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hardmultiple choiceObjective-mapped

Exhibit

Interface Gi1/0/12:
 switchport mode access
 spanning-tree portfast
 spanning-tree bpduguard enable
Status after connection: err-disabled

Exhibit: After a new switch was connected, the access-layer port went into err-disabled state immediately. Which feature most likely caused this?

Question 1hardmultiple choice
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Exhibit: After a new switch was connected, the access-layer port went into err-disabled state immediately. Which feature most likely caused this?

Full question →

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

Distractor review

Root Guard

Distractor.

B

Distractor review

UDLD aggressive

Distractor.

C

Best answer

BPDU Guard

Correct choice.

D

Distractor review

Storm control

Distractor.

Common exam trap

Common exam trap: answer the scenario, not the keyword

A frequent exam trap is mistaking Root Guard or UDLD aggressive mode as the cause of an err-disabled port immediately after connecting a new switch. Root Guard only blocks ports from becoming root ports and does not err-disable them instantly upon BPDU reception. UDLD aggressive disables ports only when unidirectional link failures occur, unrelated to BPDU traffic. Storm control limits broadcast or multicast traffic but does not cause err-disabled states due to BPDUs. The key is recognizing that BPDU Guard is specifically designed to err-disable PortFast-enabled ports upon receiving BPDUs, signaling a potential topology risk.

Technical deep dive

How to think about this question

BPDU Guard is a Cisco Catalyst switch feature designed to protect the Spanning Tree Protocol (STP) topology by disabling ports that receive Bridge Protocol Data Units (BPDUs) unexpectedly. Typically, PortFast is enabled on access ports connected to end devices to allow immediate forwarding state, bypassing the usual STP listening and learning states. However, if a BPDU is received on such a port, it indicates that another switch or device running STP is connected, which could cause loops or topology instability. When BPDU Guard is enabled on a PortFast port, the switch immediately places the port into an err-disabled state upon receiving a BPDU. This behavior prevents potential Layer 2 loops by shutting down ports that should only connect to end devices, not other switches. The port remains disabled until manually re-enabled or configured for automatic recovery. This mechanism enforces strict access-layer security and topology integrity. A common exam trap is confusing BPDU Guard with Root Guard or UDLD. Root Guard blocks ports from becoming root ports but does not err-disable the port immediately. UDLD detects unidirectional links and disables ports only under link failure conditions, unrelated to BPDUs. Storm control limits traffic rates but does not react to BPDUs. Understanding these distinctions is essential for troubleshooting err-disabled ports and designing secure network access layers.

KKey Concepts to Remember

  • BPDU Guard disables a port immediately when it receives a BPDU on a PortFast-enabled access port to prevent potential Layer 2 loops.
  • PortFast is configured on access ports to allow immediate transition to forwarding state, assuming connection to end devices, not switches.
  • Root Guard prevents a port from becoming a root port by blocking superior BPDUs but does not err-disable the port immediately.
  • UDLD aggressive mode disables a port only when unidirectional link detection fails, which is unrelated to BPDU reception.
  • Storm control limits broadcast, multicast, or unicast traffic to prevent traffic storms but does not cause err-disabled due to BPDUs.
  • An access-layer port in err-disabled state due to BPDU Guard indicates a BPDU was received on a PortFast-enabled port, signaling a misconfiguration or unauthorized switch connection.
  • BPDU Guard is a security feature designed to protect the Spanning Tree Protocol topology by shutting down ports that should not receive BPDUs.
  • Understanding the interaction between PortFast and BPDU Guard is critical for designing stable and secure access-layer switch ports.

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

A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?

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

A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?

Question 5

A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?

Question 6

A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?

FAQ

Questions learners often ask

What does this 200-301 question test?

BPDU Guard disables a port immediately when it receives a BPDU on a PortFast-enabled access port to prevent potential Layer 2 loops.

What is the correct answer to this question?

The correct answer is: BPDU Guard — An edge port with PortFast and BPDU Guard enabled is expected to shut down if it receives a BPDU. That behavior is intentional because the port is meant for end devices, not for another switch.

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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