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?
-
A1
-
B2
-
C5
-
D12
-
EDepends on the number of VLANs
Answer
Correct Answer: 1
Explanation
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:
Elect a single root bridge based on lowest bridge ID (priority + MAC).All switches exchange BPDUs to agree on the root.The rest of the topology converges around the root bridge.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