STP Basics and Concepts 3 — Questions and Answers
Question 1: Which IEEE standard introduced Rapid Spanning Tree Protocol (RSTP)?
- IEEE 802.1D
- IEEE 802.1w (Correct answer)
- IEEE 802.1s
- IEEE 802.1Q
Correct answer: IEEE 802.1w
IEEE 802.1w defined RSTP, which provides much faster convergence than the original 802.1D STP.
Question 2: What is the purpose of the Topology Change Notification (TCN) BPDU in STP?
- To elect a new root bridge
- To signal that the network topology has changed (Correct answer)
- To advertise port costs
- To reset bridge priorities
Correct answer: To signal that the network topology has changed
A TCN BPDU is sent upstream toward the root bridge to notify all switches that a topology change has occurred, prompting MAC table flushes.
Question 3: How many seconds does it take for a classic STP port to transition from Blocking to Forwarding by default?
- 15 seconds
- 20 seconds
- 30 seconds
- 50 seconds (Correct answer)
Correct answer: 50 seconds
A port spends 15 seconds in Listening and 15 seconds in Learning for a total of 30 seconds before entering Forwarding, but max age (20s) is added when recovering from indirect failures, totaling up to 50 seconds.
Question 4: Which STP port role is used to block traffic and prevent switching loops on non-root, non-designated ports?
- Root port
- Designated port
- Alternate port (Correct answer)
- Backup port
Correct answer: Alternate port
In RSTP, the alternate port is the blocking port that provides a standby path to the root bridge.
Question 5: What does STP use path cost to determine?
- The speed of BPDU transmission
- The best path to the root bridge (Correct answer)
- The number of VLANs on a trunk
- The bridge priority increment
Correct answer: The best path to the root bridge
STP calculates cumulative path costs to identify the lowest-cost path to the root bridge, which becomes the root port.
Question 6: Which condition causes a non-root switch to generate a TCN BPDU?
- A port transitions from Blocking to Forwarding or from Forwarding to Blocking (Correct answer)
- The hello timer expires
- A new VLAN is added
- The bridge priority changes
Correct answer: A port transitions from Blocking to Forwarding or from Forwarding to Blocking
A switch sends a TCN BPDU when a port that was forwarding goes down, or when a port transitions to Forwarding while another port is already forwarding.
Question 7: What is the maximum number of switches (hops) STP supports by default before a BPDU is considered stale?
- 7
- 10
- 20 (Correct answer)
- 64
Correct answer: 20
The max age default of 20 seconds limits the diameter of the spanning tree; by default this corresponds to a maximum of 7 switches in the path (with the default hello time of 2 seconds).
STP Basics and Concepts 3
Which IEEE standard introduced Rapid Spanning Tree Protocol (RSTP)?