Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
interface GigabitEthernet1/0/9 switchport mode access spanning-tree portfast spanning-tree bpduguard enable Event: %SPANTREE-2-BLOCK_BPDUGUARD: Received BPDU on PortFast enabled port. Disabling interface.
Based on the exhibit, what is the strongest reason the switchport went into err-disabled state?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
BPDU Guard disabled the PortFast-enabled access port after it received a BPDU.
This is correct because the event message explicitly identifies a BPDU Guard violation.
B
Distractor review
Port security shut down the port because the VLAN was wrong.
This is wrong because the event shown is about BPDUs, not MAC-based port security.
C
Distractor review
DHCP snooping disabled the interface because a host requested an address.
This is wrong because DHCP snooping does not produce this spanning-tree error.
D
Distractor review
EtherChannel suspended the interface because the bundle was incomplete.
This is wrong because the event message points to BPDU Guard, not EtherChannel.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is mistaking the cause of an err-disabled port for port security or DHCP snooping violations. Candidates may incorrectly assume that a VLAN mismatch or MAC address violation caused the shutdown, but the event message specifically references BPDU Guard. Another trap is confusing EtherChannel issues with BPDU Guard events; EtherChannel suspends ports due to bundle inconsistencies, not BPDU reception. Recognizing that BPDU Guard triggers only when a PortFast-enabled port receives a BPDU is essential to avoid these mistakes and correctly identify the root cause of the err-disabled state.
Technical deep dive
How to think about this question
BPDU Guard is a Cisco feature designed to protect the network topology by disabling ports that receive Bridge Protocol Data Units (BPDUs) unexpectedly. It is typically enabled on access ports configured with PortFast, which assumes the port connects directly to an end device rather than another switch. PortFast allows the port to bypass the usual STP listening and learning states, transitioning immediately to forwarding to reduce connection delays for end devices. However, if a BPDU is received on such a port, it indicates a potential topology change or misconfiguration, which could cause loops or instability. When BPDU Guard detects a BPDU on a PortFast-enabled port, it immediately places the port into an err-disabled state to prevent possible Layer 2 loops. This behavior is a strict enforcement of the network design principle that access ports should not receive BPDUs. The switch logs an event message indicating a BPDU Guard violation, which helps network administrators quickly identify the cause of the shutdown. This mechanism is critical in environments where unauthorized switches or bridging devices might be connected to access ports, potentially disrupting the spanning-tree topology. A common exam trap is confusing BPDU Guard violations with other security features like port security or DHCP snooping. Port security deals with MAC address violations and does not react to BPDUs, while DHCP snooping protects against rogue DHCP servers and does not interact with spanning-tree events. EtherChannel suspends interfaces due to bundle inconsistencies, not BPDU reception. Understanding the specific triggers and behaviors of BPDU Guard helps avoid misdiagnosing the cause of an err-disabled port and ensures proper troubleshooting and network protection.
KKey Concepts to Remember
- BPDU Guard disables a PortFast-enabled access port immediately upon receiving a BPDU to prevent potential Layer 2 loops caused by connecting another switch or bridge.
- PortFast is configured on access ports to allow immediate transition to the forwarding state, assuming the port connects to an end device, not another switch.
- When BPDU Guard detects a BPDU on a PortFast-enabled port, it error-disables the interface to protect the network topology from unexpected switches.
- Port security focuses on MAC address violations and does not trigger error-disable states due to BPDU reception or spanning-tree events.
- DHCP snooping monitors DHCP messages to prevent rogue DHCP servers but does not interact with spanning-tree or BPDU Guard mechanisms.
- EtherChannel bundles multiple physical links into one logical link and suspends interfaces only when bundle consistency fails, unrelated to BPDU Guard events.
- Spanning Tree Protocol (STP) uses BPDUs to detect loops and maintain a loop-free topology; unexpected BPDUs on PortFast ports indicate misconfiguration or unauthorized devices.
- Error-disabled ports require manual intervention or configured automatic recovery to restore connectivity after BPDU Guard triggers a shutdown.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
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Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
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Question 5
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Question 6
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FAQ
Questions learners often ask
What does this 200-301 question test?
BPDU Guard disables a PortFast-enabled access port immediately upon receiving a BPDU to prevent potential Layer 2 loops caused by connecting another switch or bridge.
What is the correct answer to this question?
The correct answer is: BPDU Guard disabled the PortFast-enabled access port after it received a BPDU. — The strongest reason is a BPDU Guard violation on a PortFast-enabled access port. In practical terms, the port was expected to face an end host, not a switching device that emits BPDUs. When BPDUs appeared, the switch treated that as a topology-policy violation and error-disabled the interface to protect the network. This is one of the most classic access-layer protection patterns on the CCNA exam.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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