The answer is that the interface GigabitEthernet0/0 is blocking traffic because it received a superior BPDU, which forced it into the alternate port role. In Spanning Tree Protocol, a port transitions to the blocking state when it hears a BPDU with a lower bridge ID or a lower path cost to the root bridge than its own, making it an alternate port that provides a redundant path but does not forward data. This scenario directly tests your understanding of STP port roles and states under the CCNA 200-301 v2 exam, where a common trap is confusing an alternate port (blocking due to a superior BPDU) with a root port (forwarding). The key takeaway is that an alternate port is not a failure—it is a deliberate loop-prevention mechanism triggered by a better path existing elsewhere. Remember the mnemonic: "Alternate blocks because it heard a better BPDU first."
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
SW3# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 00d0.97f1.8a00
Cost 4
Port 25 (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 00d0.97f3.b200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Altn BLK 4 128.25 P2p
Gi0/1 Root FWD 4 128.26 P2p
Gi0/2 Desg FWD 19 128.27 P2p
Refer to the exhibit. A network engineer is troubleshooting a connectivity issue on SW3. A host connected to the same segment as SW3's GigabitEthernet0/0 interface cannot reach any network resources. The engineer issues the show spanning-tree vlan 10 command and receives the output shown. Based on the output, what is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
SW3# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 00d0.97f1.8a00
Cost 4
Port 25 (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 00d0.97f3.b200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Altn BLK 4 128.25 P2p
Gi0/1 Root FWD 4 128.26 P2p
Gi0/2 Desg FWD 19 128.27 P2p
A
GigabitEthernet0/0 is administratively down, which prevents the host from communicating.
Why wrong: The output shows the port state as 'BLK' (blocking), not 'disable' or 'down'. Administratively down ports are not present in spanning-tree output or would show 'Disable' state.
B
The port is in the Blocking state because the switch detected a loop and moved the port to error-disabled state.
Why wrong: Error-disabled ports would show a status of 'err-disabled' in spanning-tree output, not 'BLK'. The output clearly shows the port is in the normal blocking state (BLK) as part of STP convergence, not due to an error condition.
C
The port is blocked because SW3 has a lower bridge priority than the root bridge and should be the designated port for that segment.
Why wrong: The exhibit shows the root bridge is 00d0.97f1.8a00, and SW3's port Gi0/0 has role 'Altn' (alternate), not 'Desg' (designated). A designated port would be forwarding, not blocking. SW3 is not the designated bridge for that segment.
D
The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.
The output explicitly shows role 'Altn' and state 'BLK' for Gi0/0. An alternate port is blocked because it receives better BPDUs on that interface than it can send, providing an alternate path to the root bridge. This is correct STP behavior, and the blocking state prevents the host from communicating.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.
The output shows that GigabitEthernet0/0 is in the Blocking state for VLAN 10. In Rapid PVST+ or classic STP, a port enters the Blocking state when it receives a superior BPDU (i.e., a BPDU with a lower bridge ID or lower path cost to the root), causing it to become an alternate (or backup) port rather than a designated or root port. This prevents the host from reaching network resources because the port does not forward traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
GigabitEthernet0/0 is administratively down, which prevents the host from communicating.
Why it's wrong here
The output shows the port state as 'BLK' (blocking), not 'disable' or 'down'. Administratively down ports are not present in spanning-tree output or would show 'Disable' state.
✗
The port is in the Blocking state because the switch detected a loop and moved the port to error-disabled state.
Why it's wrong here
Error-disabled ports would show a status of 'err-disabled' in spanning-tree output, not 'BLK'. The output clearly shows the port is in the normal blocking state (BLK) as part of STP convergence, not due to an error condition.
✗
The port is blocked because SW3 has a lower bridge priority than the root bridge and should be the designated port for that segment.
Why it's wrong here
The exhibit shows the root bridge is 00d0.97f1.8a00, and SW3's port Gi0/0 has role 'Altn' (alternate), not 'Desg' (designated). A designated port would be forwarding, not blocking. SW3 is not the designated bridge for that segment.
✓
The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.
Why this is correct
The output explicitly shows role 'Altn' and state 'BLK' for Gi0/0. An alternate port is blocked because it receives better BPDUs on that interface than it can send, providing an alternate path to the root bridge. This is correct STP behavior, and the blocking state prevents the host from communicating.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.Correct answer▾
Why this is correct
The output explicitly shows role 'Altn' and state 'BLK' for Gi0/0. An alternate port is blocked because it receives better BPDUs on that interface than it can send, providing an alternate path to the root bridge. This is correct STP behavior, and the blocking state prevents the host from communicating.
✗GigabitEthernet0/0 is administratively down, which prevents the host from communicating.Wrong answer — click to see why▾
Why this is wrong here
Candidates may incorrectly associate the blocked state with an administratively disabled interface.
✗The port is in the Blocking state because the switch detected a loop and moved the port to error-disabled state.Wrong answer — click to see why▾
Why this is wrong here
Candidates often confuse error-disabled state (caused by features like BPDU guard) with the standard STP blocking state.
✗The port is blocked because SW3 has a lower bridge priority than the root bridge and should be the designated port for that segment.Wrong answer — click to see why▾
Why this is wrong here
Candidates may misunderstand the root election process and assume a lower priority switch always becomes designated for all segments, ignoring the Altn role.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between a port being blocked due to normal STP operation (receiving a superior BPDU) versus being error-disabled or administratively down, leading candidates to incorrectly assume a physical or administrative issue.
Trap categories for this question
Command / output trap
The output shows the port state as 'BLK' (blocking), not 'disable' or 'down'. Administratively down ports are not present in spanning-tree output or would show 'Disable' state.
Detailed technical explanation
How to think about this question
In STP, each port on a non-root bridge transitions through listening, learning, and forwarding states; a port that receives a superior BPDU from another switch on the same segment becomes an alternate port and remains in the Blocking state to prevent loops. The 'show spanning-tree vlan 10' output reveals the port role (Altn) and state (BLK), confirming it is an alternate port. In real-world scenarios, this often occurs when redundant links exist and the switch is not the root, causing the port to block even if the link is physically up.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 200-301 question in full detail.
Switching and Network Access — This question tests Switching and Network Access — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge. — The output shows that GigabitEthernet0/0 is in the Blocking state for VLAN 10. In Rapid PVST+ or classic STP, a port enters the Blocking state when it receives a superior BPDU (i.e., a BPDU with a lower bridge ID or lower path cost to the root), causing it to become an alternate (or backup) port rather than a designated or root port. This prevents the host from reaching network resources because the port does not forward traffic.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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