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.
The role of DTP is simple: it automatically negotiates whether a link between two switches should become a trunk port or remain an access port.

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

Figure 1 – DTP Negotiation Between Switches

Instead of configuring trunk or access mode statically 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 Dynamic Trunking Protocol (DTP) behaves, let’s start with a simple scenario.

Figure 2 – Basic DTP Link Setup

We have two Cisco switches, SW1 and SW2, connected through their GigabitEthernet 0/1 interfaces.

Default Switchport Configuration

By default, Cisco switchports are configured as follows:

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

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

This configuration is automatically applied to switch interfaces unless explicitly modified.

Administrative Mode vs. Operational Mode

Each switchport maintains two distinct states:

• Administrative Mode defines how the port is configured and how it is allowed to behave.

• Operational Mode reflects the actual state of the port after any negotiation has taken place.

The administrative mode represents the port’s intent, while the operational mode shows the final result.

Behavior of Dynamic Auto Mode

When a switchport is configured in dynamic auto mode, it operates passively.

In this mode, the port:

• Listens for incoming DTP frames

• Does not send DTP frames

• Does not actively attempt to form a trunk

The port will only become a trunk if the neighboring device actively initiates the negotiation.

Resulting Link State

In this topology, both SW1 and SW2 are configured with dynamic auto.

Because both ports are passively waiting and neither side sends DTP frames, no trunk negotiation takes place. As a result, the link does not transition to trunk mode and remains an access link.

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

Since no trunk is negotiated, the interfaces stay in their default operational state:

Operational Mode: static access

This behavior is normal and expected when no device actively initiates trunking.

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

These modes give you control over whether a port should actively negotiate, passively listen, or completely disable trunking behavior.

But how do these modes behave in a real configuration?
Let’s look at a practical example.

We will start with the most common and widely used combination:

Dynamic Desirable (initiating) + Dynamic Auto (listening)

In this scenario:

• SW1 actively attempts to form a trunk

• SW2 remains passive and accepts the negotiation if requested

Figure 5 – Trunk Formation: Dynamic Desirable + Dynamic Auto

Configuration

First, we configure SW1 to use dynamic desirable mode.
Open the CLI and configure DTP with me.

On SW1:

SW1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)# interface gigabitEthernet 0/1
SW1(config-if)# switchport mode dynamic desirable
SW1(config-if)# end
%SYS-5-CONFIG_I: Configured from console by console

In dynamic desirable mode, the switchport actively sends DTP frames to initiate trunk negotiation.

On SW2, no configuration changes are required.
It remains in dynamic auto mode, listening for incoming DTP frames.

Verification

After the negotiation, we verify the trunk status on both switches.

On SW1:

SW1# show interfaces trunk
Port        Mode         Encapsulation  Status        Native vlan
Gi0/1       desirable    802.1q          trunking      1

Vlans allowed on trunk:
Gi0/1       1-1005

Vlans allowed and active in management domain:
Gi0/1       1,10,20

Vlans in spanning tree forwarding state and not pruned:
Gi0/1       1,10,20

On SW2:

You can also open the CLI here and verify on SW2, as I did, that the interface is now a trunk interface.

SW2# show interfaces trunk
Port        Mode         Encapsulation  Status        Native vlan
Gi0/1       auto         802.1q          trunking      1

Vlans allowed on trunk:
Gi0/1       1-1005

Vlans allowed and active in management domain:
Gi0/1       1,10,20

Vlans in spanning tree forwarding state and not pruned:
Gi0/1       1,10,20

The output confirms that the trunk link has been successfully established.

Explanation

• SW1 initiates the trunk by sending DTP frames because it is configured in dynamic desirable mode.

• SW2 listens for DTP frames and accepts the negotiation because it is configured in dynamic auto mode.

As a result, the trunk link is successfully formed between the two switches.

⚠️ If both sides were configured in dynamic auto, no trunk would be created.
At least one interface must be configured in dynamic desirable or trunk mode to initiate DTP negotiation.

→ You have successfully created a trunk negotiated by DTP.

Now that you know how to configure DTP and how easy it is to form a trunk, let’s dive deeper into how different DTP modes interact.

Combinations that Successfully Create a Trunk

For a trunk to form, at least one side must take the initiative by sending DTP frames.

Figure 6 – Combinations That Successfully Create a Trunk

Key points to remember

• dynamic desirable: Sends DTP frames actively to negotiate a trunk.

• trunk: Forces the interface to trunk mode and also sends DTP frames.

• dynamic auto: Only listens for DTP frames; it needs a partner to initiate.

  
  If at least one side is in dynamic desirable or trunk mode, a trunk will be successfully formed.

Situations Where No Trunk Is Formed

What happens when no one initiates or trunking is disabled?

Figure 7 – Situations Where No Trunk Is Formed

If both sides are in dynamic auto or one side is access: no DTP frames are sent and no trunk is created.

Full Summary Table

Here’s a summary of all possible DTP mode interactions:

Table 1 – DTP Mode Interaction Summary

Special Case: Trunk vs Access

If one port is set to trunk and the other to access, they won’t communicate correctly.

Why?

• The trunk port tags VLAN frames (802.1Q).

• The access port expects untagged frames.

Result: They cannot understand each other, and communication fails.

Dynamic Trunking Protocol (DTP) is a Cisco protocol that automates trunk negotiation between switches.

By default, ports are in dynamic auto mode: they listen but don’t initiate negotiation.
To form a trunk, at least one side must actively send DTP frames (dynamic desirable or trunk).

Table 2 – Summary of DTP Switchport Modes

You know the DTP modes and which combinations form a trunk. Let's continue with the DTP lab, where you'll test these modes on real switches and watch the negotiation happen.