Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
interface GigabitEthernet1/0/12 switchport mode access spanning-tree portfast spanning-tree bpduguard enable
A user connects a small unmanaged switch to an access port, and the port immediately transitions to err-disabled. Which feature most likely caused this behavior?
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
UDLD aggressive
This is wrong because UDLD aggressive is related to one-way link failures, not this spanning-tree protection response.
B
Best answer
BPDU Guard
This is correct because BPDU Guard disables a PortFast-enabled interface if a BPDU is received.
C
Distractor review
Root guard
This is wrong because root guard is intended to stop a port from becoming a root port when superior BPDUs are received.
D
Distractor review
Loop guard
This is wrong because loop guard protects against certain non-designated port failures, not this classic edge-port err-disable event.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is mistaking BPDU Guard for Root Guard or Loop Guard. Candidates often confuse Root Guard’s function of blocking superior BPDUs without disabling the port with BPDU Guard’s immediate err-disable action upon BPDU receipt. Similarly, Loop Guard protects against indirect link failures on non-designated ports but does not err-disable ports receiving BPDUs. Misunderstanding these differences can lead to incorrect answers about why a port transitions to err-disabled, especially when the question involves an access port connected to an unmanaged switch.
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. It is typically enabled on access ports configured with PortFast, which allows ports to bypass the usual STP listening and learning states and transition immediately to forwarding. This setup assumes that the port connects only to end devices, not other switches, so any BPDU received signals a potential topology issue or misconfiguration. When BPDU Guard is enabled on a PortFast-enabled port, the switch monitors incoming frames for BPDUs. If a BPDU is detected, BPDU Guard immediately places the port into an err-disabled state, effectively shutting it down to prevent possible Layer 2 loops or topology instability. This behavior is crucial in environments where unauthorized switches or devices might be connected, as it prevents the spread of incorrect spanning tree information that could disrupt the network. A common exam trap is confusing BPDU Guard with other STP protection features like Root Guard or Loop Guard. Unlike Root Guard, which blocks ports from becoming root ports but does not err-disable them, BPDU Guard actively disables the port on BPDU receipt. Similarly, Loop Guard protects against unidirectional link failures on non-designated ports but does not err-disable access ports receiving BPDUs. Understanding these distinctions helps avoid misinterpretation of port states and ensures correct configuration for network stability.
KKey Concepts to Remember
- BPDU Guard disables a PortFast-enabled access port immediately upon receiving a BPDU to prevent potential Layer 2 loops or topology changes.
- PortFast allows access ports to transition quickly to the forwarding state but assumes no switches are connected, making BPDU Guard essential for protection.
- UDLD aggressive mode detects unidirectional link failures but does not err-disable ports due to BPDU reception on access ports.
- Root Guard prevents a port from becoming a root port by blocking superior BPDUs but does not err-disable the port upon BPDU receipt.
- Loop Guard protects against indirect link failures on non-designated ports and does not trigger err-disable on access ports receiving BPDUs.
- An err-disabled state caused by BPDU Guard requires manual or automatic recovery to re-enable the port after a BPDU is detected.
- Access ports connected to end devices should have PortFast and BPDU Guard enabled to maintain stable Layer 2 topology and prevent loops.
- Receiving BPDUs on a PortFast-enabled access port indicates a misconfiguration or unauthorized switch connection, triggering BPDU Guard.
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 PortFast-enabled access port immediately upon receiving a BPDU to prevent potential Layer 2 loops or topology changes.
What is the correct answer to this question?
The correct answer is: BPDU Guard — BPDU Guard is designed to protect access ports that are expected to connect only to end devices. In simple terms, the port is being told, “You should never hear spanning-tree messages here.” When BPDU Guard detects a BPDU on a protected PortFast interface, it places the port into an err-disabled state to stop a possible switching loop or topology problem from spreading. That matches the exhibit perfectly. The port is configured as an access port with PortFast and BPDU Guard enabled.
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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