In STP, which factor has the highest priority when selecting the root port?

In Spanning Tree Protocol (STP), the root port is the port on a non-root bridge that has the lowest cost to reach the root bridge. The root cost is a critical factor in this decision because it determines the most efficient path from the switch to the root bridge. Lower costs are preferred, as they indicate shorter or more favorable paths through the network.

When switches participate in STP, they calculate the cost based on various factors, such as link speed. Each link type has a specific cost associated with it, and these costs are summed to determine the total cost to reach the root bridge. The switch with the smallest total cost will have its port selected as the root port, ensuring that traffic is always directed along the best possible path to prevent loops and minimize latency.

The other factors, including neighbor port ID, bridge ID, and port speed, can influence the overall STP process but do not directly compete for selection as the primary consideration for root port status. While the speed of a port contributes to calculating root cost, it is the cumulative cost that ultimately dictates which port becomes the root port. Thus, the root cost stands as the highest priority factor in selecting the root port.