DTP Dynamic Trunking Protocol

The DTP is used by Cisco switches to negotiate if links  between two switches should be placed into access or trunk mode. .
 
The DTP also helps what trunking protocol (802.1Q or ISL) should be used. The individual DTP modes are:
 
dynamic auto - the port will negotiate the mode automatically, however, it prefers to be an access port
dynamic desirable - the port will negotiate the mode automatically, however, it prefers to be a trunk port
 
Note: when you manually configure the trunk mode to be on using this command

switchport mode trunk

DTP will be still be sent of out of that interface.


The following combinations will give you the results below

dynamic auto + dynamic auto = access
dynamic auto + dynamic desirable = trunk
dynamic desirable + dynamic desirable = trunk
dynamic auto + trunk = trunk
dynamic desirable + trunk = trunk
dynamic auto + access = access
dynamic desirable + access =  access

when you set the interface to static access, you are effectively disabling DTP on that port ( I think the interface might sent one DTP packet when it comes up)

I personally prefer to turn off DTP on trunk ports using switchport nonegotiate command, however when doing that you need to make sure to check both ends of the trunk to be sure that  the trunk is functional. 

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