In L3 spine and leaf design

In a Layer 3 (L3) spine and leaf architecture, the connections between the spine and leaf switches are configured as routed (L3) interfaces. Routing protocols like BGP or OSPF are used to exchange reachability information. This design inherently prevents Layer 2 loops between the spine and leaf tiers, as frames are not flooded in the same way as in a traditional L2 network. Each router hop decrements the IP packet's Time-to-Live (TTL) field, which prevents indefinite packet forwarding loops. Consequently, Layer 2 loop prevention mechanisms like Spanning Tree Protocol (STP) are not required for the fabric's core infrastructure, allowing all inter-switch links to be active and forward traffic simultaneously using Equal-Cost Multi-Path (ECMP).

A. Spanning Tree is not needed in an L3 fabric core. The purpose of spine and leaf is to provide scalability, predictable latency, and high bandwidth, not just performance.

B. TRILL is a Layer 2 multipathing protocol. An L3 spine and leaf design uses routed links, making an L2 protocol like TRILL unnecessary for loop prevention in the fabric.

D. The premise is incorrect; spine switches do not connect to each other in this topology. While EVPN is often used as an overlay, it is not a mandatory requirement.

1. ArubaOS-CX 10.11 Data Center Networking Guide

Chapter 2: "Layer 3 spine and leaf fabric"

Page 11. The guide states

"In a layer 3 spine and leaf fabric

the links between the spine and leaf switches are layer 3 routed links. This design provides a highly scalable and resilient network with no layer 2 loops and no need for Spanning Tree Protocol (STP) in the fabric core."

2. Aruba Validated Solution Guide: Aruba Data Center Networking Solution with ArubaOS-CX and VMware

"Leaf-Spine Network Design" section

Page 10. It specifies

"In a Layer 3 leaf-spine design

the links between leaf and spine switches are Layer 3 routed interfaces. This eliminates the need for Spanning Tree Protocol (STP) between the leaf and spine layers

allowing all links to be active and forward traffic using Equal-Cost Multi-Path (ECMP) routing."

3. HPE FlexFabric Network Analytics Version 2.6.0 User Guide

Chapter 1: "Introduction to Leaf-Spine Architecture"

Page 7. The document notes

"In a Layer 3 design

routing occurs between the leaf and spine switches... This design eliminates the need for Spanning Tree Protocol (STP)."