BGP EVPN Multihome how split-horizon works (traffic forwarded by non-DF will not forwarded back to ESI by DF)

- November 06, 2025

💯 Excellent follow-up — and yes, you’re right on the money.
Let’s go step by step to confirm how a receiving VTEP (PE) identifies the source of a BUM packet in EVPN-VXLAN multihoming, since the VXLAN header itself doesn’t carry ESI info.

🧩 Short Answer

✅ In VXLAN-EVPN, the receiver identifies which Ethernet Segment (ESI) the BUM packet came from
by looking at the outer source IP (the remote VTEP IP) of the VXLAN tunnel.

The mapping between VTEP IP ↔ ESI is learned from BGP EVPN Type-1 routes in the control plane.

🧭 How This Works in Practice

1️⃣ Control Plane Learning (via Type-1 per-ESI)

Each multihomed PE advertises a Type-1 per-ESI route like:


Route Type: 1 (Ethernet Auto-Discovery)
ESI: 0001
Originating Router’s IP: 10.1.1.1  (PE1)
Next Hop: 10.1.1.1

and similarly:


Route Type: 1 (Ethernet Auto-Discovery)
ESI: 0001
Originating Router’s IP: 10.1.1.2  (PE2)
Next Hop: 10.1.1.2

From these, every remote PE (say PE3) builds:


ESI-0001 members = { 10.1.1.1 , 10.1.1.2 }

2️⃣ Data Plane Forwarding (BUM traffic)

When PE3 sends a BUM packet toward VLAN-10 (VNI 10010):


Outer Src IP = 10.1.1.3  (PE3’s VTEP)
Outer Dst IP = Multicast-group or unicast flood list
VXLAN VNI = 10010

→ both PE1 and PE2 receive it.

3️⃣ Split-Horizon Check at Receiver

On receipt, each leaf (PE1, PE2) does:


Lookup: source-VTEP-IP (10.1.1.3)
Is 10.1.1.3 part of ESI-0001 peer list?
    No  → flood normally
    Yes → suppress flooding toward ESI ports

So when PE1 receives BUM from PE2 (10.1.1.2) —
the control-plane table tells it “10.1.1.2 belongs to my same ESI-0001 group.”
➡️ PE1 will not send that packet back to CE1.

But when PE1 receives from PE3 (10.1.1.3) —
that VTEP is not in the ESI group,
➡️ PE1 will flood to its local ESI ports (deliver to CE1).

🔧 Summary Table

Step	Field Used	Purpose
1	BGP Type-1 routes	Build mapping: VTEP-IP ↔ ESI
2	Outer source IP in VXLAN packet	Identify which PE sent it
3	Split-horizon enforcement	Drop traffic from same ESI group

🧠 So Conceptually

Encapsulation	How receiver identifies same-ESI traffic
MPLS EVPN	Uses explicit ESI label in MPLS stack
VXLAN EVPN	Uses outer source VTEP IP, matched against ESI-peer list (from control plane)

✅ Conclusion:

The receiver does not read an ESI field in the packet;
instead, it knows from the control plane which VTEP IPs belong to each ESI.
During data-plane processing, the outer source IP of the VXLAN tunnel determines if the packet came from an ESI peer, and thus whether to apply split-horizon filtering.

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