draft-ietf-bess-evpn-l3mh-proto-00 identifies a common challenge in L3VPN multi-homing designs: how a multi-homed CE performs Layer-2 hashing across its Link Aggregation Group (LAG) members. This hashing determines the outgoing interface for all Ethernet frames, including ARP and IPv6 Neighbor Discovery (ND) messages.
As a result, ARP or ND responses from the CE may consistently be directed to a single service PE. For instance, if the hashing algorithm always selects the link to PE1, only PE1 will populate its ARP/ND table, while PE2 will not obtain the corresponding neighbor information. Consequently, when unicast traffic from the core is load-balanced toward PE2, it may lack the required adjacency information and be unable to forward traffic correctly.
To resolve this issue, the EVPN L3 multi-homing draft specifies ARP/ND route synchronization. By exchanging ARP and ND information between peering PEs using EVPN MAC/IP Advertisement routes (EVPN Route Type 2), both PEs maintain a comprehensive and consistent view of CE adjacencies. This approach ensures correct unicast forwarding behavior regardless of the hashing of ARP/ND responses by the CE.
In this test, a single CE LAG was distributed across two independent PEs, both operating in all-active mode for the same L3VPN instance. The PEs established an EVPN Ethernet Segment toward the CE, which remained fully transparent to the CE. To ensure consistent Layer-3 forwarding in this topology, the PEs synchronized ARP and IPv6 ND entries learned on their L3 interfaces using EVPN Route Type 2 advertisements.
During validation, ARP/ND entries learned on one PE were successfully advertised via EVPN and installed on the peer PE without requiring local relearning. With continuous CE-to-CE ICMP ping traffic, we have the verified data-plane behavior. When one of the CE-to-PE access links was intentionally disabled, the remaining PE immediately continued forwarding traffic for the L3VPN service. Traffic switched seamlessly to the redundant path, with no observable packet loss or convergence delay.