Layer 3 VXLAN gateway support in EVPN-VXLAN fabrics using a RIOT loopback port

Enable routing in and out of VXLAN tunnels (RIOT) on devices that don't have native RIOT support, so these devices can serve as VXLAN gateway leaf devices in EVPN-VXLAN edge-routed bridging (ERB) overlay fabrics. You configure a loopback LAG that performs RIOT operations in a two-pass process to route VXLAN tunnel traffic. The loopback LAG port must be a member of all VXLAN VLANs with IRB interfaces.