAC = IRB1 MAC.

- Inner destination MAC = M2.

- Tunnel information provided by the BD (VNI, VTEP IPs, and
MACs for the VXLAN case).

(5) When the packet arrives at NVE2:

* Based on the tunnel information (VNI for the VXLAN case), the
BD-10 context is identified for a MAC lookup.

* Encapsulation is stripped off and, based on a MAC lookup
(assuming MAC forwarding on the egress NVE), the packet is
forwarded to TS2, where it will be properly routed.

(6) Should TS2 move from NVE2 to NVE3, MAC Mobility procedures will
be applied to the MAC route M2/IP2, as defined in [RFC7432].
Route type 5 prefixes are not subject to MAC Mobility
procedures; hence, no changes in the DGW IP-VRF table will occur
for TS2 mobility -- i.e., all the prefixes will still be
pointing at IP2 as the Overlay Index. There is an indirection
for, e.g., SN1/24, which still points at Overlay Index IP2 in
the routing table, but IP2 will be simply resolved to a
different tunnel based on the outcome of the MAC Mobility
procedures for the MAC/IP Advertisement route M2/IP2.

Note that in the opposite direction, TS2 will send traffic based on
its static-route next-hop information (IRB1 and/or IRB2), and regular
EVPN procedures will be applied.

4.2. Floating IP Overlay Index Use Case

Sometimes TSs work in active/standby mode where an upstream floating
IP owned by the active TS is used as the Overlay Index to get to some
subnets behind the TS. This redundancy mode, already introduced in
Sections 2.1 and 2.2, is illustrated in Figure 6.

NVE2 DGW1
+-----------+ +---------+ +-------------+
+---TS2(VA)--| (BD-10) |-| |----| (BD-10) |
| M2/IP2 +-----------+ | | | IRB1\ |
| <-+ | | | (IP-VRF)|---+
| | | | +-------------+ _|_
SN1 vIP23 (floating) | VXLAN/ | ( )
| | | GENEVE | DGW2 ( WAN )
| <-+ NVE3 | | +-------------+ (___)
| M3/IP3 +-----------+ | |----| (BD-10) | |
+---TS3(VA)--| (BD-10) |-| | | IRB2\ | |
+-----------+ +---------+ | (IP-VRF)|---+
+-------------+

Figure 6: Floating IP Overlay Index for Redundant TS

In this use case, a GW IP is used as an Overlay Index for the same
reasons as in Section 4.1. However, this GW IP is a floating IP that
belongs to the active TS. Assuming TS2 is the active TS and owns
vIP23:

(1) NVE2 advertises the following BGP routes for TS2:

* Route type 2 (MAC/IP Advertisement route) containing: ML =
48, M = M2, IPL = 32, and IP = vIP23 (as well as BGP
Encapsulation Extended Community). The MAC and IP addresses
may be learned via ARP snooping.

* Route type 5 (IP Prefix route) containing: IPL = 24, IP =
SN1, ESI = 0, and GW IP address = vIP23. The prefix and GW
IP are learned by policy.

(2) NVE3 advertises the following BGP route for TS3 (it does not
advertise an RT-2 for M3/vIP23):

* Route type 5 (IP Prefix route) containing: IPL = 24, IP =
SN1, ESI = 0, and GW IP address = vIP23. The prefix and GW
IP are learned by policy.

(3) DGW1 and DGW2 import both received routes based on the Route
Target:

* M2 is added to the BD-10 FIB along with its corresponding
tunnel information. For the VXLAN use case, the VTEP will be
derived from the MAC/IP Advertisement route BGP next hop and
VNI from the VNI field. M2/vIP23 is added to the ARP table.

* SN1/24 is added to the IP-VRF in DGW1 and DGW2 with Overlay
Index vIP23 pointing at M2 in the local BD-10.

(4) When DGW1 receives a packet from the WAN with destination IPx,
where IPx belongs to SN1/24:

* A destination IP lookup is performed on the DGW1 IP-VRF
table, and Overlay Index = vIP23 is found. Since vIP23 is an
Overlay Index, a recursive route resolution for vIP23 is
required.

