4 — EVPN control plane¶
Lesson 3 showed how a frame crosses the fabric. This lesson answers: how does a VTEP know where to send it? That's the control plane — and it's what makes VXLAN-EVPN better than plain VXLAN.
The problem EVPN solves¶
Plain VXLAN (without EVPN) still does flood-and-learn, just like old L2: to find an unknown MAC, flood everywhere and learn from the replies. That doesn't scale and wastes bandwidth.
EVPN replaces flooding with advertising. Each VTEP tells the others which MACs and IPs live behind it — using BGP. No discovery-by-flooding.
EVPN is a BGP address family¶
EVPN isn't a new protocol — it's a new address family carried in MP-BGP:
family evpn signalingin Junos /address-family l2vpn evpnin Cisco.- Technically AFI 25 (L2VPN), SAFI 70 (EVPN).
- It rides on the same BGP sessions you already run — in the lab, the iBGP session between the leaf loopbacks.
So the leaves peer over BGP and exchange EVPN routes (covered next lesson)
that say things like "MAC aa
cc, IP 10.100.10.11, is behind VTEP 10.0.0.22,
in VNI 10100."
Two identifiers you must know: RD and RT¶
These trip people up. Learn them cold — they're guaranteed interview material.
Route Distinguisher (RD)¶
- Makes each VTEP's routes unique. Two leaves might both advertise
10.100.10.0/24; without an RD those look identical in BGP. - Prepended to the route: e.g.
10.0.0.21:1. Purely for uniqueness. - In the lab, each leaf had a different RD (
10.0.0.21:1vs10.0.0.22:1).
Route Target (RT)¶
- An extended community that controls import/export — which VTEPs/VRFs should accept a route.
- A VTEP exports routes tagged with an RT, and imports routes matching its configured RT. Same RT = same virtual network.
- In the lab, both leaves shared the same RT (
target:65000:1) — that's how they agreed to be in one VNI.
RD = uniqueness. RT = membership. Different RD per box, shared RT per VNI.
What EVPN gives you (beyond just "no flooding")¶
| Feature | What it does |
|---|---|
| MAC/IP advertising | Learn remote hosts via BGP (Type-2), not flooding |
| ARP/ND suppression | The local leaf answers ARP from its EVPN table — the ARP never crosses the fabric |
| Fast convergence | A host move is a BGP update, not a flood-and-relearn |
| Multihoming (ESI) | A host can connect to two leaves, both active (all-active) |
| Integrated Routing & Bridging (IRB) | Route between VNIs on the leaf (L3VNI), anycast gateway |
iBGP vs eBGP for the overlay¶
- iBGP (lab 01): all VTEPs in one AS. Full mesh — or, at scale, spines act as route reflectors so you don't need N² sessions.
- eBGP (lab 04): each leaf its own AS; the overlay uses eBGP-EVPN. Common in large fabrics because one protocol (eBGP) can do both underlay and overlay.
This overlay choice is the axis labs 01–04 explore.
Check yourself¶
- What does EVPN replace, and with what?
- EVPN is carried in which protocol, as what?
- Explain RD vs RT — which is unique per box, which is shared per VNI, and why?
- What is ARP suppression and why is it valuable?
→ Next: EVPN route types