Final answer:
To enable BPDU Guard on an interface with PortFast enabled, you enter 'spanning-tree bpduguard enable' in the interface configuration mode of a Cisco switch.
Step-by-step explanation:
The command to enable BPDU Guard on an interface that already has PortFast enabled in Cisco IOS is spanning-tree bpduguard enable. After entering interface configuration mode for the specified interface (in this case, F0/1), this command will activate BPDU Guard, providing additional network protection by ensuring that PortFast-enabled ports are not mistakenly connected to switches that could cause potential bridging loops.
In interface configuration mode after enabling Port-Fast on interface F0/1, you would activate BPDU Guard using the command "spanning-tree bpduguard enable." This command ensures that the interface remains a Port-Fast edge port but immediately puts it into an errdisable state if any Bridge Protocol Data Units (BPDUs) are received.
BPDU Guard enhances network stability by preventing the connection of unauthorized switches or devices that might otherwise introduce potential spanning tree issues. This proactive measure protects the network integrity, especially in scenarios where Port-Fast is enabled, prioritizing rapid connectivity for designated edge ports while safeguarding against unintended loops.
Therefore answer is a. spanning-tree bpduguard enable.