* vIP23 is resolved to M2 in the ARP table, and M2 is resolved
to the tunnel information given by the BD (remote VTEP and
VNI for the VXLAN case).

* The IP packet destined to IPx is encapsulated with:

- Inner source MAC = IRB1 MAC.

- Inner destination MAC = M2.

- Tunnel information provided by the BD FIB (VNI, VTEP IPs,
and MACs for the VXLAN case).

(5) When the packet arrives at NVE2:

* Based on the tunnel information (VNI for the VXLAN case), the
BD-10 context is identified for a MAC lookup.

* Encapsulation is stripped off and, based on a MAC lookup
(assuming MAC forwarding on the egress NVE), the packet is
forwarded to TS2, where it will be properly routed.

(6) When the redundancy protocol running between TS2 and TS3
appoints TS3 as the new active TS for SN1, TS3 will now own the
floating vIP23 and will signal this new ownership using a
gratuitous ARP REPLY message (explained in [RFC5227]) or
similar. Upon receiving the new owner's notification, NVE3 will
issue a route type 2 for M3/vIP23, and NVE2 will withdraw the
RT-2 for M2/vIP23. DGW1 and DGW2 will update their ARP tables
with the new MAC resolving the floating IP. No changes are made
in the IP-VRF table.

4.3. Bump-in-the-Wire Use Case

Figure 7 illustrates an example of inter-subnet forwarding for an IP
Prefix route that carries subnet SN1. In this use case, TS2 and TS3
are Layer 2 VA devices without any IP addresses that can be included
as an Overlay Index in the GW IP field of the IP Prefix route. Their
MAC addresses are M2 and M3, respectively, and are connected to BD-
10. Note that IRB1 and IRB2 (in DGW1 and DGW2, respectively) have IP
addresses in a subnet different than SN1.

NVE2 DGW1
M2 +-----------+ +---------+ +-------------+
+---TS2(VA)--| (BD-10) |-| |----| (BD-10) |
| ESI23 +-----------+ | | | IRB1\ |
| + | | | (IP-VRF)|---+
| | | | +-------------+ _|_
SN1 | | VXLAN/ | ( )
| | | GENEVE | DGW2 ( WAN )
| + NVE3 | | +-------------+ (___)
| ESI23 +-----------+ | |----| (BD-10) | |
+---TS3(VA)--| (BD-10) |-| | | IRB2\ | |
M3 +-----------+ +---------+ | (IP-VRF)|---+
+-------------+

Figure 7: Bump-in-the-Wire Use Case

Since TS2 and TS3 cannot participate in any dynamic routing protocol
and neither has an IP address assigned, there are two potential
Overlay Index types that can be used when advertising SN1:

a) an ESI, i.e., ESI23, that can be provisioned on the attachment
ports of NVE2 and NVE3, as shown in Figure 7 or

b) the VA's MAC address, which can be added to NVE2 and NVE3 by
policy.

The advantage of using an ESI as the Overlay Index as opposed to the
VA's MAC address is that the forwarding to the egress NVE can be done
purely based on the state of the AC in the Ethernet segment (notified
by the Ethernet A-D per EVI route), and all the EVPN multihoming
redundancy mechanisms can be reused. For instance, the mass
withdrawal mechanism described in [RFC7432] for fast failure
detection and propagation can be used. It is assumed per this
section that an ESI Overlay Index is used in this use case, but this
use case does not preclude the use of the VA's MAC address as an
Overlay Index. If a MAC is used as the Overlay Index, the control
plane must follow the procedures described in Section 4.4.3.

The model supports VA redundancy in a similar way to the one
described in Section 4.2 for the floating IP Overlay Index use case,
except that it uses the EVPN Ethernet A-D per EVI route instead of
the MAC advertisement route to advertise the location of the Overlay
Index. The procedure is explained below:

(1) Assuming TS2 is the active TS in ESI23, NVE2 advertises the
following BGP routes:

* Route type 1 (Ethernet A-D route for BD-10) containing: ESI =
ESI23 and the corresponding tunnel information (VNI field),
as well as the BGP Encapsulation Extended Community as per
[RFC8365].

