Dynamic Trunking Protocol

Trunk ports are those used to carry traffic from multiple VLANs. The port that connects two separate switches should be designated as a trunk if the switches have multiple VLANs enabled. Trunk ports will carry traffic from all VLANs, even native VLANs for which the traffic is untagged, if all VLANs are permitted. Otherwise, the trunk connection will only carry traffic from allowed VLANs. Either ISL or 802.1Q would be used to tag or encapsulate the trunk link traffic. As all switch ports are access ports by default, creating a port trunk requires manual DTP configuration.

Dynamic Trunking Protocol
Fig 1:Dynamic Trunking Protocol

Dynamic Trunking Protocol

In order to negotiate a trunk link between two switches and the encapsulation type of either 802.1q or ISL, CISCO developed the Dynamic Trunking Protocol (Generally, 802.1q is used because ISL has more overhead than 802.1q). It is a layer 2 (data link) protocol, hence it is obviously enabled by default.

The following are the settings that can be selected while configuring a switch interface:

Switchport mode access (DTP mode OFF)

This mode, also known as DTP mode OFF, permanently disables the switch interface's ability to act as a trunk port, regardless of whether its neighboring interface is one or is attempting to do so. A dedicated layer 2 access port is what the port is.

Switchport mode trunk (DTP mode ON)

The interface enters trunking mode as a result. The reason DTP mode ON is used is because the interface will become a trunk interface whether the neighboring ports are or are not.

Switchport mode dynamic auto

For the older CISCO switches, this is their default mode. The interface can become a trunk link in this mode. If the neighboring interface is configured in trunk or desired mode, the interface will transform into a trunk link. The trunk won't form if the interface mode on both switches is set to auto.

Switchport mode dynamic desirable 

The interface will actively try to turn the link into a trunk link in this mode. If the neighboring interface is set to trunk, desirable, or auto, the interface will transform into a trunk port.

Switchport nonegotiate

This mode prevents the interface from generating DTP frames. Only when the switch port mode is access or trunk is this command utilized. To create a trunk link, you must manually configure the neighboring interface as a trunk interface.

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