February 19, 2009
Dynamic Trunking Protocol (DTP) is a standard feature of Cisco switches and allows two switches to dynamically configure interfaces interconnecting each other to be trunked ports. DTP has 5 modes; Auto (default), On, Off, desirable and nonegotiate. These 5 modes all have a purpose. I have layed out the groundwork below:
- auto – The default setting allows the port to willingly convert to trunking, however, the port will not trunk unless the neighbor is set to on or desirable. When two switches are connected together and set for auto, they will NOT trunk.
- on – This setting forces the port to be a trunk regardless of the neighbor’s settings.
- off – This setting forces the port to not trunk, even if the neighbor is set to on.
- desirable – This causes the port to attempt to become a trunk, however, the neighbor would have to be set to on, desirable or auto.
- nonegotiate – This setting, forces the port to be a trunk but disables DTP frames between the two switches. This is useful when you are working with non-Cisco equipment and just want to ensure that the ports won’t do anything you do not want them to….this is my preference.
To configure this on your switches, issue the following:
switchport trunk encapsulation dot1q
switchport mode trunk
Hopefully this was somewhat worthwhile. It is very basic so I apologize but this is the easiest way to give you the needed information without boring you to death 🙂