CCNA Switching and Network Access Practice Question
Exhibit
SW3# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 00d0.97f1.8a00
Cost 4
Port 25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 00d0.97f3.b200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Altn BLK 4 128.25 P2p
Gi0/1 Root FWD 4 128.26 P2p
Gi0/2 Desg FWD 19 128.27 P2pRefer to the exhibit. A network engineer is troubleshooting a connectivity issue on SW3. A host connected to the same segment as SW3's GigabitEthernet0/0 interface cannot reach any network resources. The engineer issues the show spanning-tree vlan 10 command and receives the output shown. Based on the output, what is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between a port being blocked due to normal STP operation (receiving a superior BPDU) versus being error-disabled or administratively down, leading candidates to incorrectly assume a physical or administrative issue.
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
✓
The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.
The output shows that GigabitEthernet0/0 is in the Blocking state for VLAN 10. In Rapid PVST+ or classic STP, a port enters the Blocking state when it receives a superior BPDU (i.e., a BPDU with a lower bridge ID or lower path cost to the root), causing it to become an alternate (or backup) port rather than a designated or root port. This prevents the host from reaching network resources because the port does not forward traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
GigabitEthernet0/0 is administratively down, which prevents the host from communicating.
Why it's wrong here
Administratively down is a switchport shutdown state, not an STP state. The output shows GigabitEthernet0/0 in the BLK (blocking) state and role Altn (alternate), which can only occur when the port is administratively and operationally up, allowing BPDUs to be received and processed. A shutdown port would display as 'administratively down' in interface status and would either be absent from spanning-tree output or shown as disabled, never as BLK. Therefore, this option misinterprets a normal STP blocking state as a manual administrative configuration.
- ✗
The port is in the Blocking state because the switch detected a loop and moved the port to error-disabled state.
Why it's wrong here
Error-disabled ports would show a status of 'err-disabled' in spanning-tree output, not 'BLK'. The output clearly shows the port is in the normal blocking state (BLK) as part of STP convergence, not due to an error condition.
- ✗
The port is blocked because SW3 has a lower bridge priority than the root bridge and should be the designated port for that segment.
Why it's wrong here
The exhibit shows the root bridge is 00d0.97f1.8a00, and SW3's port Gi0/0 has role 'Altn' (alternate), not 'Desg' (designated). A designated port would be forwarding, not blocking. SW3 is not the designated bridge for that segment.
- ✓
The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.
Why this is correct
The output explicitly shows role 'Altn' and state 'BLK' for Gi0/0. An alternate port is blocked because it receives better BPDUs on that interface than it can send, providing an alternate path to the root bridge. This is correct STP behavior, and the blocking state prevents the host from communicating.
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.
✓The interface GigabitEthernet0/0 is in the Blocking state because it received a superior BPDU, making it an alternate port to the root bridge.Correct answer▾
Why this is correct
The output explicitly shows role 'Altn' and state 'BLK' for Gi0/0. An alternate port is blocked because it receives better BPDUs on that interface than it can send, providing an alternate path to the root bridge. This is correct STP behavior, and the blocking state prevents the host from communicating.
✗GigabitEthernet0/0 is administratively down, which prevents the host from communicating.Wrong answer — click to see why▾
Why this is wrong here
Candidates may incorrectly associate the blocked state with an administratively disabled interface.
✗The port is in the Blocking state because the switch detected a loop and moved the port to error-disabled state.Wrong answer — click to see why▾
Why this is wrong here
Candidates often confuse error-disabled state (caused by features like BPDU guard) with the standard STP blocking state.
✗The port is blocked because SW3 has a lower bridge priority than the root bridge and should be the designated port for that segment.Wrong answer — click to see why▾
Why this is wrong here
Candidates may misunderstand the root election process and assume a lower priority switch always becomes designated for all segments, ignoring the Altn role.
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
STP
Shielded Twisted Pair (STP) is a copper cable that wraps each twisted pair in foil or braid to block electromagnetic interference.
Key term
Blocking state
A port state in Spanning Tree Protocol (STP) where the port is prevented from forwarding traffic to avoid loops in a redundant 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.