* Route type 5 (IP Prefix route) containing: IPL = 24, IP =
SN1, ESI = ESI23, and GW IP address = 0. The EVPN Router's
MAC Extended Community defined in [RFC9135] is added and
carries the MAC address (M2) associated with the TS behind
which SN1 sits. M2 may be learned by policy; however, the
MAC in the Extended Community is preferred if sent with the
route.

(2) NVE3 advertises the following BGP route for TS3 (no AD per EVI
route is advertised):

* Route type 5 (IP Prefix route) containing: IPL = 24, IP =
SN1, ESI = 23, and GW IP address = 0. The EVPN Router's MAC
Extended Community is added and carries the MAC address (M3)
associated with the TS behind which SN1 sits. M3 may be
learned by policy; however, the MAC in the Extended Community
is preferred if sent with the route.

(3) DGW1 and DGW2 import the received routes based on the Route
Target:

* The tunnel information to get to ESI23 is installed in DGW1
and DGW2. For the VXLAN use case, the VTEP will be derived
from the Ethernet A-D route BGP next hop and VNI from the
VNI/VSID field (see [RFC8365]).

* The RT-5 coming from the NVE that advertised the RT-1 is
selected, and SN1/24 is added to the IP-VRF in DGW1 and DGW2
with Overlay Index ESI23 and MAC = M2.

(4) When DGW1 receives a packet from the WAN with destination IPx,
where IPx belongs to SN1/24:

* A destination IP lookup is performed on the DGW1 IP-VRF
table, and Overlay Index = ESI23 is found. Since ESI23 is an
Overlay Index, a recursive route resolution is required to
find the egress NVE where ESI23 resides.

* The IP packet destined to IPx is encapsulated with:

- Inner source MAC = IRB1 MAC.

- Inner destination MAC = M2 (this MAC will be obtained from
the EVPN Router's MAC Extended Community received along
with the RT-5 for SN1). Note that the EVPN Router's MAC
Extended Community is used in this case to carry the TS's
MAC address, as opposed to the MAC address of the NVE/PE.

- Tunnel information for the NVO tunnel is provided by the
Ethernet A-D route per EVI for ESI23 (VNI and VTEP IP for
the VXLAN case).

(5) When the packet arrives at NVE2:

* Based on the tunnel demultiplexer information (VNI for the
VXLAN case), the BD-10 context is identified for a MAC lookup
(assuming a MAC-based disposition model [RFC7432]), or the
VNI may directly identify the egress interface (for an MPLS-
based disposition model, which in this context is a VNI-based
disposition model).

* Encapsulation is stripped off and, based on a MAC lookup
(assuming MAC forwarding on the egress NVE) or a VNI lookup
(in case of VNI forwarding), the packet is forwarded to TS2,
where it will be forwarded to SN1.

(6) If the redundancy protocol running between TS2 and TS3 follows
an active/standby model and there is a failure, TS3 is appointed
as the new active TS for SN1. TS3 will now own the connectivity
to SN1 and will signal this new ownership. Upon receiving the
new owner's notification, NVE3's AC will become active and issue
a route type 1 for ESI23, whereas NVE2 will withdraw its
Ethernet A-D route for ESI23. DGW1 and DGW2 will update their
tunnel information to resolve ESI23. The inner destination MAC
will be changed to M3.

4.4. IP-VRF-to-IP-VRF Model

This use case is similar to the scenario described in Section 9.1 of
[RFC9135]; however, the new requirement here is the advertisement of
IP prefixes as opposed to only host routes.

In the examples described in Sections 4.1, 4.2, and 4.3, the BD
instance can connect IRB interfaces and any other Tenant Systems
connected to it. EVPN provides connectivity for:

1. Traffic destined to the IRB or TS IP interfaces, as well as

2. Traffic destined to IP subnets sitting behind the TS, e.g., SN1
or SN2.

In order to provide connectivity for (1), MAC/IP Advertisement routes
(RT-2) are needed so that IRB or TS MACs and IPs can be distributed.
Connectivity type (2) is accomplished by the exchange of IP Prefix
routes (RT-5) for IPs and subnets sitting behind certain Overlay
Indexes, e.g., GW IP, ESI, or TS MAC.

