What is EVPN?

EVPN (Ethernet VPN) is a BGP-based control plane that advertises MAC and IP addresses between switches, to build layer-2 and layer-3 networks over a fabric.

Traditional layer-2 networks learn where a MAC address lives by flooding unknown traffic and watching what comes back. EVPN, defined in RFC 7432, replaces most of that with BGP: each switch announces the hosts it has learnt locally, using the L2VPN/EVPN address family. These announcements are grouped into route types. Type 2 (MAC/IP Advertisement) says "this MAC, and optionally this IP, is behind me". Type 3 (Inclusive Multicast) says "I take part in this layer-2 segment, send me broadcast and unknown traffic". Type 5 (IP Prefix) carries routed prefixes for layer-3 designs, and types 1 and 4 support multihoming, where a host connects to two switches at once. Route targets decide which switches import which routes, much as they do in MPLS VPNs.

EVPN is only the control plane, so it needs a data plane to carry the actual frames. In data centres that is nearly always VXLAN, described for EVPN in RFC 8365, which wraps Ethernet frames in UDP so they can cross a routed leaf-spine underlay. Each layer-2 segment is identified by a VNI, and each switch acts as a VTEP. EVPN also runs over MPLS in service-provider networks. The usual confusion is treating EVPN and VXLAN as the same thing: VXLAN can run without EVPN using flood-and-learn, and EVPN can run without VXLAN. EVPN also cuts down ARP flooding and lets a host move between switches cleanly, because the new location is advertised. The EVPN and VXLAN lesson builds a fabric in FRR and shows type-2 and type-3 routes on real switches.