ICND1 Spanning Tree Protocol (STP) 1 — Questions and Answers
Question 1: What is the primary purpose of Spanning Tree Protocol (STP)?
- To enable dynamic routing between VLANs
- To prevent switching loops in Layer 2 networks (Correct answer)
- To provide load balancing across multiple uplinks
- To encrypt traffic between switches
Correct answer: To prevent switching loops in Layer 2 networks
STP prevents Layer 2 switching loops by placing redundant ports in a blocking state, stopping broadcast storms.
Question 2: Which IEEE standard defines the original Spanning Tree Protocol?
- IEEE 802.1Q
- IEEE 802.3
- IEEE 802.1D (Correct answer)
- IEEE 802.1W
Correct answer: IEEE 802.1D
IEEE 802.1D defines the original STP specification; 802.1W defines RSTP and 802.1Q defines VLANs.
Question 3: What is the default bridge priority value used by Cisco switches in STP?
- 4096
- 32768 (Correct answer)
- 61440
- 65535
Correct answer: 32768
Cisco switches default to a bridge priority of 32768, which is the midpoint of the valid range and must be a multiple of 4096.
Question 4: In STP, which port state actively forwards user traffic and learns MAC addresses?
- Blocking
- Listening
- Learning
- Forwarding (Correct answer)
Correct answer: Forwarding
Only the Forwarding state allows a port to send and receive data traffic; it is the final stable active state.
Question 5: How is the Root Bridge elected in an STP topology?
- The switch with the highest MAC address wins
- The switch with the lowest Bridge ID wins (Correct answer)
- The switch with the most active ports wins
- The switch configured as the default gateway wins
Correct answer: The switch with the lowest Bridge ID wins
The switch with the lowest Bridge ID (priority + MAC address) wins the Root Bridge election.
Question 6: Which STP port state immediately follows the Listening state in the port state transition?
- Forwarding
- Blocking
- Learning (Correct answer)
- Disabled
Correct answer: Learning
After Listening, a port transitions to Learning, where it builds the MAC address table but does not yet forward user traffic.
Question 7: What is the default STP Hello timer interval at which the Root Bridge sends BPDUs?
- 1 second
- 2 seconds (Correct answer)
- 5 seconds
- 10 seconds
Correct answer: 2 seconds
The Hello timer defaults to 2 seconds, defining how frequently the Root Bridge multicasts Bridge Protocol Data Units to neighbors.
ICND1 Spanning Tree Protocol (STP) 1
What is the primary purpose of Spanning Tree Protocol (STP)?