In some cases, IP Prefix routes may be advertised for subnets and IPs
sitting behind an IRB. This use case is referred to as the "IP-VRF-
to-IP-VRF" model.

[RFC9135] defines an asymmetric IRB model and a symmetric IRB model
based on the required lookups at the ingress and egress NVE. The
asymmetric model requires an IP lookup and a MAC lookup at the
ingress NVE, whereas only a MAC lookup is needed at the egress NVE;
the symmetric model requires IP and MAC lookups at both the ingress
and egress NVE. From that perspective, the IP-VRF-to-IP-VRF use case
described in this section is a symmetric IRB model.

Note that in an IP-VRF-to-IP-VRF scenario, out of the many subnets
that a tenant may have, it may be the case that only a few are
attached to a given IP-VRF of the NVE/PE. In order to provide inter-
subnet connectivity among the set of NVE/PEs where the tenant is
connected, a new SBD is created on all of them if a recursive
resolution is needed. This SBD is instantiated as a regular BD (with
no ACs) in each NVE/PE and has an IRB interface that connects the SBD
to the IP-VRF. The IRB interface's IP or MAC address is used as the
Overlay Index for a recursive resolution.

Depending on the existence and characteristics of the SBD and IRB
interfaces for the IP-VRFs, there are three different IP-VRF-to-IP-
VRF scenarios identified and described in this document:

1. Interface-less model: no SBD and no Overlay Indexes required.

2. Interface-ful with an SBD IRB model: requires SBD as well as GW
IP addresses as Overlay Indexes.

3. Interface-ful with an unnumbered SBD IRB model: requires SBD as
well as MAC addresses as Overlay Indexes.

Inter-subnet IP multicast is outside the scope of this document.

4.4.1. Interface-less IP-VRF-to-IP-VRF Model

Figure 8 depicts the Interface-less IP-VRF-to-IP-VRF model.

NVE1(M1)
+------------+
IP1+----| (BD-1) | DGW1(M3)
| \ | +---------+ +--------+
| (IP-VRF)|----| |-|(IP-VRF)|----+
| / | | | +--------+ |
+---| (BD-2) | | | _+_
| +------------+ | | ( )
SN1| | VXLAN/ | ( WAN )--H1
| NVE2(M2) | GENEVE/| (___)
| +------------+ | MPLS | +
+---| (BD-2) | | | DGW2(M4) |
| \ | | | +--------+ |
| (IP-VRF)|----| |-|(IP-VRF)|----+
| / | +---------+ +--------+
SN2+----| (BD-3) |
+------------+

Figure 8: Interface-less IP-VRF-to-IP-VRF Model

In this case:

a) The NVEs and DGWs must provide connectivity between hosts in SN1,
SN2, and IP1 and hosts sitting at the other end of the WAN -- for
example, H1. It is assumed that the DGWs import/export IP and/or
VPN-IP routes to/from the WAN.

b) The IP-VRF instances in the NVE/DGWs are directly connected
through NVO tunnels, and no IRBs and/or BD instances are
instantiated to connect the IP-VRFs.

c) The solution must provide Layer 3 connectivity among the IP-VRFs
for Ethernet NVO tunnels -- for instance, VXLAN or GENEVE.

d) The solution may provide Layer 3 connectivity among the IP-VRFs
for IP NVO tunnels -- for example, GENEVE (with IP payload).

In order to meet the above requirements, the EVPN route type 5 will
be used to advertise the IP prefixes, along with the EVPN Router's
MAC Extended Community as defined in [RFC9135] if the advertising
NVE/DGW uses Ethernet NVO tunnels. Each NVE/DGW will advertise an
RT-5 for each of its prefixes with the following fields:

* RD as per [RFC7432].

* Ethernet Tag ID = 0.

* IP prefix length and IP address, as explained in the previous
sections.

* GW IP address = 0.

* ESI = 0.

* MPLS label or VNI corresponding to the IP-VRF.

