Cisco CCNA Spanning Tree Protocol (STP) 3 — Questions and Answers
Question 1: Which Rapid STP (802.1w) port role replaces the STP blocking state and provides an alternate path to the root bridge?
- Alternate port (Correct answer)
- Backup port
- Discarding port
- Edge port
Correct answer: Alternate port
In RSTP, the Alternate port is in the Discarding state and provides a redundant path to the root bridge if the root port fails.
Question 2: What is the purpose of the STP Topology Change Notification (TCN) BPDU?
- To inform other switches that the Layer 2 topology has changed so MAC tables can be flushed (Correct answer)
- To elect a new root bridge after a link failure
- To block redundant ports in the network
- To synchronize VLAN databases across switches
Correct answer: To inform other switches that the Layer 2 topology has changed so MAC tables can be flushed
TCN BPDUs signal topology changes so that switches reduce their MAC address aging timer to quickly flush stale entries.
Question 3: A network has three switches with default bridge priorities. Switch A has MAC 00:00:00:00:00:01, Switch B has MAC 00:00:00:00:00:02, and Switch C has MAC 00:00:00:00:00:03. Which switch becomes the root bridge?
- Switch A (Correct answer)
- Switch B
- Switch C
- They rotate the root bridge role
Correct answer: Switch A
With equal priorities, the switch with the lowest MAC address wins; 00:00:00:00:00:01 is the lowest, making Switch A the root bridge.
Question 4: Which STP port state is equivalent to the RSTP 'Discarding' state?
- Blocking (Correct answer)
- Listening
- Learning
- Forwarding
Correct answer: Blocking
RSTP combines the STP Blocking and Listening states into a single Discarding state where neither forwarding nor MAC learning occurs.
Question 5: What is the STP path cost for a 1 Gbps link using the IEEE 802.1D-2004 revised cost values?
- 4 (Correct answer)
- 19
- 100
- 2000
Correct answer: 4
The revised IEEE 802.1D-2004 cost for a 1 Gbps link is 4, while the original 1998 standard assigned it a cost of 4 as well (but 10 Gbps=2, 100 Mbps=19).
Question 6: Which command verifies the STP role and state of all ports on a Cisco switch for VLAN 10?
- show spanning-tree vlan 10 (Correct answer)
- show spanning-tree detail
- show interface trunk
- show vlan brief
Correct answer: show spanning-tree vlan 10
The 'show spanning-tree vlan 10' command displays the root bridge, bridge priority, port roles, port states, and costs for VLAN 10.
Question 7: Which RSTP port type connects to another switch's designated port and can achieve rapid transition to Forwarding?
- Point-to-point port (Correct answer)
- Shared port
- Edge port
- Alternate port
Correct answer: Point-to-point port
RSTP point-to-point ports (full-duplex links to other switches) can negotiate rapid transitions using the proposal/agreement mechanism.
Cisco CCNA Spanning Tree Protocol (STP) 3
Which Rapid STP (802.1w) port role replaces the STP blocking state and provides an alternate path to the root bridge?