In this section, you’ll learn the core concepts behind Common Spanning Tree (CST), how it works, and how switches use it to prevent loops in a Layer 2 network. You’ll explore the algorithm, port roles, and timers involved in STP, and understand what happens when the network topology changes.

By the end of this section, you'll be able to manually calculate the STP topology for a given network.

The section includes the following lessons:

• Introduction to Common STP (CST) - A high-level overview of CST and how it fits into the bigger picture of Layer 2 loop prevention.
• The Spanning-Tree Algorithm - Learn how switches use the STA to elect a root bridge and build a loop-free topology.
• Spanning-Tree Port Roles - Understand the function of root, designated, and blocking ports.
• STP Port States and Timers - A closer look at the different port states and the timers that control convergence.
• STP Topology Changes (TCN) - Learn what triggers a topology change and how STP handles it to maintain stability.
• Lab #2.1 – Manual STP Computation - Practice computing the STP root bridge, port roles, and final topology step by step.
• Lab #2.2 – Manual STP – Topology 2 - A second scenario to reinforce your understanding through hands-on calculation.