Each RT-5 will be sent with a Route Target identifying the tenant
(IP-VRF) and may be sent with two BGP extended communities:

* The first one is the BGP Encapsulation Extended Community, as per
[RFC9012], identifying the tunnel type.

* The second one is the EVPN Router's MAC Extended Community, as per
[RFC9135], containing the MAC address associated with the NVE
advertising the route. This MAC address identifies the NVE/DGW
and MAY be reused for all the IP-VRFs in the NVE. The EVPN
Router's MAC Extended Community must be sent if the route is
associated with an Ethernet NVO tunnel -- for instance, VXLAN. If
the route is associated with an IP NVO tunnel -- for instance,
GENEVE with an IP payload -- the EVPN Router's MAC Extended
Community should not be sent.

The following example illustrates the procedure to advertise and
forward packets to SN1/24 (IPv4 prefix advertised from NVE1):

(1) NVE1 advertises the following BGP route:

* Route type 5 (IP Prefix route) containing:

- IPL = 24, IP = SN1, Label = 10.

- GW IP = set to 0.

- BGP Encapsulation Extended Community [RFC9012].

- EVPN Router's MAC Extended Community that contains M1.

- Route Target identifying the tenant (IP-VRF).

(2) DGW1 imports the received routes from NVE1:

* DGW1 installs SN1/24 in the IP-VRF identified by the RT-5
Route Target.

* Since GW IP = ESI = 0, the label is a non-zero value, and the
local policy indicates this interface-less model, DGW1, will
use the label and next hop of the RT-5, as well as the MAC
address conveyed in the EVPN Router's MAC Extended Community
(as the inner destination MAC address) to set up the
forwarding state and later encapsulate the routed IP packets.

(3) When DGW1 receives a packet from the WAN with destination IPx,
where IPx belongs to SN1/24:

* A destination IP lookup is performed on the DGW1 IP-VRF
table. The lookup yields SN1/24.

* Since the RT-5 for SN1/24 had a GW IP = ESI = 0, a non-zero
label, and a next hop, and since the model is interface-less,
DGW1 will not need a recursive lookup to resolve the route.

* The IP packet destined to IPx is encapsulated with: inner
source MAC = DGW1 MAC, inner destination MAC = M1, outer
source IP (tunnel source IP) = DGW1 IP, and outer destination
IP (tunnel destination IP) = NVE1 IP. The source and inner
destination MAC addresses are not needed if IP NVO tunnels
are used.

(4) When the packet arrives at NVE1:

* NVE1 will identify the IP-VRF for an IP lookup based on the
label (the inner destination MAC is not needed to identify
the IP-VRF).

* An IP lookup is performed in the routing context, where SN1
turns out to be a local subnet associated with BD-2. A
subsequent lookup in the ARP table and the BD FIB will
provide the forwarding information for the packet in BD-2.

The model described above is called an "interface-less" model since
the IP-VRFs are connected directly through tunnels, and they don't
require those tunnels to be terminated in SBDs instead, as in
Sections 4.4.2 or 4.4.3.

4.4.2. Interface-ful IP-VRF-to-IP-VRF with SBD IRB

Figure 9 depicts the Interface-ful IP-VRF-to-IP-VRF with SBD IRB
model.

NVE1
+------------+ DGW1
IP10+---+(BD-1) | +---------------+ +------------+
| \ | | | | |
|(IP-VRF)-(SBD)| |(SBD)-(IP-VRF)|-----+
| / IRB(M1/IP1) IRB(M3/IP3) | |
+---+(BD-2) | | | +------------+ _+_
| +------------+ | | ( )
SN1| | VXLAN/ | ( WAN )--H1
| NVE2 | GENEVE/ | (___)
| +------------+ | MPLS | DGW2 +
+---+(BD-2) | | | +------------+ |
| \ | | | | | |
|(IP-VRF)-(SBD)| |(SBD)-(IP-VRF)|-----+
| / IRB(M2/IP2) IRB(M4/IP4) |
SN2+----+(BD-3) | +---------------+ +------------+
+------------+

