Dynamic Trunking Protocol or DTP is used on Cisco IOS switches to negotiate if the interface should become an access port or trunk. By default DTP is enabled and the switch interfaces will be in “dynamic auto” or “dynamic desirable” mode. This means that whenever you receive a DTP packet that requests to form a trunk, your interface will be in trunk mode.
The following context sensitive help indicates the DTP mode in which you can configure a switch port.
SW2(config-if)#switchport mode dynamic ? auto Set trunking mode dynamic negotiation parameter to AUTO desirable Set trunking mode dynamic negotiation parameter to DESIRABLE