Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A switch port connected to an end host is configured with both PortFast and BPDU Guard. What is the most likely outcome if a small switch is connected there and starts sending BPDUs?
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
The port is error-disabled by BPDU Guard.
This is correct because BPDU Guard disables the edge port when a BPDU is received.
B
Distractor review
The port automatically becomes the root port.
This is wrong because BPDU Guard does not promote the port into root-port status.
C
Distractor review
The port converts into a trunk for the attached switch.
This is wrong because BPDU Guard does not negotiate trunking in response to BPDUs.
D
Distractor review
The port ignores the BPDU because PortFast disables STP entirely.
This is wrong because PortFast does not eliminate BPDU Guard protection or all STP awareness.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that PortFast disables STP entirely or that a port with PortFast enabled will ignore BPDUs. This misconception leads to the incorrect belief that connecting a switch to such a port will not cause any issues. In fact, PortFast only speeds up port activation but does not disable BPDU processing. BPDU Guard is designed to detect BPDUs on these ports and err-disable them to prevent loops. Misunderstanding this interaction can cause candidates to select incorrect answers that suggest the port remains active or changes roles, which is not true.
Technical deep dive
How to think about this question
PortFast is a Cisco feature that expedites the transition of a switch port into the forwarding state by skipping the usual Spanning Tree Protocol (STP) listening and learning phases. This feature is designed for ports connected directly to end devices such as PCs or servers, where no STP topology changes are expected. However, PortFast does not disable STP entirely; the port still participates in STP and can receive BPDUs if they arrive. BPDU Guard complements PortFast by adding a layer of protection against accidental topology loops. When BPDU Guard is enabled on a PortFast port, it continuously monitors for incoming BPDUs. If a BPDU is detected, which indicates that another switch or STP device is connected, BPDU Guard immediately places the port into an error-disabled state. This action prevents potential Layer 2 loops and topology instability by shutting down the port until manual or automatic recovery is performed. A common exam trap is misunderstanding the interaction between PortFast and BPDU Guard. Some might incorrectly assume that PortFast disables STP or that the port will ignore BPDUs. In reality, PortFast only accelerates port state transitions, and BPDU Guard enforces strict topology protection by disabling the port upon BPDU receipt. Practically, this means connecting a switch to a PortFast and BPDU Guard enabled port will cause the port to shut down, preventing network loops and preserving STP integrity.
KKey Concepts to Remember
- PortFast immediately transitions a switch port into the forwarding state, bypassing the usual STP listening and learning states to speed up host connectivity.
- BPDU Guard monitors PortFast-enabled ports and places a port into an error-disabled state if any Bridge Protocol Data Units (BPDUs) are received, protecting the network topology.
- Switch ports configured with PortFast are intended for end devices and should not receive BPDUs, which are normally sent by switches participating in STP.
- When BPDU Guard detects a BPDU on a PortFast port, it disables the port to prevent potential Layer 2 loops caused by unexpected switches connected to edge ports.
- PortFast does not disable STP entirely; it only accelerates port transition states but still allows BPDU Guard to enforce topology protection.
- The error-disabled state caused by BPDU Guard requires manual or automatic recovery to re-enable the port after a BPDU violation.
- BPDU Guard is a critical feature in STP topology protection, especially in environments where edge ports connect to end hosts rather than other switches.
- Connecting a switch to a PortFast and BPDU Guard enabled port violates the assumption of an edge port and triggers BPDU Guard to err-disable the port.
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?
PortFast immediately transitions a switch port into the forwarding state, bypassing the usual STP listening and learning states to speed up host connectivity.
What is the correct answer to this question?
The correct answer is: The port is error-disabled by BPDU Guard. — The most likely outcome is that the port is placed into an error-disabled state by BPDU Guard. In plain language, PortFast treats the port as an edge port for faster startup, while BPDU Guard protects that assumption. If the switch receives a BPDU on a port that was supposed to connect only to an end device, it reacts by shutting the port down to help prevent accidental loops or topology problems. This is one of the most recognizable edge-port protection behaviors in switching. The correct answer is the one focused on err-disable due to unexpected BPDU receipt.
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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