DTP (Dynamic Trunking Protocol)
Some Cisco switches have a proprietary protocol that lets them automatically negotiate trunking with a neighboring device. This protocol is called Dynamic Trunking Protocol (DTP).
DTP is a Cisco proprietary protocol. DTP manages trunk negotiation only if the port on the neighbor switch is configured in a trunk mode that supports DTP.
Switches from other vendors do not support DTP.
DTP Configuration
The switchport mode command has additional options for negotiating the interface mode. Please refer to the table below
| Access Mode |
| – Puts the interface into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. – The interface becomes a nontrunk interface, regardless of whether the neighboring interface is a trunk interface. |
| Dynamic Auto Mode |
| – Makes the interface able to convert the link to a trunk link. – The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. |
| Dynamic Desirable Mode |
| – Makes the interface actively attempt to convert the link to a trunk link. – The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or dynamic auto mode. |
| Trunk Mode |
| – Puts the interface into permanent trunking mode and negotiates to convert the neighboring link into a trunk link. – The interface becomes a trunk interface even if the neighboring interface is not a trunk interface. |
Enter interface configuration for the port to be assigned on the Trunk
S1(config)# interface [interface-id]
Set the switchport mode to a DTP mode
S1(config-if)# switchport mode [mode]
Assigning Dynamic Auto mode on a switchport
S1> enable S1# Configure Terminal S1(config)# interface fa0/1 S1(config-if)# switchport mode dynamic auto S1(config-if)# exit S1(config)#
| Dynamic Auto | Dynamic Desirable | Trunk | Access | |
|---|---|---|---|---|
| Dynamic Auto | Access | Trunk | Trunk | Access |
| Dynamic Desirable | Trunk | Trunk | Trunk | Access |
| Trunk | Trunk | Trunk | Trunk | Limited Conn |
| Access | Access | Access | Limited Conn | Access |
[NOTE] The table above shows the results of a DTP configuration options on opposite ends of a trunk link. It is recommended that you configure trunk links statically.
Stop DTP Negotiation
To enable trunking from a Cisco device to another vendor device that does not support DTP, you will need to use a command which causes the interface to become a trunk but will not generate DTP Frames
Commands to Stop DTP Negotiation
S1(config-if)# switchport mode trunk S1(config-if)# switchport nonegotiate
and if you wish to re-enable DTP Negotiation you can simply configure your interface to use dynamic auto or dynamic desirable again
Commands to Stop DTP Negotiation
S1(config-if)# switchport mode dynamic auto