EVPN Interconnect Architecture for Resilient Multi-Domain Fabrics#

Patrice Brissette — Cisco Systems

• Why: Multi-site EVPN over heterogeneous underlays (IPv4 WAN meeting IPv6 DC fabrics) is exactly the DC/WAN interconnect problem the PM owns. This is a coherent answer to the two failure modes that plague DCI overlays: control-plane stability during fabric isolation, and L2 loops across sites.
• What: A hierarchical multi-site design where each fabric stays an independent EVPN domain and Border Gateways provide DCI: dual physical-IP VTEPs per BGW (one for the DC domain, one for the WAN domain) so tunnels are terminated and re-originated at the BGW — creating a clean scaling and policy boundary. Key mechanics: Inter-site ESI for all-active BGW multi-homing, DF/NDF election with local-bias BUM forwarding rules and explicit per-bridge egress filtering to prevent loops, Normalized-VNI translation to resolve overlapping VNIs between independently administered sites, downstream-assigned VNI option, host mobility via /32 advertisement re-origination, and step-by-step packet walks for L2 unicast, BUM, and L3 flows.
• How: Failure-isolation walkthroughs for both WAN loss and DC loss show the alternate BGW-to-BGW path preserving host reachability; decoupling DC from WAN VTEP space means either side can change without re-coordinating.
• Where applicable: Any multi-site EVPN/VXLAN program; not tied to specific silicon, though presented by Cisco's lead EVPN architect (and SONiC routing workgroup chair), so feature availability on non-Cisco gear (I-ESI, normalized VNI) should be verified. Concepts (re-origination boundary, VNI normalization, DF local-bias) transfer across vendors.