The Primary Effect of the Spanning-Tree Portfast Command

D.

The primary effect of the "spanning-tree portfast" command in Cisco networking is to configure a switch port as a Portfast port.

By default, when a port on a Cisco switch comes up, it goes through several stages of the Spanning Tree Protocol (STP) before it is put into the forwarding state. This is to prevent loops in the network caused by multiple active paths. However, in some cases, such as when connecting an end device like a PC or printer, these STP stages can cause unnecessary delays in the time it takes for the device to come online.

Portfast is a feature that bypasses the normal STP stages for designated switch ports, allowing them to immediately transition to the forwarding state when the link comes up. This can significantly reduce the amount of time it takes for end devices to connect to the network, especially in environments with a large number of switches or devices.

To enable Portfast on a switch port, the "spanning-tree portfast" command is used in interface configuration mode. The command can be applied to individual ports or to an entire VLAN. When a Portfast-enabled port receives a BPDU (Bridge Protocol Data Unit), it will automatically disable Portfast and go through the normal STP stages to prevent loops.

In summary, the primary effect of the "spanning-tree portfast" command is to minimize the time it takes for end devices to connect to the network by bypassing the normal STP stages for designated switch ports.