08 - Le protocole STP [Français] - Etude de cas N° 2

Introduction

This tutorial provides a detailed explanation of the Spanning Tree Protocol (STP) as discussed in the video by Mohamed QARA. STP is crucial in networking to prevent loops created by redundancy at the physical layer, ensuring a single path remains available between end devices. This guide will walk you through the essential components of STP using a practical case study approach.

Step 1: Understand the Role of STP

• STP is designed to prevent loops in a network, which can cause broadcast storms and network congestion.
• It operates at the Data Link Layer (Layer 2) of the OSI model.
• Key concepts include the Root Bridge, Root Ports, Designated Ports, and Non-Designated Ports.

Step 2: Election of the Root Bridge

• The first step in STP is electing a Root Bridge, which serves as the central point of the spanning tree.
• This process is based on the Bridge ID, which consists of the Bridge Priority and MAC address.
• To elect the Root Bridge:
  1. Each switch sends Bridge Protocol Data Units (BPDUs) to its neighbors.
  2. The switch with the lowest Bridge ID becomes the Root Bridge.

Practical Tip: Lower the Bridge Priority of a switch you want to be the Root Bridge by configuring it in the command line.

Step 3: Choosing Root Ports

• Once the Root Bridge is elected, each switch determines its Root Port, which is the port leading to the Root Bridge.
• To choose the Root Port:
  1. Evaluate the cost of paths to the Root Bridge.
  2. Select the port with the lowest path cost as the Root Port.

Common Pitfall: Ensure that switch configurations are consistent; otherwise, cost calculations may lead to incorrect selections.

Step 4: Selecting Designated Ports

• Each network segment needs a Designated Port to manage traffic towards the Root Bridge.
• To select Designated Ports:
  1. For each segment, compare the Bridge IDs of all switches connected to that segment.
  2. The switch with the lowest Bridge ID becomes the Designated Port for that segment.

Real-World Application: Designated Ports help optimize network traffic flow and reduce collisions.

Step 5: Identifying Non-Designated Ports

• Ports that are neither Root Ports nor Designated Ports are classified as Non-Designated Ports.
• These ports are put into a blocking state to prevent loops, thus maintaining a single active path.
• To identify Non-Designated Ports:
  1. Review the port roles assigned during the selection process.
  2. Ensure Non-Designated Ports are properly configured to block traffic.

Practical Tip: Regularly monitor port statuses to ensure that the STP configuration is functioning correctly.

Conclusion

In conclusion, the Spanning Tree Protocol is vital in maintaining a loop-free network environment. By following the steps of electing the Root Bridge, choosing Root Ports, Designated Ports, and identifying Non-Designated Ports, you can effectively configure and manage your network. For further learning, consider exploring advanced STP configurations or related protocols like Rapid Spanning Tree Protocol (RSTP).