Figure 9: Interface-ful with SBD IRB Model

In this model:

a) As in Section 4.4.1, the NVEs and DGWs must provide connectivity
between hosts in SN1, SN2, and IP10 and in hosts sitting at the
other end of the WAN.

b) However, the NVE/DGWs are now connected through Ethernet NVO
tunnels terminated in the SBD instance. The IP-VRFs use IRB
interfaces for their connectivity to the SBD.

c) Each SBD IRB has an IP and a MAC address, where the IP address
must be reachable from other NVEs or DGWs.

d) The SBD is attached to all the NVE/DGWs in the tenant domain BDs.

e) The solution must provide Layer 3 connectivity for Ethernet NVO
tunnels -- for instance, VXLAN or GENEVE (with Ethernet payload).

EVPN type 5 routes will be used to advertise the IP prefixes, whereas
EVPN RT-2 routes will advertise the MAC/IP addresses of each SBD IRB
interface. Each NVE/DGW will advertise an RT-5 for each of its
prefixes with the following fields:

* RD as per [RFC7432].

* Ethernet Tag ID = 0.

* IP prefix length and IP address, as explained in the previous
sections.

* GW IP address = IRB-IP of the SBD (this is the Overlay Index that
will be used for the recursive route resolution).

* ESI = 0.

* Label value should be zero since the RT-5 route requires a
recursive lookup resolution to an RT-2 route. It is ignored on
reception, and the MPLS label or VNI from the RT-2's MPLS Label1
field is used when forwarding packets.

Each RT-5 will be sent with a Route Target identifying the tenant
(IP-VRF). The EVPN Router's MAC Extended Community should not be
sent in this case.

The following example illustrates the procedure to advertise and
forward packets to SN1/24 (IPv4 prefix advertised from NVE1):

(1) NVE1 advertises the following BGP routes:

* Route type 5 (IP Prefix route) containing:

- IPL = 24, IP = SN1, Label = SHOULD be set to 0.

- GW IP = IP1 (SBD IRB's IP).

- Route Target identifying the tenant (IP-VRF).

* Route type 2 (MAC/IP Advertisement route for the SBD IRB)
containing:

- ML = 48, M = M1, IPL = 32, IP = IP1, Label = 10.

- A BGP Encapsulation Extended Community [RFC9012].

- Route Target identifying the SBD. This Route Target may
be the same as the one used with the RT-5.

(2) DGW1 imports the received routes from NVE1:

* DGW1 installs SN1/24 in the IP-VRF identified by the RT-5
Route Target.

- Since GW IP is different from zero, the GW IP (IP1) will
be used as the Overlay Index for the recursive route
resolution to the RT-2 carrying IP1.

(3) When DGW1 receives a packet from the WAN with destination IPx,
where IPx belongs to SN1/24:

* A destination IP lookup is performed on the DGW1 IP-VRF
table. The lookup yields SN1/24, which is associated with
the Overlay Index IP1. The forwarding information is derived
from the RT-2 received for IP1.

* The IP packet destined to IPx is encapsulated with: inner
source MAC = M3, inner destination MAC = M1, outer source IP
(source VTEP) = DGW1 IP, and outer destination IP
(destination VTEP) = NVE1 IP.

(4) When the packet arrives at NVE1:

* NVE1 will identify the IP-VRF for an IP lookup based on the
label and the inner MAC DA.

* An IP lookup is performed in the routing context, where SN1
turns out to be a local subnet associated with BD-2. A
subsequent lookup in the ARP table and the BD FIB will
provide the forwarding information for the packet in BD-2.

The model described above is called an "interface-ful with SBD IRB"
model because the tunnels connecting the DGWs and NVEs need to be
terminated into the SBD. The SBD is connected to the IP-VRFs via SBD
IRB interfaces, and that allows the recursive resolution of RT-5s to
GW IP addresses.

4.4.3. Interface-ful IP-VRF-to-IP-VRF with Unnumbered SBD IRB

Figure 10 depicts the Interface-ful IP-VRF-to-IP-VRF with unnumbered
SBD IRB model. Note that this model is similar to the one described
in Section 4.4.2, only without IP addresses on the SBD IRB
interfaces.

