Quick Answer
The answer is BPDU guard. This feature prevents loops on PortFast ports by automatically disabling a port if it receives any BPDU, which is exactly what happens when a switch is accidentally connected to a port configured for a host. PortFast assumes the connected device is an end host that will never generate BPDUs, so when another switch sends them, BPDU guard detects the violation and errdisables the port, breaking the potential loop. On the JNCIA-Junos exam, this scenario tests your understanding of how to secure the spanning-tree edge port concept; a common trap is confusing BPDU guard with root guard, but remember that BPDU guard reacts to any BPDU, while root guard only prevents a port from becoming a root port. A useful memory tip: BPDU guard is the “bouncer” that kicks out any switch that tries to talk BPDU on a host-only port.
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer enables Spanning Tree PortFast on a switch port connected to a host. Later, another switch is connected to that same port, causing a loop. What feature could have prevented this?
Loop guard
Why wrong: Loop guard prevents alternate ports from becoming designated in case of BPDU loss.
BPDU guard
BPDU guard disables the port upon receiving a BPDU, preventing loops on PortFast ports.
Root guard
Why wrong: Root guard prevents a port from becoming a root port, but does not stop BPDUs.
UplinkFast
Why wrong: UplinkFast is a Cisco proprietary feature for fast convergence.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
BPDU guard
BPDU guard is the correct answer because it disables a port configured with PortFast if a BPDU is received, preventing loops when a switch is accidentally connected. In this scenario, PortFast was enabled for a host, but connecting another switch caused BPDUs to be sent, which BPDU guard detects and shuts down the port to break the loop.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Loop guard
Why it's wrong here
Loop guard prevents alternate ports from becoming designated in case of BPDU loss.
BPDU guard
Why this is correct
BPDU guard disables the port upon receiving a BPDU, preventing loops on PortFast ports.
Related concept
Read the scenario before looking for a memorised answer.
Root guard
Why it's wrong here
Root guard prevents a port from becoming a root port, but does not stop BPDUs.
UplinkFast
Why it's wrong here
UplinkFast is a Cisco proprietary feature for fast convergence.
Common exam traps
The trap here is that candidates confuse BPDU guard with Loop guard, thinking both prevent loops, but Loop guard addresses unidirectional link failures, not the accidental connection of a switch to a PortFast port.
Detailed technical explanation
BPDU guard works by monitoring for any BPDU on a PortFast-enabled interface; upon receipt, it places the port into an errdisable state (or a similar error state in Junos, such as 'blocked' or 'disabled'). In Junos, this is configured with 'edge' (PortFast equivalent) and 'bpdu-block-on-edge' under the spanning-tree interface hierarchy. A real-world scenario is a data center where an access port intended for a server is mistakenly patched to another switch, causing a loop that BPDU guard immediately contains.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: BPDU guard — BPDU guard is the correct answer because it disables a port configured with PortFast if a BPDU is received, preventing loops when a switch is accidentally connected. In this scenario, PortFast was enabled for a host, but connecting another switch caused BPDUs to be sent, which BPDU guard detects and shuts down the port to break the loop.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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