Cisco
400-007 · Question #80
400-007 Question #80: Real Exam Question with Answer & Explanation
The correct answer is C: Loop guard. Loop Guard works alongside UDLD to prevent STP topology changes caused by missing BPDUs, covering the failure scenarios that UDLD alone cannot address in time.
Question
You want to mitigate failures that are caused by STP loops that occur before UDLD detects the failure or that are caused by a device that is no longer sending BPDUs. Which mechanism do you use along with UDLD?
Options
- ARoot guard
- BBPDU guard
- CLoop guard
- DBPDU filtering
Explanation
Loop Guard works alongside UDLD to prevent STP topology changes caused by missing BPDUs, covering the failure scenarios that UDLD alone cannot address in time.
Common mistakes.
- A. Root Guard prevents inferior BPDUs from causing a port's switch to accept a new root bridge, which addresses root placement stability rather than loop prevention from missing BPDUs.
- B. BPDU Guard is designed for PortFast-enabled access ports and disables a port if any BPDU is received, which is the opposite behavior needed and does not prevent loops from missing BPDUs.
- D. BPDU Filtering suppresses BPDU transmission and reception on a port, effectively disabling STP on that segment and increasing loop risk rather than mitigating it.
Concept tested. STP Loop Guard and UDLD complementary protection
Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10596-84.html
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