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L2VPN — Pseudowire, VPLS & EVPN

Carrying Layer 2 across an MPLS/IP core: pseudowires (PWE3, VC label, tLDP, E-Line), VPLS (multipoint, split-horizon, flood-and-learn, its scaling pain), and EVPN (BGP MAC learning, Type 1/2/3/4 routes, all-active ESI multihoming, ARP suppression, MPLS or VXLAN).

Sometimes customers want their Layer 2 carried across a provider core — the same broadcast domain or the same "wire" in two cities. Three technologies do this over MPLS/IP, in order of capability: pseudowire (point-to-point), VPLS (multipoint, data-plane learning), and EVPN (multipoint, BGP control plane). EVPN is where everything has converged.

The progression: a pseudowire emulates one wire; VPLS emulates a whole switch by meshing pseudowires and flooding to learn MACs; EVPN keeps the switch model but learns MACs in BGP — fixing VPLS's flooding, scaling, and multihoming problems.

Pseudowire (PWE3 / L2 circuit)

A point-to-point emulation of an L2 link (Ethernet, VLAN, ATM, Frame Relay, PPP) across an MPLS core — the MEF E-Line service. Two labels: the outer transport/tunnel label to the far PE, the inner VC label identifying the pseudowire.

Signaling: targeted LDP (tLDP) between the two PEs (Martini, RFC 4447) or BGP (Kompella). The PW type says what's emulated (raw vs tagged Ethernet).
Control word (optional) carries sequencing/flags and prevents mis-hashing frames as IP/MPLS in the core.
No MAC learning — it's just a wire: whatever comes in one end goes out the other.

See LDP (tLDP) and MPLS (label stack).

VPLS

Multipoint L2 VPN — the core emulates one big switch/LAN (MEF E-LAN). PEs form a full mesh of pseudowires and bridge between them.

Split-horizon rule: a frame learned from one PW is never forwarded to another PW (only to local attachment circuits) — that's how a full mesh stays loop-free without STP.
MAC learning is in the data plane, and BUM traffic is flooded to every PE.
Signaling: LDP-VPLS (RFC 4762) or BGP-VPLS (RFC 4761, which adds auto-discovery of PEs). H-VPLS adds a hierarchy (spoke PWs into hub PEs) to avoid an N² full mesh.

Why VPLS hurts at scale

Full mesh of PWs (N²), data-plane flood-and-learn (every PE learns every MAC, lots of BUM), and — the big one — no all-active multihoming: a site dual-homed to two PEs can't safely use both without loops. Convergence on failure is slow. These are exactly the pains EVPN removes.

EVPN (Layer 2)

EVPN keeps the multipoint L2 service but moves MAC learning into MP-BGP (L2VPN EVPN address family) — a control plane for the bridge, like BGP is for routing. Data plane is MPLS or VXLAN. It's the standard for both DC fabrics and provider L2VPN.

EVPN route	Purpose
Type 2	MAC/IP advertisement — PEs learn remote MACs (and IPs, enabling ARP suppression) via BGP instead of flooding.
Type 3	Inclusive Multicast — auto-discovery + builds the BUM flood list (ingress replication or multicast).
Type 1	Ethernet Auto-Discovery — per-ES; enables fast withdraw (mass MAC withdrawal on link loss) and aliasing.
Type 4	Ethernet Segment — discovers PEs on the same multihomed segment and elects the Designated Forwarder.

All-active multihoming via the Ethernet Segment Identifier (ESI): a site dual-homed to two PEs uses both links, with DF election for BUM and split-horizon to prevent loops — the headline VPLS fix.
MAC mobility (sequence numbers) handles a VM moving between PEs cleanly; aliasing load-balances to a multihomed MAC.
Services map to MEF: E-LAN (multipoint), E-Line (EVPN-VPWS), E-Tree (hub-and-spoke).

See MP-BGP / EVPN and VLAN & VXLAN (EVPN as the VXLAN control plane).

Pseudowire vs VPLS vs EVPN

	Pseudowire	VPLS	EVPN
Topology	Point-to-point	Multipoint	Multipoint
MAC learning	— (just a wire)	Data plane (flood/learn)	Control plane (BGP)
Discovery	tLDP / BGP	LDP or BGP	BGP (Type 3)
Multihoming	n/a	Single-active, hard	All-active (ESI)
BUM / ARP	—	Flood everywhere	Reduced; ARP suppression
Data plane	MPLS	MPLS	MPLS or VXLAN
MEF service	E-Line	E-LAN	E-LAN / E-Line / E-Tree

Likely interview questions

Pseudowire vs VPLS vs EVPN in one sentence each.
How does a two-label PW packet traverse the core? (transport + VC label)
What is the VPLS split-horizon rule and why is it needed? (loop-free full mesh, no STP)
Why doesn't VPLS scale, and what does EVPN fix? (N² mesh, flood/learn, no all-active multihoming)
How does EVPN learn MACs, and which route type carries them? (MP-BGP, Type 2)
How does EVPN do all-active multihoming? (ESI + Type 1/4, DF election, split-horizon)
Can EVPN run over VXLAN and MPLS? (yes — same control plane, different data plane)
What are E-Line / E-LAN / E-Tree?

Related: MP-BGP / EVPN · VLAN & VXLAN · MPLS · LDP.