The answer is that Gi0/2 is in a blocking state due to loop prevention. This conclusion is drawn from the show spanning-tree output, which reveals that the root bridge has a MAC address of 0011.2233.4455 and a priority of 32778, while SW1’s bridge ID shows the same priority but a higher MAC address of 0011.2233.4466, confirming SW1 is not the root. Since Gi0/1 is the root port (cost 19) and Gi0/3 is a designated port, Gi0/2 becomes an alternate port placed in blocking state to break the redundant loop—a core function of spanning tree root bridge identification. On the ENCOR 350-401 exam, this tests your ability to read STP output and differentiate root, designated, and alternate ports; a common trap is assuming the local switch is the root when it has the same priority, but the lower MAC address always wins. Remember the mnemonic: “Lower MAC, lower cost, root boss.”
350-401 Infrastructure Security Practice Question
This 350-401 practice question tests your understanding of infrastructure security. 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 SW1:
SW1# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 0011.2233.4455
Cost 19
Port 1 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0011.2233.4466
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Gi0/2 is in blocking state due to loop prevention.
The output shows that the root bridge has address 0011.2233.4455, and SW1's bridge ID is 0011.2233.4466. Since the root bridge has a lower MAC address (and same priority), SW1 is not the root. Gi0/1 is the root port, Gi0/2 is an alternate port (blocking), and Gi0/3 is a designated port. This indicates a redundant topology with a loop, and STP is blocking Gi0/2 to prevent it.
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
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
SW1 is the root bridge for VLAN 10.
Why it's wrong here
The root bridge has address 0011.2233.4455, not SW1's address 0011.2233.4466.
✓
Gi0/2 is in blocking state due to loop prevention.
Why this is correct
Gi0/2 is an alternate port in blocking state, which is normal STP behavior to prevent loops.
Related concept
Access ports place end devices into a single VLAN.
✗
Gi0/3 is a root port.
Why it's wrong here
Gi0/3 is a designated port (Desg), not a root port.
✗
The root bridge has a higher priority than SW1.
Why it's wrong here
Both have the same priority (32768 + 10 = 32778), so the root is chosen by lower MAC address.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
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
How to think about this question
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.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
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
How this comes up in practice
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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.
Infrastructure Security — This question tests Infrastructure Security — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Gi0/2 is in blocking state due to loop prevention. — The output shows that the root bridge has address 0011.2233.4455, and SW1's bridge ID is 0011.2233.4466. Since the root bridge has a lower MAC address (and same priority), SW1 is not the root. Gi0/1 is the root port, Gi0/2 is an alternate port (blocking), and Gi0/3 is a designated port. This indicates a redundant topology with a loop, and STP is blocking Gi0/2 to prevent it.
What should I do if I get this 350-401 question wrong?
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.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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