Dynamic Trunking Protocol

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The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. It works on the Layer 2 of the OSI model. VLAN trunks formed using DTP may utilize either IEEE 802.1Q or Cisco ISL trunking protocols.

DTP should not be confused with VTP, as they serve different purposes. VTP communicates VLAN existence information between switches. DTP aids with trunk port establishment. Neither protocol transmits the data frames that trunks carry.

Switch port modes[edit]

The following switch port mode settings exist:

  • auto — causes the port to passively be willing to convert to trunking. The port will not trunk unless the neighbor is set to on or desirable . This is the default mode. Note that auto-auto (both ends default) links will not become trunks.
  • on — forces the link into permanent trunking, even if the neighbor doesn't agree.
  • off — forces the link to permanently not trunk, even if the neighbor doesn't agree.
  • desirable — causes the port to actively attempt to become a trunk, subject to neighbor agreement (neighbor set to on, desirable, or auto.)
  • nonegotiate — Disables the sending of DTP frames on the port. For use when the DTP frames confuse the neighboring (non-Cisco) 802.1Q switch. It does not set the port to trunking mode automatically. Trunking is determined based on whether the on or off keywords have been entered on the device. This command generates an error if dynamic modes are configured.[1] [2]'

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