STP failures are most often caused by missing BPDUs or bad links.

Problem	Solution
Missing BPDUs	Loop Guard
One-way (unidirectional) link	UDLD
Duplex mismatch	Fix duplex
CAM flapping	Check for loops

Commands

show spanning-tree
show spanning-tree inconsistentports

Exam memory hook ⚠️

No BPDU = suspected loop

Pitfalls 🚨

• Only looking at forwarding ports
• Forgetting to check duplex