Dynamic Trunk Protocol (DTP)- Overview

Here is a quick overview/cheat-sheet of Dynamic Trunk Protocol (DTP).  The table below shows which combination of port modes will create a successful trunk via DTP.  (and before anyone says anything, I do realize that the modes with no-negotiate do not actually use DTP)

	Access + No-negotiate	Access	Dynamic Auto	Dynamic Desirable	Trunk	Trunk + No-negotiate
Access + No-negotiate	No	No	No	No	No	No
Access	No	No	No	No	No	No
Dynamic Auto	No	No	No	Yes	Yes	No
Dynamic Desirable	No	No	Yes	Yes	Yes	No
Trunk	No	No	Yes	Yes	Yes	Yes
Trunk + No-negotiate	No	No	No	No	Yes	Yes

Here are a few important points regarding DTP.

• The default port mode on 3550 switches is Dynamic Desirable.
• The default port mode on 3560 switches is Dynamic Auto.
• If both switches support ISL and Dot1q trunks, and neither is specified, ISL will be preferred.
• The no-negotiate option is used when you do not want to send out DTP frames.
• The no-negotiate option is only usable with the Access and Trunk modes.

I have seen Cisco documentation that Dynamic Auto is the default switchport mode.  The Cisco Press Exam Certification Guide says that Dynamic Desirable is the default mode.  It seems to be depndant on the switch model.  New models appear to use Dynamic Auto.  So I guess I would stick with that as the default mode of a switchport if you were asked on a test (unless a specific model is mentioned).