Dynamic Trunking Protocol (DTP)

The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. It works on the Layer 2 of the OSI model. If a port can become a trunk, it may also have the ability to trunk automatically, and in some cases even negotiate what type of trunking to use on the port. DTP provides this ability to negotiate the trunking method with the other device.
There are a couple of other potential issues that arise when you start trunking.
- The first issue is that both ends of a trunk cable had better agree they're trunking, or they're going to be interpreting trunk frames as normal frames. To resolve this, Cisco came up with a protocol for switches to communicate intentions. The first version of it was VTP, VLAN Trunking Protocol, which worked with ISL. The newer version works with 802.1q as well, and is called Dynamic Trunking Protocol (DTP).
- The second issue is creating VLAN's.

Switch port modes

- auto : causes the port to passively be willing to convert to trunking. The port will not trunk unless the neighbor is set to on or desirable . This is the default mode.
- on : forces the link into permanent trunking, even if the neighbor doesn't agree.
- off : forces the link to permanently not trunk, even if the neighbor doesn't agree.
- desirable : causes the port to actively attempt to become a trunk, subject to neighbor agreement.
- nonegotiate : forces the port to permanently trunk but not send DTP frames.

Protocol Structure of DTP

On a Catalyst set-based switch, the syntax for setting up a link as a trunk is:

set trunk mod_num/port_num [on | desirable | auto | nonegotiate] [isl | dot1q | negotiate] [vlan_range]
Use this command to set the specified port or ports to trunking.