Dynamic Trunking Protocol (DTP)

The most common method of establishing a trunk port is statically setting it in the configuration using switchport mode trunk. Cisco does provide a method where the port can be negotiated as a trunk automatically with Dynamic Trunking Protocol (DTP)

Dynamic trunk ports are established by the switch by sending DTP packets across the link to negotiate whether the other end can become a trunking port.

If both ports can reach a successful negotiation, the port will become a trunk port with DTP. DTP will re-advertise itself across the link to monitor the status of the link as a trunk.

DTP can be disabled on a port with the command switchport nonegotiate

There are three modes DTP can use in setting a switch port as a trunk:

• Trunk mode statically places the switch port as at trunk and DTP advertises this fact across the link to try establish a dynamic trunk. This is configured with the interface command switchport mode trunk
• Dynamic desirable mode is the switch acts an access port. DTP functions on this port and actively tries to establish a dynamic trunk. If it successful in this negotiation a trunk will be created. This is configured in interface configuration mode with the command switchport mode dynamic desirable
• Dynamic auto mode, the switch again acts an access mode. DTP does not actively try to establish a trunk link in this mode but it does listen for DTP packets. If a DTP packet is recieved on the link the interface will then try to establish a trunk port. This is configured with the command switchport dynamic auto command in interface configuration mode