What command should be used to configure a trunk interface before applying STP settings?

To configure a trunk interface properly before applying Spanning Tree Protocol (STP) settings, utilizing the command to specify the trunk encapsulation method is essential. When you use the command for trunk encapsulation—such as "switchport trunk encapsulation dot1q"—you are explicitly defining the trunking protocol to be used on that interface. This is a critical step because a trunk interface must be capable of carrying traffic for multiple VLANs, and the encapsulation method (such as IEEE 802.1Q) determines how VLAN information is tagged and processed on the trunk link.

In many switching environments, especially when integrating with different types of hardware or older networking standards, ensuring that the encapsulation method is explicitly set can help to prevent issues related to VLAN traffic being improperly forwarded or blocked. This step lays the groundwork for seamless data transmission across VLANs before STP parameters are introduced to manage the topology and avoid loops in the network.

Other commands may serve different purposes: switching the mode to dynamic will not establish a trunk; configuring access VLAN is relevant only for access ports, and setting the interface as access simply designates it to carry traffic from a single VLAN, thus contradicting the intention of establishing a trunk connection.