How EVPN Transforms VXLAN Control Planes: A Deep Dive into BGP EVPN Routing

Introduction

EVPN (Ethernet VPN) is an MP‑BGP‑based technology that defines new BGP route types and can be used as the control plane for VXLAN. This article introduces EVPN concepts, route formats, and deployment scenarios.

EVPN Basics

EVPN solves the flooding problem of the original VXLAN design by providing automatic VTEP discovery, MAC learning, and route distribution. It extends BGP with EVPN‑specific NLRI and route types.

MP‑BGP Basics

MP‑BGP (Multi‑Protocol BGP) extends BGP‑4 to support multiple address families such as IPv6 and L2VPN. EVPN leverages MP‑BGP by defining an EVPN address family and new NLRI types.

EVPN Route Types

EVPN defines five route types. Types 1‑4 are defined in RFC 7432; Type 5 is defined in later drafts. The article focuses on the most common Types 2, 3, and 5.

Type 2 – MAC/IP Route

Used for VTEP‑to‑VTEP MAC and IP learning. The NLRI format is shown in Figure 1‑2.

[Leaf1]
bridge-domain 10
 vxlan vni 10    //二层VNI
evpn
  route-distinguisher 10:1
  vpn-target 0:10 export-extcommunity    //EVPN实例的ERT
  vpn-target 100:5000 export-extcommunity
  vpn-target 0:10 import-extcommunity
#
interface Nve1
 source 1.1.1.1   //Leaf1的VTEP IP地址
 vni 10 head-end peer-list protocol bgp
#

Type 3 – Inclusive Multicast Ethernet Tag (IMET) Route

Advertises VTEP IP, VNI, and MPLS label to build head‑end replication lists and establish VXLAN tunnels.

Type 5 – IP Prefix Route

Distributes subnet routes (or host routes) across the overlay. The NLRI format is shown in Figure 1‑4.

EVPN as VXLAN Control Plane

EVPN enables automatic VTEP discovery, MAC learning, and route distribution, reducing flooding and simplifying network expansion. The control‑plane workflow includes tunnel establishment, MAC learning via Type 2 routes, head‑end replication via Type 3 routes, and host/subnet route publishing via Types 2 and 5.

MAC Learning

VTEPs exchange Type 2 routes to learn remote host MAC and IP addresses, eliminating data‑plane flooding.

Head‑End Replication List

Type 3 routes allow VTEPs to automatically create replication lists for broadcast, multicast, and unknown‑unicast traffic.

Host and Subnet Route Publishing

Type 2 routes can carry host IP information; Type 5 routes carry subnet prefixes. These routes are used by distributed gateways to forward traffic across different subnets.

VXLAN Forwarding Process

Describes same‑subnet known unicast, BUM (broadcast, unknown‑unicast, multicast), and cross‑subnet forwarding, with step‑by‑step packet flows.

ARP Broadcast Suppression

Two mechanisms are presented: ARP‑to‑unicast, which converts ARP broadcasts to unicast using learned host information, and ARP proxy (layer‑2 reply), which answers ARP requests directly at the L2 gateway.