Cisco Dynamic Trunking Protocol (DTP) Explained

By | 21st February 2017



Understanding Dynamic Trunking Protocol (DTP)

The Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol that is automatically enabled on Catalyst 2960 and Catalyst 3560 Series switches. DTP is used to negotiate forming a trunk link between two or more Cisco devices before actually forming the trunk connection. The main benefit of DTP is to increase traffic on a trunked link.
A Non-Cisco network device or Switch from other vendors do not support DTP. DTP manages trunk negotiation only if the port on the neighbor switch is configured in a trunk mode that supports DTP.

Switch trunk interfaces support different trunking modes. An interface can be set to Trunking, nontrunking, or to Negotiate. Ethernet Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which operates on a point-to-point basis between network devices.

The default DTP configuration for Cisco Catalyst 2960 and 3560 switches is in “dynamic auto” or “dynamic desirable” mode.

For you to enable trunking between a Cisco switch to a non-Cisco switch or device that does not support DTP, use the switchport mode trunk and switchport nonegotiate interface configuration mode commands. This causes the interface to become a trunk but not generate DTP frames.

In order to avoid misconfigurations and wrongly forwarded of DTP frames by a non-Cisco device, turn off DTP on interfaces on a Cisco switch connected to devices that do not support DTP.

In the diagram below:

The Fa0/1 ports on Cisco switch1 and Cisco Switch2 are set to dynamic auto, dynamic desirable so the negotiation results in the trunking state. This creates an active trunk link.

dtp with dynamic auto and desirable

dtp with dynamic desirable and desirable

How to configure dynamic desirable mode

Sw1#configure terminal
Sw1(config)#interface fa0/1
Sw1(config-if)#switchport mode dynamic desirable
Sw1(config-if)#exit
Sw1(config)#exit
Sw1#

How to configure dynamic auto mode

Sw1#configure terminal
Sw1(config)#interface fa0/1
Sw1(config-if)#switchport mode dynamic auto
Sw1(config-if)#exit
Sw1(config)#exit
Sw1#

In the diagram below,  the link between switches Switch1 and Switch2 becomes a trunk because the Fa0/1 ports on Cisco switch1 and Non-Cisco switch2 are configured to ignore all DTP advertisements and to come up in and stay in trunk port mode.

How to configure trunk mode

Sw1#configure terminal
Sw1(config)#interface fa0/1
Sw1(config-if)#switchport mode trunk
Sw1(config-if)#exit
Sw1(config)#exit
Sw1#

How to configure trunk mode with nonegotiate

Sw1#configure terminal
Sw1(config)#interface fa0/1
Sw1(config-if)#switchport mode trunk
Sw1(config-if)#switchport nonegotiate
Sw1(config-if)#exit
Sw1(config)#exit
Sw1#

 

In the diagram below: The F0/1 ports on Cisco switch1 and Cisco Switch2 are set to dynamic auto, so the negotiation results in the access mode state. This creates an inactive trunk link.

DTP Negotiated Interface Modes Explained:
Switches support different trunking modes with the help of DTP:
• Switchport mode access: This puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface, regardless of whether the neighboring interface is a trunk interface.
• Switchport mode dynamic auto: This enables the interface to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. The default switchport mode for newer Cisco switches Ethernet interfaces is dynamic auto. Note that if two Cisco switches are left to the common default setting of auto, a trunk will never form.
• Switchport mode dynamic desirable: This places the interface in an active attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode.
• Switchport mode trunk Puts the interface into permanent trunking mode and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface.
• Switchport nonegotiate: This mode prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.

Also, Read VLAN Trunking Protocol