A leaf-spine topology is fundamentally a Layer 3 routed fabric designed to overcome the limitations of traditional Layer 2 networks that rely on Spanning Tree Protocol (STP). The core principle of leaf-spine is to use all available links for forwarding traffic, which is achieved through routing protocols (like BGP or OSPF) and Equal-Cost Multi-Path (ECMP). STP, in contrast, prevents loops by blocking redundant links, which would negate the high-bandwidth, low-latency, and multi-pathing benefits of the leaf-spine architecture. Therefore, a leaf-spine network does not use STP for its core loop-avoidance mechanism; it relies on routing.

A. This statement is true. The design requires every leaf switch to connect to every spine switch to create a full-mesh, non-blocking fabric between the layers.

B. This is a primary benefit. The path for east-west traffic is always leaf-spine-leaf, ensuring predictable and consistent low latency between any two hosts in the data center.

D. This is a core feature. ECMP is used to load-balance traffic across all the available, equal-cost paths between the leaf switches, maximizing bandwidth utilization and resiliency.

1. Nokia Data Center Fabric Solution Guide, Release 21.10, Section 2.1, "Data center network evolution": This document contrasts traditional STP-based networks with modern leaf-spine fabrics. It states, "To overcome the limitations of STP, modern data center networks are built as a Layer 3 fabric of interconnected switches... The leaf-spine topology uses a Layer 3 routing protocol... to provide ECMP (Equal Cost Multipath) forwarding." This explicitly confirms that leaf-spine uses L3 routing and ECMP instead of STP.

2. Nokia SR Linux and Fabric Services System Architecture Overview, Section 2.1, "IP fabric underlay": This document describes the foundation of the Nokia fabric: "The data center fabric is built on a two-stage leaf-spine topology... The underlay is an IP fabric that uses an interior gateway protocol (IGP) or BGP to provide reachability between the nodes." This reinforces the concept of an IP (Layer 3) fabric, which does not use STP for its core operation.