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Dynamic Trunking Protocol

Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol used to negotiate the trunking mode between two switches. DTP allows switches to negotiate whether to form a trunk or not automatically (this is access mode) and, if so, which trunking protocol to use, such as 802.1Q or ISL. DTP can operate in two modes:

  • Dynamic Desirable: In this mode, the local switch port actively tries to create a trunk link with the neighboring switch. If the neighboring switch port is set to "Dynamic Auto" mode or is manually configured as a trunk port, the link will be formed as a trunk. However, if the neighboring switch port is an access port, the link will be formed as an access link.
  • Dynamic Auto: In this mode, the switch port will be configured as a trunk if the neighboring switch port is set to "Dynamic Desirable" mode or manually configured as a trunk. However, if the neighboring switch port is configured as an access port or set to "Dynamic Auto," the link will be formed as an access link.

DTP operates on Cisco Catalyst switches and is enabled by default on most interfaces. However, a typical recommendation is to disable DTP using the switchport nonegotiate command on interfaces where it is unnecessary, to reduce the risk of unintentional trunk formation and VLAN hopping attacks.