Comprehensive and Detailed In-Depth Explanation:
Understanding the Spanning Tree Protocol (STP) and Root Bridge Selection
Spanning Tree Protocol (STP) is used to prevent loops in a Layer 2 network by electing a Root Bridge and determining a loop-free topology.
The Root Bridge is elected based on the lowest Bridge ID (BID), which consists of:
Priority Value (Lower is preferred, default is 32768).
MAC Address (Lower MAC wins in case of a tie).
The optimal forwarding path is chosen based on cost, which is usually determined by the link bandwidth.
Why the Path is Not Optimal?
The network engineer states that all links have the same cost, meaning that the STP Root Bridge election determines the topology rather than cost differences.
The fact that the path between PC1 and PC2 is suboptimal suggests that the Root Bridge may not be ideally placed.
Determining the Likely Root Bridge:
The Root Bridge is the switch with the lowest BID (priority + MAC).
All switches calculate the shortest path to the Root Bridge and place ports in either Forwarding or Blocking mode.
From the diagram, the traffic is flowing through SW2 → SW4 → SW3 instead of directly via SW2 → SW3.
This suggests that the Root Bridge is likely SW1 or SW4 because:
Root Bridges do not block any ports.
If SW1 or SW4 is the Root Bridge, the spanning tree topology might force the path to take a longer route.
Key Takeaways:
Root Bridge selection influences the entire Layer 2 forwarding path.
The lowest Bridge ID (priority + MAC) determines the Root Bridge.
If a suboptimal path is taken, the Root Bridge is likely placed in a location that forces certain links to block.
In this scenario, SW1 or SW4 is the most probable Root Bridge, forcing traffic through an unintended path.
HCIP-Datacom-Advanced Routing & Switching Technology References:
Huawei HCIP-Datacom Official Certification Guide, Chapter on Spanning Tree Protocol (STP) and Root Bridge Selection.
Huawei Datacom Training Materials, Section on STP Convergence and Network Optimization.
Huawei Datacom Configuration Guide, Command Reference for STP Root Bridge Selection and Path Calculation.
