In an EVPN-VXLAN ERB (Edge Routed Bridging) architecture, Layer 2 VLANs can be stretched across data centers using VXLAN, while Layer 3 communication between different VLANs requires routing at the edge. Since the Blue VLAN must remain the same across DC and DC2, it must be stretched using VXLAN. However, inter-VLAN communication (between the Red and Green VLANs) requires Layer 3 routing, which is typically handled by a services block, such as the SRX Series chassis cluster shown in the diagram.
To enable inter-VLAN communication, a transit VNI (VXLAN Network Identifier) must be introduced, allowing the Red and Green VLANs to communicate across the two data centers. The interconnect between the SRX Series devices ensures that Layer 3 routing can be performed while maintaining VXLAN encapsulation between the two sites.