This 350-401 practice question tests your understanding of spanning tree protocol. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer runs the following command on Switch SW3:
SW3# show spanning-tree vlan 30
VLAN0030 Spanning tree enabled protocol ieee Root ID Priority 24606 Address aabb.cc00.0400 Cost 12 Port 2 (GigabitEthernet0/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30) Address aabb.cc00.0500 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Desg FWD 4 128.1 P2p Gi0/2 Root FWD 12 128.2 P2p Gi0/3 Desg FWD 4 128.3 P2p
Based on this output, what is the root path cost from SW3 to the root bridge for VLAN 30?
4
Why wrong: Incorrect. 4 is the cost of Gi0/1 and Gi0/3, not the root path cost.
12
Correct. The root path cost is 12, as shown in the Root ID section.
16
Why wrong: Incorrect. There is no indication of 16.
20
Why wrong: Incorrect. 20 is the Max Age, not a cost.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
12
The root path cost is the cost of the path from this switch to the root bridge. It is shown in the Root ID section as 'Cost 12'. This is the cumulative cost via the root port (Gi0/2, which has a cost of 12).
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
4
Why it's wrong here
Incorrect. 4 is the cost of Gi0/1 and Gi0/3, not the root path cost.
12
Why this is correct
Correct. The root path cost is 12, as shown in the Root ID section.
Related concept
Access ports place end devices into a single VLAN.
16
Why it's wrong here
Incorrect. There is no indication of 16.
20
Why it's wrong here
Incorrect. 20 is the Max Age, not a cost.
Common exam traps
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.
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FAQ
Spanning Tree Protocol — This question tests Spanning Tree Protocol — Access ports place end devices into a single VLAN..
The correct answer is: 12 — The root path cost is the cost of the path from this switch to the root bridge. It is shown in the Root ID section as 'Cost 12'. This is the cumulative cost via the root port (Gi0/2, which has a cost of 12).
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 18, 2026
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