CCNA Switching and Network Access Practice Question
Network Topology
You are connected to SW1 via the console. The network has three switches connected in a triangle: SW1 (G0/1 to SW2 G0/1), SW1 (G0/2 to SW3 G0/1), and SW2 (G0/2 to SW3 G0/2). SW1 is the root bridge. A PC is connected to SW3's G0/3 port, which should be an edge port. However, the PC has been sending BPDUs, causing the port to go err-disabled. Configure SW3 to prevent this in the future: enable PortFast and BPDU Guard on G0/3. Then, verify that the port recovers from err-disabled state and that a specific blocked port on SW2 is identified. Use the provided show output to determine the current state and necessary commands.
⚠ Common exam trap
A common trap is forgetting that err-disabled ports require a manual shutdown/no shutdown cycle to recover. Simply enabling BPDU Guard or saving the configuration does not restore the port. Always remember to reset the interface after correcting the cause.
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
✓
interface g0/3 spanning-tree portfast spanning-tree bpduguard enable shutdown no shutdown
The PC connected to SW3's G0/3 was sending BPDUs, causing the port to go err-disabled due to BPDU Guard. To prevent this, enable PortFast and BPDU Guard on that interface. First, move to interface configuration mode for G0/3, then issue 'spanning-tree portfast' and 'spanning-tree bpduguard enable'. After configuration, the port will remain err-disabled until manually recovered by issuing 'shutdown' followed by 'no shutdown'. The blocked port on SW2 is G0/2, as shown by the 'Altn BLK' role/status in its spanning-tree output.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
interface g0/3 spanning-tree portfast spanning-tree bpduguard enable shutdown no shutdown
Why this is correct
This sequence correctly enables PortFast and BPDU Guard on the interface, then manually recovers the err-disabled port by issuing shutdown followed by no shutdown. PortFast makes the port an edge port, and BPDU Guard disables it if a BPDU is received, preventing future err-disable events.
- ✗
interface g0/3 spanning-tree portfast spanning-tree bpduguard enable no shutdown
Why it's wrong here
This is incorrect because after enabling BPDU Guard, the port is already in err-disabled state and will not recover without a manual shutdown/no shutdown cycle. Simply issuing 'no shutdown' without first shutting down the interface will not clear the err-disabled state.
- ✗
interface g0/3 spanning-tree portfast spanning-tree bpduguard enable end copy running-config startup-config
Why it's wrong here
The command sequence enables PortFast and BPDU Guard, but then exits to privileged exec and saves the running configuration. This does not address the immediate operational issue: the port is currently in err-disabled state because BPDU Guard detected a BPDU. Saving the configuration only persists the intended PortFast/BPDU Guard settings; it does not clear the error condition. To recover, the port must be manually cycled with shutdown followed by no shutdown, or the interface must be brought out of err-disabled via other means (e.g., errdisable recovery). Therefore, this sequence leaves the port offline.
- ✗
interface g0/3 spanning-tree portfast spanning-tree bpduguard enable shutdown
Why it's wrong here
After enabling BPDU Guard, issuing only shutdown places the port administratively down. While shutdown does clear the err-disabled state (as part of the manual recovery process), the port remains down because no subsequent no shutdown is issued. The standard recovery sequence requires both shutdown and no shutdown to transition from err-disabled to up/up. Hence, this incomplete sequence fails to restore connectivity.
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.
✓interface g0/3 spanning-tree portfast spanning-tree bpduguard enable shutdown no shutdownCorrect answer▾
Why this is correct
This sequence correctly enables PortFast and BPDU Guard on the interface, then manually recovers the err-disabled port by issuing shutdown followed by no shutdown. PortFast makes the port an edge port, and BPDU Guard disables it if a BPDU is received, preventing future err-disable events.
✗interface g0/3 spanning-tree portfast spanning-tree bpduguard enable no shutdownWrong answer — click to see why▾
Why this is wrong here
The err-disabled state requires a manual interface reset (shutdown followed by no shutdown) to recover; a single 'no shutdown' command is insufficient.
Why candidates choose this
Candidates may think that 'no shutdown' alone can recover an err-disabled port, but the port must first be shut down to clear the error condition.
✗interface g0/3 spanning-tree portfast spanning-tree bpduguard enable end copy running-config startup-configWrong answer — click to see why▾
Why this is wrong here
Saving the configuration preserves the settings but does not affect the current operational state of the interface; the port stays err-disabled.
Why candidates choose this
Candidates may confuse configuration tasks with operational recovery, thinking that saving the config will also reset the interface.
✗interface g0/3 spanning-tree portfast spanning-tree bpduguard enable shutdownWrong answer — click to see why▾
Why this is wrong here
The shutdown command disables the interface but does not automatically re-enable it; the err-disabled state is cleared only after a shutdown/no shutdown cycle.
Why candidates choose this
Candidates might think that shutting down the interface alone will clear the err-disabled condition, but the port remains administratively down without a 'no shutdown'.
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?”
Visual reference
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Related to this question
Learn chapter
Interpreting Packet Capture Output for Layer 2/3 Troubleshooting
Key term
Bridge Protocol Data Unit
A Bridge Protocol Data Unit (BPDU) is a special message that network switches exchange to detect and prevent loops in an Ethernet network.
Key term
BPDU Guard
BPDU Guard is a Cisco switch feature that protects the network from unauthorized devices by disabling a port if it receives a Bridge Protocol Data Unit (BPDU).
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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