Comprehensive and Detailed 150 to 250 words of Explanation From [SR Linux EVPN and Data Center Interconnect/Course Guide/topics]:

For Layer 2 data center interconnect using an integrated gateway, VPLS 100 on the Nokia 7750 SR represents the WAN-side Layer 2 VPN service. The VPLS requires an EVI because EVPN uses the EVI to identify the L2 service instance in the control plane. It also requires BGP EVPN signaling with the appropriate route targets so the local and remote service instances can import the correct EVPN routes. Because the service is transported across the WAN, the BGP/EVPN instance must be associated with the MPLS transport tunnels toward the remote gateway, dcgw20. A VXLAN instance with a VNI is not configured in the 7750 SR VPLS 100 for this integrated WAN gateway service. The VXLAN/VNI mapping is used inside the SR Linux data center fabric where MAC-VRF services are transported over VXLAN. On the WAN side, the VPLS service is carried using MPLS/EVPN mechanisms, not a VXLAN VNI configured directly under the VPLS. Reference: integrated gateway DCI, L2 EVPN/VPLS interworking, EVI and route-target operation.