Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol that negotiates both the status of
trunk ports and the trunk encapsulation of trunk ports.
A switch port on a Cisco Catalyst switch supports a number of trunking modes. The trunking mode
defines how the port negotiates using DTP to set up a trunk link with its peer port. Identify the commands used to configure the trunking mode:
■ The command switchport mode trunk configures the local switch port to advertise to the
remote port that it is dynamically changing to a trunking state. The local port is considered to
be in an unconditional (always on) trunking state.
■ The command switchport mode dynamic auto configures the local switch port to advertise to
the remote switch port that it is able to trunk but does not request to go to the trunking state.
The local port transitions to the trunking state only if the remote port trunk mode has been configured to be on or desirable. If both ports on the switches are set to auto, they do not negotiate
to be in a trunking state. They negotiate to be in an access state.
■ The command switchport mode dynamic desirable configures the local switch port to advertise to the remote switch port that it is able to trunk and asks the remote switch port to go to the
trunking state. If the local port detects that the remote has been configured in on, desirable, or
auto mode, the local port ends up in a trunking state. If the remote switch port is in the nonegotiate mode, the local switch port remains as a nontrunking port.
■ You can turn off DTP for the trunk so that the local port does not send out DTP frames to the
remote port. The command switchport nonegotiate disables DTP. The local port is then considered to be in an unconditional trunking state. Why would you use this command instead of
the switchport mode trunk command?
Use this feature when you need to configure a trunk with a switch from another switch vendor.
In below Table, the arguments for the switchport mode command are listed for the local side of the link
down the first column and for the remote side of the link across the first row. Indicate whether the link
will transition to access mode or trunk mode after the two switches have sent DTP messages.

DTP