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 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 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