NVE1
+------------+ DGW1
IP1+----+(BD-1) | +---------------+ +------------+
| \ | | | | |
|(IP-VRF)-(SBD)| (SBD)-(IP-VRF) |-----+
| / IRB(M1)| | IRB(M3) | |
+---+(BD-2) | | | +------------+ _+_
| +------------+ | | ( )
SN1| | VXLAN/ | ( WAN )--H1
| NVE2 | GENEVE/ | (___)
| +------------+ | MPLS | DGW2 +
+---+(BD-2) | | | +------------+ |
| \ | | | | | |
|(IP-VRF)-(SBD)| (SBD)-(IP-VRF) |-----+
| / IRB(M2)| | IRB(M4) |
SN2+----+(BD-3) | +---------------+ +------------+
+------------+

Figure 10: Interface-ful with Unnumbered SBD IRB Model

In this model:

a) As in Sections 4.4.1 and 4.4.2, the NVEs and DGWs must provide
connectivity between hosts in SN1, SN2, and IP1 and in hosts
sitting at the other end of the WAN.

b) As in Section 4.4.2, the NVE/DGWs are connected through Ethernet
NVO tunnels terminated in the SBD instance. The IP-VRFs use IRB
interfaces for their connectivity to the SBD.

c) However, each SBD IRB has a MAC address only and no IP address
(which is why the model refers to an "unnumbered" SBD IRB). In
this model, there is no need to have IP reachability to the SBD
IRB interfaces themselves, and there is a requirement to limit
the number of IP addresses used.

d) As in Section 4.4.2, the SBD is composed of all the NVE/DGW BDs
of the tenant that need inter-subnet forwarding.

e) As in Section 4.4.2, the solution must provide Layer 3
connectivity for Ethernet NVO tunnels -- for instance, VXLAN or
GENEVE (with Ethernet payload).

This model will also make use of the RT-5 recursive resolution. EVPN
type 5 routes will advertise the IP prefixes along with the EVPN
Router's MAC Extended Community used for the recursive lookup,
whereas EVPN RT-2 routes will advertise the MAC addresses of each SBD
IRB interface (this time without an IP).

Each NVE/DGW will advertise an RT-5 for each of its prefixes with the
same fields as described in Section 4.4.2, except:

* GW IP address = set to 0.

Each RT-5 will be sent with a Route Target identifying the tenant
(IP-VRF) and the EVPN Router's MAC Extended Community containing the
MAC address associated with the SBD IRB interface. This MAC address
may be reused for all the IP-VRFs in the NVE.

The example is similar to the one in Section 4.4.2:

(1) NVE1 advertises the following BGP routes:

* Route type 5 (IP Prefix route) containing the same values as
in the example in Section 4.4.2, except:

- GW IP = SHOULD be set to 0.

- EVPN Router's MAC Extended Community containing M1 (this
will be used for the recursive lookup to an RT-2).

* Route type 2 (MAC route for the SBD IRB) with the same values
as in Section 4.4.2, except:

- ML = 48, M = M1, IPL = 0, Label = 10.

(2) DGW1 imports the received routes from NVE1:

* DGW1 installs SN1/24 in the IP-VRF identified by the RT-5
Route Target.

- The MAC contained in the EVPN Router's MAC Extended
Community sent along with the RT-5 (M1) will be used as
the Overlay Index for the recursive route resolution to
the RT-2 carrying M1.

(3) When DGW1 receives a packet from the WAN with destination IPx,
where IPx belongs to SN1/24:

* A destination IP lookup is performed on the DGW1 IP-VRF
table. The lookup yields SN1/24, which is associated with
the Overlay Index M1. The forwarding information is derived
from the RT-2 received for M1.

* The IP packet destined to IPx is encapsulated with: inner
source MAC = M3, inner destination MAC = M1, outer source IP
(source VTEP) = DGW1 IP, and outer destination IP
(destination VTEP) = NVE1 IP.

(4) When the packet arrives at NVE1:

* NVE1 will identify the IP-VRF for an IP lookup based on the
label and the inner MAC DA.

