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. A key principle to apply: spanning Tree Protocol (STP) Modes. 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.
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
Configure PortFast on interface Gi0/2 to bring it up immediately.
Why wrong: PortFast is used on access ports to bypass STP convergence. It does not resolve a blocked uplink port and could cause loops.
B
Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge.
Why wrong: Lowering STP priority on SwitchC makes it more likely to become the root bridge, not less. This would not fix the blocking issue if the root bridge is using a different STP mode.
C
Enable BPDU Guard on interface Gi0/2 to prevent BPDU attacks.
Why wrong: BPDU Guard is used to protect against rogue switches, not to fix a blocked port due to STP mode mismatch.
D
Configure the spanning-tree mode to PVST+ instead of Rapid PVST+.
Changing spanning-tree mode to PVST+ ensures compatibility with the root bridge's STP mode, allowing the blocked port to transition to forwarding.
The answer is to change the STP priority on SwitchC to a lower value, such as 24576, to make it the root bridge for VLAN 10. This is correct because STP root bridge election troubleshooting often reveals that a non-root switch with a blocked port cannot forward traffic to the gateway; in this scenario, SwitchC’s port to the root bridge (SwitchA) is in a blocking state, which prevents VLAN 10 hosts from reaching their gateway. On the CCNA 200-301 v2 exam, this tests your understanding of how STP elects a root bridge based on the lowest bridge priority, and a common trap is assuming a blocked port indicates a physical fault rather than a logical STP state. The key insight is that lowering SwitchC’s priority below SwitchA’s ensures SwitchC becomes the root, placing its gateway-facing port in a forwarding state. Memory tip: “Lower priority wins the root—blocked ports mean you’re not the boss.”
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
✓
Configure the spanning-tree mode to PVST+ instead of Rapid PVST+.
The issue is that the port connecting to the root bridge is in a blocking state. In Rapid PVST+, if the root bridge is using PVST+, incompatibility can cause ports to be blocked. Configuring the spanning-tree mode to PVST+ on all switches ensures consistent STP operation, allowing the port to transition to forwarding and restoring connectivity.
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 on access ports to bypass STP convergence. It does not resolve a blocked uplink port and could cause loops.
✗
Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge.
Why it's wrong here
Lowering STP priority on SwitchC makes it more likely to become the root bridge, not less. This would not fix the blocking issue if the root bridge is using a different STP mode.
When this WOULD be correct
In a well-designed switched network, the root bridge should be centrally located to optimize traffic flow. By adjusting the STP priority on SwitchC to a lower value (e.g., 24576), it becomes the root bridge, which may be the intended design if SwitchA is misconfigured. This corrects the STP topology and allows the VLAN 10 gateway to be reachable from SwitchC.
✗
Enable BPDU Guard on interface Gi0/2 to prevent BPDU attacks.
Why it's wrong here
BPDU Guard is used to protect against rogue switches, not to fix a blocked port due to STP mode mismatch.
✓
Configure the spanning-tree mode to PVST+ instead of Rapid PVST+.
Why this is correct
Changing spanning-tree mode to PVST+ ensures compatibility with the root bridge's STP mode, allowing the blocked port to transition to forwarding.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
✓Configure the spanning-tree mode to PVST+ instead of Rapid PVST+.Correct answer▾
Why this is correct
Changing spanning-tree mode to PVST+ ensures compatibility with the root bridge's STP mode, allowing the blocked port to transition to forwarding.
✗Configure PortFast on interface Gi0/2 to bring it up immediately.Wrong answer — click to see why▾
Why this is 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.
Why candidates choose this
Students may confuse PortFast as a general solution for STP issues, thinking it can quickly bring up any blocked port, but it only applies to edge ports and does not affect root bridge selection.
✗Change the STP priority on SwitchC to a lower value (e.g., 24576) to ensure it is not the root bridge.Wrong answer — click to see why▾
Why this is wrong here
Lowering STP priority on SwitchC makes it more likely to become the root bridge, not less. This would not fix the blocking issue if the root bridge is using a different STP mode.
★ When this WOULD be the correct answer
In a well-designed switched network, the root bridge should be centrally located to optimize traffic flow. By adjusting the STP priority on SwitchC to a lower value (e.g., 24576), it becomes the root bridge, which may be the intended design if SwitchA is misconfigured. This corrects the STP topology and allows the VLAN 10 gateway to be reachable from SwitchC.
✗Enable BPDU Guard on interface Gi0/2 to prevent BPDU attacks.Wrong answer — click to see why▾
Why this is wrong here
BPDU Guard is used to protect against unauthorized switches by disabling a port if a BPDU is received, but it does not fix the root bridge election issue. The port is blocking due to STP, not due to BPDU violations.
Why candidates choose this
Students may think that enabling BPDU Guard can prevent unwanted BPDUs that cause blocking, but it actually disables the port upon receiving any BPDU, which would worsen connectivity.
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
Candidates often mistake a blocked port as a problem to fix with PortFast or priority changes, but compatibility between Rapid PVST+ and PVST+ can cause STP issues. Always verify the STP mode used across the network.
Detailed technical explanation
How to think about this question
STP uses Bridge ID (priority + MAC address) to elect the root bridge; a lower priority value (in increments of 4096) makes a switch more likely to become root. In PVST+ or Rapid PVST+, each VLAN has its own STP instance, so the root bridge for VLAN 10 can be different from other VLANs. The administrator can use the 'spanning-tree vlan 10 priority 24576' command to force SwitchC to become root for that VLAN, ensuring its designated port to the gateway is forwarding.
KKey Concepts to Remember
Spanning Tree Protocol (STP) Modes
Root Bridge
Blocking State
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
Spanning Tree Protocol (STP) Modes
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.
Visual reference
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 — Spanning Tree Protocol (STP) Modes.
What is the correct answer to this question?
The correct answer is: Configure the spanning-tree mode to PVST+ instead of Rapid PVST+. — The issue is that the port connecting to the root bridge is in a blocking state. In Rapid PVST+, if the root bridge is using PVST+, incompatibility can cause ports to be blocked. Configuring the spanning-tree mode to PVST+ on all switches ensures consistent STP operation, allowing the port to transition to forwarding and restoring connectivity.
What should I do if I get this 200-301 question wrong?
Review spanning Tree Protocol (STP) Modes, then practise related 200-301 questions on the same topic to reinforce the concept.
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?
Spanning Tree Protocol (STP) Modes
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Variation 1. A network administrator is troubleshooting connectivity issues in a switched network. Users on VLAN 10 report intermittent connectivity to the server farm. The network uses Rapid PVST+ as the spanning-tree protocol. The administrator examines the switch that is the root bridge for VLAN 10 and notices that one of the uplink interfaces to an access switch is in a blocking state. What is the most likely cause of this issue?
hard
A.Change the port type of Gi0/3 to trunk to allow multiple VLANs.
B.Configure spanning-tree portfast on Gi0/3 to speed up convergence.
✓ C.Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.
D.Enable BPDU guard on Gi0/3 to prevent unauthorized switches from affecting the network.
Why C: In Rapid PVST+, the root bridge for a VLAN should have all its ports in a forwarding state. If an uplink interface on the root bridge is blocking, it indicates that another switch is being elected as the root bridge for VLAN 10, likely because it has a lower spanning-tree priority. By checking and adjusting the priority on other switches, the administrator can ensure the intended switch becomes the root bridge, resolving the intermittent connectivity caused by suboptimal path selection.
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