CCNA Switching and Network Access Practice Question
Exhibit
SwitchA# 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 32778 (priority 32768 sys-id-ext 10)
Address 0011.2233.4455
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 Desg FWD 4 128.2 P2p
Gi0/3 Altn BLK 4 128.3 P2pA 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?
⚠ Common exam trap
Cisco often tests the misconception that a blocking port on a root bridge indicates a physical or configuration issue with that specific port, when in fact it signals that the switch is not the root bridge due to a lower priority on another switch.
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
✓
Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Change the port type of Gi0/3 to trunk to allow multiple VLANs.
Why it's wrong here
Configuring Gi0/3 as a trunk only affects VLAN tagging and the set of VLANs carried, not the STP port role. The blocking state indicates an STP decision—likely that the current root bridge has a higher bridge priority than another switch for VLAN 10—so the port cannot forward even after trunk mode is set. Trunking would be relevant if VLANs were being pruned, but it cannot change the root bridge election or port cost calculations.
- ✗
Configure spanning-tree portfast on Gi0/3 to speed up convergence.
Why it's wrong here
Portfast is intended solely for access ports connected to end stations, allowing them to bypass the listening and learning states to converge quickly. Applying it to an inter-switch uplink like Gi0/3 is dangerous because it could create a temporary bridging loop during STP recalculation. Even if Portfast were enabled, the port would still block after the switch receives the superior BPDU that elected another switch as root; it only skips the forward-delay timers, not the STP decision itself.
- ✓
Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.
Why this is correct
The root bridge is elected based on the lowest bridge priority. If another switch has a lower priority, it becomes the root, causing ports on the current root to block. Verifying and adjusting priorities will ensure the correct root bridge election.
- ✗
Enable BPDU guard on Gi0/3 to prevent unauthorized switches from affecting the network.
Why it's wrong here
BPDU guard is an STP protection feature typically enabled on PortFast-configured access ports; when it receives a BPDU, it disables the port to prevent rogue switches. On an uplink connecting two switches, BPDUs are expected and necessary for STP operation, so enabling BPDU guard would shut down Gi0/3 and disrupt the link entirely. It does nothing to influence root bridge selection or why Gi0/3 is in a blocking state, since that state is determined by root bridge priorities and path costs.
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.
✓Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.Correct answer▾
Why this is correct
The root bridge is elected based on the lowest bridge priority. If another switch has a lower priority, it becomes the root, causing ports on the current root to block. Verifying and adjusting priorities will ensure the correct root bridge election.
✗Change the port type of Gi0/3 to trunk to allow multiple VLANs.Wrong answer — click to see why▾
Why this is wrong here
Changing the port type to trunk does not affect spanning-tree root bridge election or port roles. The blocking state is determined by spanning-tree topology, not by trunk configuration.
Why candidates choose this
Students might think that a trunk port is needed for multiple VLANs and that misconfiguration could cause blocking, but the issue is about root bridge election, not port type.
✗Configure spanning-tree portfast on Gi0/3 to speed up convergence.Wrong answer — click to see why▾
Why this is wrong here
Portfast is intended for access ports connected to end devices to bypass listening/learning states; it is not used on uplinks and does not resolve a blocking state caused by spanning-tree topology.
Why candidates choose this
Portfast is often associated with faster convergence, so a student might incorrectly assume it can fix a blocking state, but it only applies to edge ports.
✗Enable BPDU guard on Gi0/3 to prevent unauthorized switches from affecting the network.Wrong answer — click to see why▾
Why this is wrong here
BPDU guard is used on access ports to protect against rogue switches by disabling the port if a BPDU is received. It does not affect root bridge election or port roles on uplinks.
Why candidates choose this
BPDU guard is a common security feature, and students might think it could prevent unwanted topology changes, but it is not applicable to uplink ports in this scenario.
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?”
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Forwarding state
A Spanning Tree Protocol port state where the port is fully operational, forwarding both data frames and BPDUs, and is part of the active loop-free topology.
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.