Final answer:
A Cisco switch uses the Dynamic Trunking Protocol (DTP) to automatically detect trunk ports.
Step-by-step explanation:
A Cisco switch uses the Dynamic Trunking Protocol (DTP) to automatically detect trunk ports.
DTP is a Cisco proprietary protocol that allows switches to determine if they should form a trunk link with the connected device.
By default, a Cisco switch is in Dynamic Desirable mode, which means it actively tries to negotiate trunking with connected devices. If the connected device supports trunking and is also in a compatible mode, such as Dynamic Auto, the ports will form a trunk.