07 - Le protocole STP [Français] - Etude de cas N° 1

Introduction

This tutorial provides a comprehensive overview of the Spanning Tree Protocol (STP), a crucial protocol in networking that prevents loops in switched networks. By following the steps outlined, you will understand how to identify and configure the root bridge, root ports, designated ports, and non-designated ports through a practical case study approach.

Step 1: Understanding the Role of STP

• STP is designed to prevent network loops that can occur due to redundancy in physical connections.
• It ensures that there is only one active path between network devices, promoting efficient data flow.

Step 2: Election of the Root Bridge

• Identify the Root Bridge: The root bridge is the central point in the STP topology.
• Bridge ID: Each bridge has a unique Bridge ID, which is a combination of the bridge priority (default is 32768) and MAC address.
• Election Process:
  • Compare Bridge IDs of all switches.
  • The switch with the lowest Bridge ID becomes the root bridge.
• Practical Tip: Adjust the bridge priority to influence which switch becomes the root bridge. A lower priority value increases the chances of a switch being elected.

Step 3: Choosing the Root Ports

• Definition: The root port is the port on a non-root bridge that has the lowest cost path to the root bridge.
• Cost Calculation: Each port has a cost associated with it based on bandwidth:
  • 10 Mbps = 100
  • 100 Mbps = 19
  • 1 Gbps = 4
  • 10 Gbps = 2
• Selection Process:
  • Each switch calculates the cost to reach the root bridge through its ports.
  • The port with the lowest cost is designated as the root port.
• Common Pitfall: Ensure that port costs are consistently configured; otherwise, STP may not operate correctly.

Step 4: Choosing Designated Ports

• Definition: The designated port is the port on a network segment that has the lowest cost to the root bridge and is responsible for forwarding traffic towards the root bridge.
• Selection Criteria:
  • Compare the costs of all ports on the segment leading to the root bridge.
  • The port with the lowest cost becomes the designated port for that segment.
• Practical Advice: Ensure that there is only one designated port per network segment to avoid loops.

Step 5: Identifying Non-Designated Ports

• Definition: Non-designated ports are those that do not forward traffic to the root bridge and are in a blocking state to prevent loops.
• Process:
  • Any port that is neither a root port nor a designated port is considered a non-designated port.
  • These ports remain in a blocking state unless a failure occurs in the network topology.

Conclusion

In this tutorial, we explored the fundamentals of the Spanning Tree Protocol, focusing on the election of the root bridge, selection of root ports, designated ports, and non-designated ports. Understanding these concepts is essential for maintaining efficient and loop-free networks. As a next step, consider practicing these configurations in a lab environment or simulation tool to reinforce your understanding of STP in real-world networking scenarios.