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Question 559 of 1,052
hardmultiple choiceObjective-mapped
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 interfaces status
Port Name Status Vlan Duplex Speed Type
Gi0/1 err-disabled 10 auto auto 10/100/1000BaseTX
Gi0/2 connected 10 a-full a-100 10/100/1000BaseTX
Gi0/3 connected 1 a-full a-100 10/100/1000BaseTX
SwitchC# show interfaces gigabitEthernet 0/1
GigabitEthernet0/1 is down, line protocol is down (err-disabled)
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
SwitchC# show errdisable recovery
ErrDisable Reason Timer Status
----------------- --------------
arp-inspection Disabled
bpduguard Enabled
channel-misconfig Disabled
...
SwitchC# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 32778
Address aaaa.bbbb.cccc
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 aaaa.bbbb.cccc
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Desg FWD 4 128.3 P2p
SwitchC# show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
spanning-tree portfast
spanning-tree bpduguard enableA network administrator is troubleshooting connectivity loss in a switched network. All switches run Rapid PVST+. A host connected to an access port on SwitchC can no longer reach the default gateway. The access port is configured with PortFast and BPDU Guard. The administrator checks the interface status and finds it in an err-disabled state. 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.
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
✓
BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it.
The interface Gi0/1 is in an err-disabled state because BPDU Guard detected a BPDU on a PortFast-enabled port, which indicates an unauthorized switch or hub connection. BPDU Guard transitions the port to err-disabled to prevent a Layer 2 loop. The correct action is to configure BPDU Guard filtering on the port to allow BPDUs while keeping PortFast enabled, or to remove BPDU Guard if the connected device is legitimate. Option A is wrong because the root bridge is correctly elected (SwitchC is root). Option C is incorrect because Rapid PVST+ is configured and operational. Option D is wrong because the port is configured as an access port, not a trunk, and the issue is not related to trunk misconfiguration.
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.
- ✗
The root bridge election failed, causing a loop.
Why it's wrong here
The root bridge is correctly elected (SwitchC is root for VLAN 10).
- ✓
BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it.
Why this is correct
BPDU Guard is enabled on Gi0/1, and a BPDU was received, causing the port to go err-disabled.
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.
- ✗
Rapid PVST+ is not compatible with PortFast.
Why it's wrong here
Rapid PVST+ and PortFast are compatible; PortFast is designed for access ports in Rapid PVST+.
- ✗
The port is configured as a trunk but should be an access port.
Why it's wrong here
The port is configured as an access port (switchport mode access), not a trunk.
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.
✓BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it.Correct answer▾
Why this is correct
BPDU Guard is enabled on Gi0/1, and a BPDU was received, causing the port to go err-disabled.
✗The root bridge election failed, causing a loop.Wrong answer — click to see why▾
Why this is wrong here
The root bridge election is not the issue; the port is err-disabled due to BPDU Guard.
✗Rapid PVST+ is not compatible with PortFast.Wrong answer — click to see why▾
Why this is wrong here
PortFast works with Rapid PVST+; the issue is BPDU Guard, not compatibility.
✗The port is configured as a trunk but should be an access port.Wrong answer — click to see why▾
Why this is wrong here
The port is already an access port; trunk misconfiguration is not the cause.
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: BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it. — The interface Gi0/1 is in an err-disabled state because BPDU Guard detected a BPDU on a PortFast-enabled port, which indicates an unauthorized switch or hub connection. BPDU Guard transitions the port to err-disabled to prevent a Layer 2 loop. The correct action is to configure BPDU Guard filtering on the port to allow BPDUs while keeping PortFast enabled, or to remove BPDU Guard if the connected device is legitimate. Option A is wrong because the root bridge is correctly elected (SwitchC is root). Option C is incorrect because Rapid PVST+ is configured and operational. Option D is wrong because the port is configured as an access port, not a trunk, and the issue is not related to trunk misconfiguration.
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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