The correct conclusion is that GigabitEthernet0/3 is in the Blocking state to prevent a loop, as indicated by the "Altn BLK" role and status in the output. This is because SW2 is not the root bridge—its bridge priority of 32778 is higher than the root priority of 24596—so it must have one root port (Gi0/1) to reach the root bridge, one designated port (Gi0/2) on its segment, and any remaining redundant ports become alternate ports that are placed in the blocking state to break potential loops. On the ENCOR 350-401 exam, this type of show spanning-tree output interpretation tests your ability to quickly identify root bridge status, port roles, and STP states, with a common trap being to assume a switch with a lower bridge ID is always the root—remember that the root is determined by the lowest bridge priority, not the local switch’s ID. A useful memory tip: "Alternate = Blocking, Designated = Forwarding, Root = Forwarding" helps you map roles to states at a glance.
350-401 Spanning Tree Protocol Practice Question
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 SW2:
SW2# show spanning-tree vlan 20
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 24596
Address aabb.cc00.0200
Cost 4
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 20)
Address aabb.cc00.0300
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
✓
GigabitEthernet0/3 is in the Blocking state to prevent a loop.
SW2 is not the root bridge (its bridge priority 32778 is higher than root priority 24596). Gi0/1 is the root port (cost 4 to root), Gi0/2 is a designated port, and Gi0/3 is an alternate port in blocking state. The root bridge is at MAC aabb.cc00.0200.
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.
✗
SW2 is the root bridge for VLAN 20.
Why it's wrong here
Incorrect. The root ID shows a different bridge (aabb.cc00.0200) with lower priority.
✓
GigabitEthernet0/3 is in the Blocking state to prevent a loop.
Why this is correct
Correct. Gi0/3 is shown as Altn BLK (Alternate, Blocking), which prevents a loop.
Related concept
Access ports place end devices into a single VLAN.
✗
The root port cost is 8.
Why it's wrong here
Incorrect. The root cost is 4, as shown in the Root ID section.
✗
SW2's bridge priority is 32768.
Why it's wrong here
Incorrect. The bridge priority is 32778 (32768 + 20 for VLAN 20).
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.
Trap categories for this question
Command / output trap
Incorrect. The root ID shows a different bridge (aabb.cc00.0200) with lower priority.
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.
Spanning Tree Protocol — This question tests Spanning Tree Protocol — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: GigabitEthernet0/3 is in the Blocking state to prevent a loop. — SW2 is not the root bridge (its bridge priority 32778 is higher than root priority 24596). Gi0/1 is the root port (cost 4 to root), Gi0/2 is a designated port, and Gi0/3 is an alternate port in blocking state. The root bridge is at MAC aabb.cc00.0200.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer runs the following command on Switch SW5:
SW5# show spanning-tree vlan 50
VLAN0050
Spanning tree enabled protocol ieee
Root ID Priority 24626
Address aabb.cc00.0800
Cost 4
Port 1 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 50)
Address aabb.cc00.0900
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 Root FWD 4 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Desg FWD 4 128.3 P2p
Gi0/4 Altn BLK 4 128.4 P2p
Based on this output, how many ports are in the Forwarding state?
easy
A.1
B.2
✓ C.3
D.4
Why C: The output shows three ports in Forwarding state: Gi0/1 (Root FWD), Gi0/2 (Desg FWD), and Gi0/3 (Desg FWD). Gi0/4 is in Blocking state (Altn BLK).
Variation 2. 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?
hard
A.4
✓ B.12
C.16
D.20
Why B: 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).
Variation 3. A network engineer runs the following command on Switch SW6:
SW6# show spanning-tree vlan 60
VLAN0060
Spanning tree enabled protocol ieee
Root ID Priority 24636
Address aabb.cc00.0a00
Cost 8
Port 1 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 60)
Address aabb.cc00.0b00
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 Root FWD 8 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Altn BLK 4 128.3 P2p
Gi0/4 Desg FWD 4 128.4 P2p
Based on this output, what is the bridge priority of the root bridge for VLAN 60?
medium
A.24576
✓ B.24636
C.32768
D.32828
Why B: The root bridge priority is shown in the Root ID section as 24636. This includes the system ID extension of 60, so the base priority is 24576 (24636 - 60 = 24576).
Variation 4. A network engineer runs the following command on Switch SW4:
SW4# show spanning-tree vlan 40
VLAN0040
Spanning tree enabled protocol ieee
Root ID Priority 24616
Address aabb.cc00.0600
Cost 8
Port 1 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 40)
Address aabb.cc00.0700
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 Root FWD 8 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Desg FWD 4 128.3 P2p
Based on this output, which port is the root port?
easy
✓ A.GigabitEthernet0/1
B.GigabitEthernet0/2
C.GigabitEthernet0/3
D.There is no root port because SW4 is the root bridge.
Why A: The root port is the port that provides the best path to the root bridge. In the output, Gi0/1 is listed as 'Root FWD', indicating it is the root port.
Variation 5. A network engineer runs the following command on Switch SW7:
SW7# show spanning-tree vlan 70
VLAN0070
Spanning tree enabled protocol ieee
Root ID Priority 24646
Address aabb.cc00.0c00
Cost 4
Port 1 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 70)
Address aabb.cc00.0d00
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 Root FWD 4 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Altn BLK 4 128.3 P2p
Based on this output, which port is the alternate port?
easy
A.GigabitEthernet0/1
B.GigabitEthernet0/2
✓ C.GigabitEthernet0/3
D.There is no alternate port.
Why C: The alternate port is the port that provides an alternative path to the root bridge and is placed in blocking state. In the output, Gi0/3 is shown as 'Altn BLK'.
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