350-601 Network Practice Question
A data center switch has multiple access ports configured with spanning-tree portfast. A new server is connected to one of these ports and immediately causes a network loop. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that PortFast alone prevents loops, but the trap here is that PortFast only speeds up initial convergence; without BPDUguard, a PortFast port can still participate in STP and cause a loop if it receives a BPDU.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The port is not configured with BPDUguard.
Spanning Tree Protocol (STP) PortFast immediately transitions a port to the forwarding state, bypassing the listening and learning phases. However, PortFast alone does not protect against loops if a rogue device (like a server) inadvertently connects two switch ports or sends BPDUs. BPDUguard must be explicitly configured on the port to disable it upon receiving any BPDU, preventing a loop. Without BPDUguard, the switch will process the BPDU and may re-enter STP convergence, potentially causing a loop if the server is misconfigured or bridging traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The portfast feature is disabled by default on the switch.
Why it's wrong here
Portfast must be explicitly configured; it is not enabled by default.
- ✗
The server sends BPDUs.
Why it's wrong here
Portfast does not prevent BPDU reception; BPDUguard does.
- ✓
The port is not configured with BPDUguard.
Why this is correct
Without BPDUguard, the port stays up when a BPDU is received, potentially causing a loop.
- ✗
The switch is running MSTP instead of PVST+.
Why it's wrong here
MSTP itself does not cause loops; the problem is lack of BPDUguard.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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