“Stretching Layer 2 domains across WAN links is notoriously risky. If you don’t control broadcast and multicast flooding, a loop on-premise will crash your cloud.”

At IBM, we used NSX Autonomous Edge L2VPN clients to extend on-premise VLANs to IBM Cloud during multi-phase workload migrations.

Broadcast Suppression & MAC Learning

To prevent WAN link saturation, the NSX L2VPN client suppresses ARP broadcast traffic locally:

• Local ARP Proxy: Responds to ARP requests on behalf of remote hosts using a local IP-to-MAC table.
• Optimized Egress Routing (OER): Ensures local default gateway traffic routes locally rather than trombone-routing across the WAN.

[!IMPORTANT] Always enable ARP Proxy on stretched L2VPN gateways to eliminate unnecessary broadcast traffic across expensive WAN links.

The Verdict

Key Takeaway

Contain Stretched L2 Broadcast Domains.

Stretching Layer 2 subnets should be a temporary migration mechanism. Ensure local ARP Proxy and egress routing optimizations are active to protect WAN performance.