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352-001 · Question #84
352-001 Question #84: Real Exam Question with Answer & Explanation
The correct answer is B: loop guard. Loop guard detects unidirectional STP failures by monitoring non-designated ports for incoming BPDUs and placing them into a loop-inconsistent state when BPDU reception stops, preventing accidental forwarding.
Question
Which mechanism should be added to a network design to identify unidirectional Spanning Tree Protocol failures through BPDU loss?
Options
- AUDLD
- Bloop guard
- CBPDU guard?
- Droot guard
Explanation
Loop guard detects unidirectional STP failures by monitoring non-designated ports for incoming BPDUs and placing them into a loop-inconsistent state when BPDU reception stops, preventing accidental forwarding.
Common mistakes.
- A. UDLD detects physical unidirectional link conditions by exchanging protocol PDUs, but it operates independently of STP and does not specifically react to BPDU loss within the spanning tree topology the way loop guard does.
- C. BPDU guard disables a PortFast-enabled access port upon receiving any BPDU, protecting against unauthorized switch connections, but it does not monitor for the absence of BPDUs or detect unidirectional STP failures.
- D. Root guard prevents a port from accepting superior BPDUs that would change the root bridge position, protecting the STP topology from rogue root bridges, but it does not monitor BPDU reception or respond to unidirectional link failures.
Concept tested. STP loop guard detection of unidirectional BPDU loss
Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10596-84.html
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