Dynamic Trunking Protocol
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Dynamic Trunking Protocol (herein referred to as “DTP”), is a proprietary protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two switches participating in a VLAN, and for negotiating the type of trunking encapsulation (802.1Q) to be used. It exists on layer 2 of the OSI model.
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[edit] Time Saving
On large networks, where many Cisco switches are used, manual configuration of each switch with the correct VLAN information on a switch-by-switch basis is time-consuming and may be risky. Should you set, say, VLAN #128 as the VLAN used by Human Resources on one switch, and that same VLAN (#128) as the one used by your Finance Dept. on another, you may create confusion when you need to troubleshoot further down the line, and also even render your VLAN security plan useless.
With DTP (and VTP), you can create or delete a VLAN on just one “Master” switch, and have the switch pass that information to any or all of your other switches under the same administrative control. This group of switches is referred to a “VTP domain”.
[edit] Removing Individual Devices From DTP Participation
If you do not want one of your switches to listen to the “Master” switch, and have it ignore the propogated VLAN information, you can set it to “Transparent” operation mode, whereby it will only use its' own internal VLAN configuration, and will not pass on its' own VLAN configuration to other switches.
[edit] Drawbacks
Network engineers working with DTP and VTP need to be especially careful when adding a new VLAN-enabled switch to their existing infrastructure - if the new switch is configured as the VTP/DTP “Master”, it will distribute its' (possibly blank or incorrect) VLAN configuration, irretrievably overwriting those of all the other switches!