* An IP lookup is performed in the routing context, where SN1
turns out to be a local subnet associated with BD-2. A
subsequent lookup in the ARP table and the BD FIB will
provide the forwarding information for the packet in BD-2.

The model described above is called an "interface-ful with unnumbered
SBD IRB" model (as in Section 4.4.2) but without the SBD IRB having
an IP address.

5. Security Considerations

This document provides a set of procedures to achieve inter-subnet
forwarding across NVEs or PEs attached to a group of BDs that belong
to the same tenant (or VPN). The security considerations discussed
in [RFC7432] apply to the intra-subnet forwarding or communication
within each of those BDs. In addition, the security considerations
in [RFC4364] should also be understood, since this document and
[RFC4364] may be used in similar applications.

Contrary to [RFC4364], this document does not describe PE/CE route
distribution techniques but rather considers the CEs as TSs or VAs
that do not run dynamic routing protocols. This can be considered a
security advantage, since dynamic routing protocols can be blocked on
the NVE/PE ACs, not allowing the tenant to interact with the
infrastructure's dynamic routing protocols.

In this document, the RT-5 may use a regular BGP next hop for its
resolution or an Overlay Index that requires a recursive resolution
to a different EVPN route (an RT-2 or an RT-1). In the latter case,
it is worth noting that any action that ends up filtering or
modifying the RT-2 or RT-1 routes used to convey the Overlay Indexes
will modify the resolution of the RT-5 and therefore the forwarding
of packets to the remote subnet.

6. IANA Considerations

IANA has registered value 5 in the "EVPN Route Types" registry
[EVPNRouteTypes] defined by [RFC7432] as follows:

+=======+=============+===========+
| Value | Description | Reference |
+=======+=============+===========+
| 5 | IP Prefix | RFC 9136 |
+-------+-------------+-----------+

Table 3

7. References

7.1. Normative References

[EVPNRouteTypes]
IANA, "EVPN Route Types",
<https://www.iana.org/assignments/evpn>.

[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>.

[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>.

[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>.

[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>.

[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.

[RFC9135] Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in Ethernet VPN
(EVPN)", RFC 9135, DOI 10.17487/RFC9135, October 2021,
<https://www.rfc-editor.org/info/rfc9135>.

7.2. Informative References

[IEEE-802.1Q]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks -- Bridges and Bridged Networks",
DOI 10.1109/IEEESTD.2018.8403927, IEEE Std 802.1Q, July
2018,
<https://standards.ieee.org/standard/802_1Q-2018.html>.

[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>.

[RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
DOI 10.17487/RFC5227, July 2008,
<https://www.rfc-editor.org/info/rfc5227>.

[RFC5798] Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798,
DOI 10.17487/RFC5798, March 2010,
<https://www.rfc-editor.org/info/rfc5798>.

[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>.

[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>.

[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.

[RFC8926] Gross, J., Ed., Ganga, I., Ed., and T. Sridhar, Ed.,
"Geneve: Generic Network Virtualization Encapsulation",
RFC 8926, DOI 10.17487/RFC8926, November 2020,
<https://www.rfc-editor.org/info/rfc8926>.

Acknowledgments

The authors would like to thank Mukul Katiyar, Jeffrey Zhang, and
Alex Nichol for their valuable feedback and contributions. Tony
Przygienda and Thomas Morin also helped improve this document with
their feedback. Special thanks to Eric Rosen for his detailed
review, which really helped improve the readability and clarify the
concepts. We also thank Alvaro Retana for his thorough review.

Contributors

In addition to the authors listed on the front page, the following
coauthors have also contributed to this document:

Senthil Sathappan
Florin Balus
Aldrin Isaac
Senad Palislamovic
Samir Thoria

Authors' Addresses

Jorge Rabadan (editor)
Nokia
777 E. Middlefield Road
Mountain View, CA 94043
United States of America

Email: [email protected]

Wim Henderickx
Nokia

Email: [email protected]

John Drake
Juniper

Email: [email protected]

Wen Lin
Juniper

Email: [email protected]

Ali Sajassi
Cisco

Email: [email protected]