spanning-tree bpduguard enable
Shut the port down if it receives a BPDU — protects PortFast ports.
Enforced: plug a switch into the port and it goes err-disabled at once ('show interfaces status' reads err-disabled). 'shutdown' then 'no shutdown' brings it back only once the switch is gone — otherwise the next BPDU shuts it again.
Don't just read it — type spanning-tree bpduguard enable in a real network simulator in your browser and watch it take effect. No install, no account needed to start.
Frequently asked
What does the "spanning-tree bpduguard enable" command do?
Shut the port down if it receives a BPDU — protects PortFast ports.
What's the common gotcha with "spanning-tree bpduguard enable"?
Enforced: plug a switch into the port and it goes err-disabled at once ('show interfaces status' reads err-disabled). 'shutdown' then 'no shutdown' brings it back only once the switch is gone — otherwise the next BPDU shuts it again.