Cisco CCNA Spanning Tree Protocol (STP) 2 — Questions and Answers
Question 1: Which STP port state forwards data frames but does NOT learn MAC addresses?
- Listening (Correct answer)
- Learning
- Forwarding
- Blocking
Correct answer: Listening
The Listening state participates in frame forwarding decisions but does not populate the MAC address table.
Question 2: A switch has a bridge priority of 32768 and a MAC address of 00:1A:2B:3C:4D:5E. What is its Bridge ID?
- 32768:00:1A:2B:3C:4D:5E (Correct answer)
- 00:1A:2B:3C:4D:5E:32768
- 32768 only
- The MAC address only
Correct answer: 32768:00:1A:2B:3C:4D:5E
The Bridge ID is a combination of the bridge priority (2 bytes) followed by the MAC address (6 bytes).
Question 3: What is the default STP forward delay timer value?
- 15 seconds (Correct answer)
- 20 seconds
- 10 seconds
- 30 seconds
Correct answer: 15 seconds
The default forward delay is 15 seconds, which applies to both the Listening and Learning states.
Question 4: Which type of STP port provides the best path from a non-root switch back toward the root bridge?
- Root port (Correct answer)
- Designated port
- Alternate port
- Backup port
Correct answer: Root port
Each non-root switch has exactly one root port, which is the port with the lowest cost path to the root bridge.
Question 5: In PVST+, how is the extended system ID used in the bridge priority field?
- The VLAN ID is added to the bridge priority (Correct answer)
- The VLAN ID replaces the bridge priority
- The VLAN ID is subtracted from the bridge priority
- The VLAN ID is not used in PVST+
Correct answer: The VLAN ID is added to the bridge priority
PVST+ uses the lower 12 bits of the 16-bit priority field as the extended system ID (VLAN ID), so valid priorities must be multiples of 4096.
Question 6: Which STP feature automatically sets a port to the Forwarding state when connected to an end device, bypassing the Listening and Learning states?
- PortFast (Correct answer)
- BPDU Guard
- Loop Guard
- Root Guard
Correct answer: PortFast
PortFast allows a port to transition directly to Forwarding, skipping Listening and Learning, and is intended for ports connected to end hosts.
Question 7: What happens when a switch running STP receives a superior BPDU on its root port?
- It updates its root bridge information and may recalculate the topology (Correct answer)
- It immediately blocks the port
- It ignores the BPDU since it already has a root port
- It sends a TCN BPDU to the root bridge
Correct answer: It updates its root bridge information and may recalculate the topology
A superior BPDU (lower bridge ID) causes the switch to update its stored topology information and potentially trigger STP recalculation.
Cisco CCNA Spanning Tree Protocol (STP) 2
Which STP port state forwards data frames but does NOT learn MAC addresses?