Spanning Tree Protocol and root bridge election In a switched network with many devices, how many root bridges are elected in a Spanning Tree Protocol (STP) topology?

Introduction / Context:
Spanning Tree Protocol (STP) is used to prevent switching loops in a network by organizing switches into a loop-free logical topology. The root bridge plays a central role by acting as the reference point for path calculations.

Given Data / Assumptions:

• We are considering a single spanning-tree instance (common in IEEE 802.1D).
• The network has dozens of switches.

Concept / Approach:

STP always elects exactly one root bridge per spanning-tree instance. All other switches calculate the shortest path to this root bridge. In PVST+ (Per-VLAN Spanning Tree), each VLAN has its own root bridge, but per instance the answer remains one.

Step-by-Step Solution:

Verification / Alternative check:

Cisco's STP documentation confirms: one root bridge per spanning-tree instance.

Why Other Options Are Wrong:

• 2, 5, 12: arbitrary counts, incorrect.
• Depends on VLANs: true only when considering multiple VLANs, but in a single STP instance, it is always 1.

Common Pitfalls:

• Forgetting that multiple VLANs can each have a root, leading to confusion.

Final Answer:

1