Cisco DTP (Dynamic Trunking Protocol) Negotiation

In this tutorial we’ll take a look at DTP (Dynamic Trunking Protocol) negotiation. DTP is normally used on Cisco IOS switches to negotiate if the interface should become an access port or trunk.

By default DTP is enabled and the interfaces of your switches will be in “dynamic auto” or “dynamic desirable” mode. This means that whenever you receive a DTP packet that requests to form a trunk, your interface will be in trunk mode. If you are unfamiliar with DTP and the different interface settings then you might want to read my “How to configure Trunk on Cisco Catalyst Switch” lesson before continuing.


Let’s take a look at DTP negotiation and how to disable it. I’ll be using two switches for this:

Cisco SW1 SW2 802.1Q Trunk

I didn’t configure anything on my switches, let’s see what the default settings are:

SW1#show interfaces fa0/24 switchport        
Name: Fa0/24
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
SW2#show interfaces fastEthernet 0/24 switchport 
Name: Fa0/24
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On

Without configuring anything on the interfaces, we are using dynamic auto mode and as a result the interfaces are in access mode.

Depending on the switch model and IOS version, the default might be “dynamic auto” or “dynamic desirable”. The switches in my example are Cisco Catalyst 3560 switches.

There are two ways to disable DTP negotiation:

  • Configure the interface for access mode.
  • Use the switchport nonegotiate command on the interface.

Configuring the interface for trunking does not disable DTP negotiation, let me give you an example. First we’ll configure the interfaces for access mode:

SW1(config)#interface fastEthernet 0/24
SW1(config-if)#switchport mode access
SW2(config)#interface fastEthernet 0/24
SW2(config-if)#switchport mode access 

When we look again at the switchport settings we can see that DTP negotiation is now disabled:

SW1#show interfaces fastEthernet 0/24 switchport 
Name: Fa0/24
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off

So configuring an interface yourself to use access mode disables DTP negotiation. How about creating a trunk ourselves?

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Forum Replies

  1. Rene,

    Watching you switching videos and I like the background of your terminal. Tale me, which terminal program and font are you using. I would like to use such a background.

     

  2. Hi Rene,

    Can you explain me what does it mean Trunking negotiation is ON does it means that this particular mode is set to send DTP frames . to the my knowledge only Dynamic desirable and ON mode will send out DTP frames and other modes are doesn’t (Dynamic Auto and Access)

  3. Hello Ankit

    When we say that DTP negotiation is ON, it means that the port is in a state where, if the proper DTP packets are sent/received, the port may change its trunking functionality. Negotiation is ON in the following states: Dynamic Desirable, Dynamic Auto or Trunk.

    DTP negotiation is OFF when a port is in one of the following states: Access or Non-negotiate.

    As for the exchanging of DTP frames, these are sent when a port is in the following states: Dynamic Desirable and Trunk.

    DTP frames are NOT sent when a port is configured as Dynamic Auto, Non-neg

    ... Continue reading in our forum

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