Disabling DTP Negotiation - switchport nonegotiate
In the previous post we covered how Layer 2 interfaces on Cisco devices use DTP frames to select a trunking operational mode when their trunking administrative mode is set to dynamic auto or dynamic desirable.
An interface that is staticlly configured in access or trunk mode responds to incoming DTP frames to allow a neighboring (dynamic) interface to select the appropriate trunking operational mode for itself.
Is there a way to disable DTP negotiation on an interface?
The switchport nonegotiate command
The switchport nonegotiate command disables DTP negotiation on a Layer 2 interface. The command is available in the Interface Configuration Mode.
This command is only accepted for interfaces that are statically configured in access or trunk mode. DTP negotiation cannot be disabled on an interface that is configured in dynamic auto or dynamic desirable mode.
The following is an example of how DTP can be disabled on an interface:
|access||Set access mode characteristics of the interface|
|mode||Set trunking mode of the interface|
|nonegotiate||Device will not engage in negotiation protocol on this interface|
|trunk||Set trunking characteristics of the interface|
Switchport nonegotiate on a dynamic interface
The default trunking administrative mode on a Layer 2 interface on a Cisco device is dynamic auto. (In older firmware versions, it used to be dynamic desirable.)
Let's see what happens when we attempt to enter the switchport nonegotiate command on an interface configured in dynamic mode.
When we try to disable DTP on a Layer 2 interface that has its trunking administrative mode to dynamic, we receive an error message stating that the command has been rejected due to a conflict between "nonegotiate" and "dynamic" status.
We cannot disable DTP on a dynamic interface because the interface must use DTP to determine its trunking operational mode. An interface must be statically configured in access or trunk mode before we can disable DTP on it.
What happens when we disable DTP on an interface?
When an interface in access or trunk mode has DTP disabled on it (by using the switchport nonegotiate command), it will not participate in DTP negotiation, and will not respond to incoming DTP frames. Any DTP frames that it receives will simply be ignored.