What is the purpose of the command spanning-tree portfast default?

The command spanning-tree portfast default is used to enable PortFast globally on all access ports on a Cisco switch. When PortFast is enabled on an access port, the port transitions directly to the forwarding state when it comes up, significantly reducing the time it takes for the port to start forwarding traffic. This is especially useful for ports connected to end devices like computers or printers, where the delay associated with the standard spanning tree process is unnecessary.

By applying this command, network administrators can streamline the boot process for end devices, improving network efficiency and performance. The other options do not accurately describe the purpose of the command; for example, it does not apply to trunk ports, nor does it change the overall configuration of spanning tree in a way that affects all VLANs or disable spanning tree itself.