CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
Cisco often tests the misconception that PortFast and BPDU Guard are incompatible with Rapid PVST+, but in reality, PortFast is a port-level feature that works identically across all spanning-tree variants, and BPDU Guard is the mechanism that causes the err-disabled state when a BPDU is received.
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.
B is correct because BPDU Guard is designed to protect the spanning-tree topology by disabling a PortFast-enabled port if it receives a BPDU, placing the port in err-disabled state. Option A is incorrect: a root bridge election failure would not cause a port to err-disable; loops do not directly trigger this state without BPDU Guard. Option C is incorrect because PortFast and BPDU Guard work with all spanning-tree variants including Rapid PVST+. Option D is incorrect: a trunk misconfiguration alone would not cause err-disable unless BPDU Guard detects a BPDU on a PortFast port.
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 did not fail; SwitchC is the correctly elected root for VLAN 10 based on the lowest bridge priority and MAC address. A failed root election would cause all switches to claim root status and generate BPDU storms, not a single port being placed into err-disabled state. The actual cause is BPDU Guard, which administratively disables the port upon receiving an unexpected BPDU.
- ✓
BPDU Guard detected a BPDU on a PortFast-enabled port and disabled it.
Why this is correct
BPDU Guard is a security feature that monitors PortFast-enabled ports for incoming BPDUs. When a BPDU is received, it immediately transitions the port to err-disabled state to prevent a potential switching loop, as a valid access port should never receive BPDUs. This exactly matches the symptom: Gi0/1 is down/err-disabled after BPDU Guard was enabled. The port will remain disabled until manually re-enabled or errdisable recovery is configured.
- ✗
Rapid PVST+ is not compatible with PortFast.
Why it's wrong here
Rapid PVST+ fully supports PortFast as a per-port enhancement. PortFast bypasses the listening and learning states but does not disable spanning-tree operation; it is designed for access ports that connect end hosts. Therefore, claiming that Rapid PVST+ and PortFast are incompatible is incorrect; they are commonly used together in enterprise campus networks.
- ✗
The port is configured as a trunk but should be an access port.
Why it's wrong here
The interface is configured as an access port using 'switchport mode access', so the premise of this option is false. BPDU Guard is independent of trunk or access mode; it is tied to PortFast, and it will shut down the port regardless of the port mode if a BPDU is received. Therefore, the problem is not a misconfigured trunk but the intentional BPDU Guard protection triggered by an unexpected BPDU.
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 a security feature that monitors PortFast-enabled ports for incoming BPDUs. When a BPDU is received, it immediately transitions the port to err-disabled state to prevent a potential switching loop, as a valid access port should never receive BPDUs. This exactly matches the symptom: Gi0/1 is down/err-disabled after BPDU Guard was enabled. The port will remain disabled until manually re-enabled or errdisable recovery is configured.
✗The root bridge election failed, causing a loop.Wrong answer — click to see why▾
Why this is wrong here
The root bridge election did not fail; SwitchC is correctly elected as root for VLAN 10. A loop would not cause an err-disabled state; loops typically cause high CPU or port flapping, not err-disable.
Why candidates choose this
Students may associate connectivity loss with root bridge issues or loops, but the err-disabled state specifically points to a port security feature like BPDU Guard.
✗Rapid PVST+ is not compatible with PortFast.Wrong answer — click to see why▾
Why this is wrong here
Rapid PVST+ and PortFast are fully compatible; PortFast is specifically designed to work with Rapid PVST+ to allow access ports to transition immediately to forwarding state. There is no incompatibility.
Why candidates choose this
Some might think that Rapid PVST+ requires BPDUs on all ports, but PortFast is a standard feature that bypasses the listening/learning states and works with any spanning-tree mode.
✗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 configured as an access port (switchport mode access), not a trunk. A trunk misconfiguration would not cause an err-disabled state due to BPDU Guard; it might cause a different issue like VLAN mismatch.
Why candidates choose this
Students might confuse access port configuration with trunk issues, especially if they think the default gateway is on a different VLAN. However, the err-disabled state is directly caused by BPDU Guard.
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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Interpreting Packet Capture Output for Layer 2/3 Troubleshooting
Key term
Default gateway
A default gateway is a network device, typically a router, that acts as the exit point for traffic from a local network to other networks, including the internet.
Key term
Err-disabled
Err-disabled is a switch port state that occurs when the switch detects a critical error on that port and automatically shuts it down to protect the network.
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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.