Dynamic Trunking Protocol
Encyclopedia
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.
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
for the purpose of negotiating trunking
Trunking
In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...
on a link between two VLAN
Virtual LAN
A virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
-aware switches
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
, and for negotiating the type of trunking encapsulation
Encapsulation (networking)
In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects....
to be used. It works on the Layer 2
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. VLAN trunks formed using DTP may utilize either IEEE 802.1Q
IEEE 802.1Q
IEEE 802.1Q is the networking standard that supports Virtual LANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames...
or Cisco ISL
Cisco Inter-Switch Link
Cisco Inter-Switch Link is a Cisco Systems proprietary protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches....
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
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 — forces the port to permanently trunk but not send DTP frames. For use when the DTP frames confuse the neighboring (non-Cisco) 802.1Q switch. The neighboring switch must be set to trunking manually.