David Sudjiman

The way L2 interface becomes trunk or non trunk depends whether we statically configure it or using DTP to dynamically decide. In real world most of people will use trunk or access combined with nonegotiate. However the options are always there. Here are simply the combinations we can have while creating the trunking. The easy way to understand it is to check whether the interface is sending DTP or not.

# switchport mode access

Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface even if the neighboring interface is not a trunk interface.

# switchport mode access
# switchport nonegotiate

Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.

# switchport mode trunk
# switchport nonegotiate

Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.
Not sending DTP packets and puts the interface into permanent trunk mode no matter whether the neighboring will set to trunk or not.

# switchport mode trunk

Puts the interface into permanent trunking mode and negotiates to convert the link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface.
Sending DTP packets and puts the interface becomes a trunk interface even if the neighboring interface is not a trunk interface.

# switchport mode dynamic auto

Set the interface to a trunk link if the neighboring interface is set to trunk or desirable mode.
Not sending DTP and will only set the interface to trunk upon receiving DTP packets from its neighbor.

# switchport mode dynamic desirable

Set the interface to a trunk link if the neighboring interface is set to trunk, desirable, or auto mode.
Sending DTP and will only set the interface to trunk if neighboring interface will react to DTP packets and set the interface to trunk.

# switchport trunk encapsulation Dot1q

Specifies 802.1Q encapsulation on the trunk link.

# switchport trunk encapsulation isl

Specifies ISL encapsulation on the trunk link.

# switchport trunk encapsulation negotiate

Specifies that the interface negotiate with the neighboring interface to become an ISL (preferred) or 802.1Q trunk, depending on the configuration and capabilities of the neighboring interface.


And below is there default setting to L2 interface.

Reference
Configuring Interface Characteristics