STP Clinical & Scientific Knowledge 2 — Questions and Answers
Question 1: Which STP port state actively forwards frames AND learns MAC addresses?
- Blocking
- Listening
- Learning
- Forwarding (Correct answer)
Correct answer: Forwarding
Only the Forwarding state both learns MAC addresses and actively forwards data frames.
Question 2: What is the default Hello Time interval in 802.1D STP?
- 1 second
- 2 seconds (Correct answer)
- 5 seconds
- 10 seconds
Correct answer: 2 seconds
The default Hello Time is 2 seconds, meaning the root bridge sends BPDUs every 2 seconds.
Question 3: A non-root bridge receives a superior BPDU on its root port. What action does STP take?
- It ignores the BPDU
- It triggers a topology change
- It updates its root port and recalculates the topology (Correct answer)
- It immediately blocks the port
Correct answer: It updates its root port and recalculates the topology
When a non-root bridge receives a superior BPDU, it updates its root port selection and recalculates path costs.
Question 4: Which STP mechanism prevents a port from immediately transitioning to Forwarding after a link comes up?
- PortFast
- Forward Delay timer (Correct answer)
- MaxAge timer
- BPDUGuard
Correct answer: Forward Delay timer
The Forward Delay timer (default 15 seconds) holds ports in Listening and Learning states before transitioning to Forwarding.
Question 5: In STP, what does a Topology Change Notification (TCN) BPDU cause receiving switches to do?
- Reload their firmware
- Age out MAC table entries faster (Correct answer)
- Elect a new root bridge
- Block all non-designated ports
Correct answer: Age out MAC table entries faster
TCN BPDUs cause switches to shorten their MAC address aging timer from 300 seconds to the Forward Delay (15 seconds).
Question 6: Which STP variant uses one instance of spanning tree per VLAN?
- CST (Common Spanning Tree)
- MSTP
- PVST+ (Correct answer)
- 802.1D
Correct answer: PVST+
PVST+ (Per-VLAN Spanning Tree Plus) runs a separate STP instance for each VLAN, allowing per-VLAN load balancing.
Question 7: What is the STP port role of a port on the root bridge that connects to another switch?
- Root Port
- Designated Port (Correct answer)
- Alternate Port
- Backup Port
Correct answer: Designated Port
All ports on the root bridge are Designated Ports because the root bridge always has the best path cost of zero.
STP Clinical & Scientific Knowledge 2
Which STP port state actively forwards frames AND learns MAC addresses?