A three-stage Clos network, commonly known as a spine-and-leaf architecture, is a foundational design for modern data center fabrics. A key principle is that endpoint devices, such as servers and storage, connect to the leaf switches (the "edge" of the fabric). The spine layer serves as a high-speed backbone, interconnecting all leaf switches. For traffic entering or leaving the data center (North-South traffic), specialized "border leaf" devices are used. These border devices connect to external networks (like the Internet or WAN) and handle the ingress and egress of traffic for the entire fabric.

A. Spines are the core of the fabric and should not have external connections, as this would create a bottleneck and add complexity.

D. Servers connect to leaf devices to provide access to the fabric; connecting them directly to spines would violate the Clos architecture principles.

1. Juniper Networks, "Data Center Fabric Architecture Design Guide", Chapter 2: "Data Center Fabric Architecture". This guide explicitly states, "In a spine-and-leaf architecture, servers and storage devices connect to leaf devices."

2. Juniper Networks, "EVPN-VXLAN Data Center Fabric and DCI", Chapter 2: "Campus and Data Center Designs". This document describes the role of border leaf devices: "Border leaf devices provide Layer 3 connectivity between the data center and external networks."