DTP

DTP: Cisco Dynamic Trunking Protocol

Dynamic Trunking Protocol (DTP), a Cisco proprietary protocol in the VLAN group, is for negotiating trunking on a link between two devices and for negotiating the type of trunking encapsulation (802.1Q) to be used.

There are different types of trunking protocols. 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. This ability to negotiate the trunking method with the other device is called dynamic 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. End stations will be extremely puzzled by the extra tag information in the frame header, their driver stacks won't understand it, and the end systems may lock up or fail in odd ways. Not fun to troubleshoot! 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), so you can tell it apart from VTP.

The second issue is creating VLAN's. If you have to configure VLAN's individually, switch by switch, you have a lot of work to do. Worse, there's a danger of inconsistency, whereby VLAN 100 is Engineering on one switch, and Accounting on another. Oops! That would be a source of confusion in troubleshooting, and might also defeat your carefully crafted VLAN security scheme. This also is addressed by VTP/DTP. You can create or delete a VLAN on one switch, and have the information propagate automatically to a group of switches under the same (your) administrative control. This group of switches would be a VTP domain.

'Protocol Structure - DTP: Cisco Dynamic Trunking Protocol'

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. The first set of keyword arguments govern the DTP modes:

Mode	What the Mode Does
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 (neighbor set to on, desirable, or auto )
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. Note that auto-auto (both ends default) links will not become trunks.
nonegotiate	Forces the port to permanently trunk but not send DTP frames. For use when the DTP frames confuse the neighboring (non-Cisco) 802.1q switch. You must manually set the neighboring switch to trunking.