Dynamic Trunking Protocol (DTP)

If you’re here, it’s because you want to understand what Dynamic Trunking Protocol (DTP) is and how it works.

DTP has a simple role: negotiate automatically if a link between two switches should become a trunk port or stay as an access port.

In order to do so, DTP frames are exchanged to decide between neighbors switches if the link will form a trunk.

Figure 1 – DTP Negotiation Between Switches

Instead of configuring trunk or access mode staticaly on both sides manually, DTP takes care of the configuration for you !

Dynamic Trunking Protocol is a proprietary protocol created by Cisco and works only between Cisco devices.

To understand how DTP behaves by default, let's look at a basic example:

Figure 2 – Basic DTP Link Setup

Here, we have two switches connected through their GigabitEthernet 0/1 interfaces.

Default Configuration on Switches

Let’s check their default configuration:

On SW1:

SW1#show interfaces gigabitEthernet 0/1 switchport
Name: Gig0/1
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
// Details omitted for clarity

On SW2:

SW2#show interfaces gigabitEthernet 0/1 switchport
Name: Gig0/1
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
// Details omitted for clarity

What Does This Configuration Mean?

By default, the interfaces operate with these important settings:

Administrative Mode: dynamic auto

This is the default DTP mode applied automatically to switch ports.
In dynamic auto the port listens for DTP frames but does not initiate trunking.

It simply waits for a DTP frame that says:

"Hey, let's become a trunk!"

If it receives such a frame, it agrees and turns into a trunk port.
Otherwise, the port stay passive.

Operational Mode: static access

Since both ports on our switches are set to dynamic auto, they are both listening but neither is sending DTP frame.
As a result, no trunk is negotiated and the ports stay in access mode by default.

Negotiation of Trunking: On
This means that DTP is enabled on the ports and ready to negotiate but again only if one side takes the initiative.

Figure 3 – No Trunk Formed with Dynamic Auto on Both Sides

Since both ports are passively waiting for a DTP frame and neither initiates
no trunk is formed and the link remains in access mode !

Now that you know how DTP behaves by default, it’s time to look at the other DTP modes that enable the negotiation of the trunk port.

☕ Take a sip of coffee or a glass of water let’s continue

DTP gives you flexibility to decide how your switchports behave.

Each switchport can be configured with one of the following modes:

Figure 4 – DTP Negotiation Modes Summary Table