In this lesson, 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:
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: OnWithout configuring anything on the interfaces, we are using dynamic auto mode and as a result the interfaces are in access mode.
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 accessSW2(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: OffSo configuring an interface yourself to use access mode disables DTP negotiation. How about creating a trunk ourselves?
Dear René,
Is it to avoid a security issue that we disable DTP ? If someone bring a rogue switch and plug it et voilà we negociate a trunk ?
Thks,
Prince
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.
Very good information.
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 forumHi,
I have one question. So the trunk negotiation would works if the two switches belong to the same VTP domain. The switch will throw an error says “Unable to perform trunk negotiation on port Et0/1 because of VTP domain mismatch.” My understanding is VTP will work after a trunk link establishes. So if I want to do a trunk between two VTP domain, I have to do the trunk in a nonnegotiable way , correct? Since trunk is the prerequisite for VTP to work, why Cisco won’t allow DTP to work if the two switches are not in the same VTP domain? It’s kinda like which
... Continue reading in our forum