Which feature must be configured to connect a classical Ethernet network to a data center network so that the data center network appears to be one large switch.

Ethernet VPN (EVPN) is a standards-based control plane for VXLAN that allows for building and connecting multiple Layer 2 domains over a common Layer 3 underlay. It uses BGP to distribute MAC address and IP reachability information, which makes the underlying data center network fabric behave like a single, large logical switch to connected devices. This enables seamless Layer 2 connectivity for classical Ethernet networks across the fabric, providing workload mobility and flexible network design without the limitations of traditional Spanning Tree Protocol.

A. VPC+ is a feature of the legacy FabricPath technology used for connecting classical Ethernet devices to a FabricPath domain, not the modern standard for creating a large logical switch.
B. UDLD (Unidirectional Link Detection) is a Layer 2 protocol that detects and disables one-way links; it does not create a network fabric or logical switch.
D. OSPF is a Layer 3 routing protocol used to exchange IP routing information within the underlay, not to create a Layer 2 overlay that appears as a single switch.

1. Cisco Press, Designing Cisco Data Center Infrastructure (DCID 300-610) Official Cert Guide, Chapter 10, "VXLAN EVPN," Section "VXLAN EVPN Control Plane," states, "BGP EVPN is a standards-based control plane that enables the advertisement of not only Layer 2 MAC addresses but also Layer 3 IP addresses."

2. Cisco, Cisco Nexus 9000 Series NX-OS VXLAN Configuration Guide, Release 10.2(x), Chapter: "Configuring VXLAN BGP EVPN," Section: "Information About VXLAN BGP EVPN," describes how EVPN provides a unified control plane for Layer 2 and Layer 3 forwarding in a VXLAN overlay network.