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CCNA Practice Question: A network administrator is troubleshooting…

This 200-301 practice question tests your understanding of 200-301 exam topics. 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

SwitchC# show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol rstp
  Root ID    Priority    32778
             Address     0011.2233.4455
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    40968  (priority 40960 sys-id-ext 10)
             Address     00a1.b2c3.d4e5
             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 LRN 4         128.1    P2p
Gi0/2               Root BLK 4         128.2    P2p

A network administrator is troubleshooting connectivity issues in a switched network. Hosts on VLAN 10 connected to SwitchC cannot reach the VLAN 10 gateway, which is connected to SwitchA. The administrator checks the STP status on SwitchC and sees that the port connecting to the root bridge is in a blocking state. The administrator also notices that the VLAN 10 gateway is reachable from SwitchA, but not from SwitchC. What is the most likely cause of this issue?

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.

Question 1hardmultiple choice
Full question →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge.

The issue is that SwitchC believes it is the root bridge for VLAN 10 (as indicated by 'This bridge is the root'), but its Bridge ID priority (40968) is higher than the actual root bridge's priority (32778). This is a classic symptom of a bridge ID priority misconfiguration. The root bridge election in STP/Rapid PVST+ uses the lowest bridge ID (priority + VLAN ID). SwitchC has a priority of 40960 + 10 = 40970 (displayed as 40968 due to a typo in the exhibit), which is higher than the actual root's 32768 + 10 = 32778. This causes SwitchC to incorrectly assume it is the root, leading to port role conflicts and blocking of the correct root port (Gi0/2 is shown as Root BLK, but it should be the root port). The correct fix is to adjust the priority on SwitchC to be higher (numerically lower) than the current root, or to set it to a lower priority on the intended root switch.

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.

  • ✗

    Configure PortFast on interface Gi0/2 to bring it up immediately.

    Why it's wrong here

    PortFast is used to bypass the listening and learning states on access ports, but it does not resolve the root bridge election issue. The port is blocking due to STP topology inconsistency, not because of slow convergence.

  • ✓

    Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge.

    Why this is correct

    By setting the priority to 24576, SwitchC's bridge ID becomes 24586 (24576+10), which is lower than the current root's 32778. This will cause SwitchC to become the root bridge if that is the intent, or by setting it to a higher priority (like 40960) it would lose the election. However, the correct action is to make SwitchC's priority higher (numerically lower) to correct the misconfiguration. In this case, the intended root (SwitchA) should have a lower priority, or SwitchC should have a higher priority (e.g., 32768) to not be root. Actually, the correct answer is to adjust the priority so that SwitchC is not root. Setting it to 24576 would make it root, which might be the desired outcome if SwitchA is misconfigured. But given the symptom, the most direct fix is to ensure the correct root bridge has the lowest priority. The exhibit shows SwitchC's priority is 40960, which is too high, so lowering it to a value less than the current root (32768) would make it root, but that might not be the intended design. The typical fix is to set the priority on the desired root switch to a lower value. However, since the question asks for the most likely cause, the answer is to correct the priority on SwitchC to match the intended root. Given the options, B is correct because it addresses the priority misconfiguration.

    Clue confirmation

    The clue word "most likely" in the question point toward this answer.

    Related concept

    Access ports place end devices into a single VLAN.

  • ✗

    Enable BPDU Guard on interface Gi0/2 to prevent BPDU attacks.

    Why it's wrong here

    BPDU Guard is used to protect against unauthorized switches, but it does not fix the root bridge election issue. The port is blocking due to STP, not due to BPDU violations.

  • ✗

    Configure the spanning-tree mode to PVST+ instead of Rapid PVST+.

    Why it's wrong here

    Both PVST+ and Rapid PVST+ use the same bridge ID election process. Changing the mode would not resolve the priority misconfiguration.

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.

✓Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge.Correct answer▾

Why this is correct

By setting the priority to 24576, SwitchC's bridge ID becomes 24586 (24576+10), which is lower than the current root's 32778. This will cause SwitchC to become the root bridge if that is the intent, or by setting it to a higher priority (like 40960) it would lose the election. However, the correct action is to make SwitchC's priority higher (numerically lower) to correct the misconfiguration. In this case, the intended root (SwitchA) should have a lower priority, or SwitchC should have a higher priority (e.g., 32768) to not be root. Actually, the correct answer is to adjust the priority so that SwitchC is not root. Setting it to 24576 would make it root, which might be the desired outcome if SwitchA is misconfigured. But given the symptom, the most direct fix is to ensure the correct root bridge has the lowest priority. The exhibit shows SwitchC's priority is 40960, which is too high, so lowering it to a value less than the current root (32768) would make it root, but that might not be the intended design. The typical fix is to set the priority on the desired root switch to a lower value. However, since the question asks for the most likely cause, the answer is to correct the priority on SwitchC to match the intended root. Given the options, B is correct because it addresses the priority misconfiguration.

✗Configure PortFast on interface Gi0/2 to bring it up immediately.Wrong answer — click to see why▾

Why this is wrong here

PortFast would not fix the root bridge mis-election; it would only cause the port to transition to forwarding faster, but the underlying STP problem would remain.

✗Enable BPDU Guard on interface Gi0/2 to prevent BPDU attacks.Wrong answer — click to see why▾

Why this is wrong here

BPDU Guard would err-disable the port if a BPDU is received, which is not the case here.

✗Configure the spanning-tree mode to PVST+ instead of Rapid PVST+.Wrong answer — click to see why▾

Why this is wrong here

The STP mode does not affect the root bridge election logic; the issue is with the priority values, not the protocol version.

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: 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.

What to study next

Got this wrong? Here's your next step.

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 200-301 questions on switching, trunking, and access-port configuration.

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FAQ

Questions learners often ask

What does this 200-301 question test?

Access ports place end devices into a single VLAN.

What is the correct answer to this question?

The correct answer is: Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge. — The issue is that SwitchC believes it is the root bridge for VLAN 10 (as indicated by 'This bridge is the root'), but its Bridge ID priority (40968) is higher than the actual root bridge's priority (32778). This is a classic symptom of a bridge ID priority misconfiguration. The root bridge election in STP/Rapid PVST+ uses the lowest bridge ID (priority + VLAN ID). SwitchC has a priority of 40960 + 10 = 40970 (displayed as 40968 due to a typo in the exhibit), which is higher than the actual root's 32768 + 10 = 32778. This causes SwitchC to incorrectly assume it is the root, leading to port role conflicts and blocking of the correct root port (Gi0/2 is shown as Root BLK, but it should be the root port). The correct fix is to adjust the priority on SwitchC to be higher (numerically lower) than the current root, or to set it to a lower priority on the intended root switch.

What should I do if I get this 200-301 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 200-301 questions on switching, trunking, and access-port configuration.

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?

Access ports place end devices into a single VLAN.

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