Dynamic Trunking Protocol (DTP)

1. What is 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.

2. How DTP Works

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

3. DTP Negotiation Modes

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

👉 But how do these modes behave in practice?

When I first learned DTP, it took me a bit of time too to understand each mode and that’s completely normal.

Let’s start with the most common combination:
Dynamic Desirable (sending) + Dynamic Auto (listening).

We’ll configure SW1 in dynamic desirable mode and leave SW2 in dynamic auto.
Let’s see how they interact and if they successfully negotiate a trunk!

Figure 5 – Trunk Formation: Dynamic Desirable + Dynamic Auto

Here’s you can see an animation showing how DTP frames are sent out from SW1’s G0/1 interface if dynamic desirable mode is enabled !

Configuration

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

The dynamic desirable mode actively sends DTP frames actively to initiate trunk negotiation.

On SW2, we don’t change anything:

It stays in dynamic auto mode, ready to listen for incoming DTP frames.

Now, let’s verify if the trunk is up.

Verification

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:

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

As you can see, the trunk link is successfully formed.

• SW1 is sending DTP frames (dynamic desirable).

• SW2 is listening and accepting (dynamic auto).

That’s exactly how DTP works.

⚠️Remember:
if both sides are in dynamic auto, no trunk will form.
You need at least one side in dynamic desirable or trunk mode to initiate.

→ Congratulations, you’ve just created a trunk negotiated by DTP.

4. How DTP Modes Interact

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.

Important:
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.

5. Conclusion

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