DTP

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Overview

{loadposition adposition5}Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol, that when enabled and configured correctly, uses advertisements to contact the switch on the other end of the link, and auto-negotiate a switchport to either an access or trunk link. When a switchport on either end of the link is misconfigured you will end up with a broken link (see chart below). DTP is enabled by default on the Cisco switches that are commonly used in the CCNA curriculum. There are four switchport modes that DTP will negotiate with in order to determine whether the link will be a trunk or an access link, the four modes are: Access, Trunk, Dynamic Auto, and Dynamic Desirable. The default switchport mode when DTP is enabled is Dynamic Auto. If both switches on either end of a link have DTP enabled, and both switchports are by default in Dynamic Auto mode then the resulting link modes will be Access on both ends of the link. By contrast, if one switchport, on one end of the link, is in Dynamic Auto mode, and the other switchport on the other end of the link, is configured for Trunk mode, then the DTP negotiation will result in the Dynamic Auto switchport changing its mode to Trunk mode and the link will become a trunk. See the chart below for the result when two DTP enabled Cisco switches negotiate switchport modes. Since only Cisco switches support DTP, when connecting to a non-Cisco switch DTP should be disabled.

 

 

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The following chart shows how the link will auto negotiate when DTP is enabled on both switches and different DTP modes are configured on either end of the link. When DTP is enabled by default on a switch, the default switchport mode is Dynamic Auto.

DTP auto-negotiation resulting link states
Port Mode Access Trunk Dynamic Auto Dynamic Desirable
Access access not recommended access access
Trunk not recommended trunk trunk trunk
Dynamic Auto access trunk access trunk
Dynamic Desirable access trunk trunk trunk

 

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Video Tutorial – Packet Tracer

In the following video tutorial, I demonstrate in Packet Tracer, how two Cisco switches running DTP, are able to auto-negotiate the link to either a trunking or access state. If you have Packet Tracer and would like to follow along with the demonstration you can download my PT files here: DTP-1-begin.zipDTP-1-Finished.zip 

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Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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