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Dynamic Trunking Protocol (DTP)

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Trunk ports are ports that are used to carry the traffic of more than one VLAN. The port which connects two different switches and the switches have more than one VLAN configured then that port should be made trunk. If all the VLANs are allowed then trunk ports will carry traffic of all the VLANs including native VLANs for which the traffic goes untagged otherwise only the allowed VLANs traffic will be carried by the trunk link. The trunk link traffic would be encapsulated or tagged either by ISL or 802.1Q. 

By default, all switch ports are access ports therefore to make a port trunk, the user should manually make it trunk by using DTP. 

Dynamic Trunking Protocol – 
Dynamic Trunking Protocol is CISCO proprietary protocol used for negotiating a trunk link between two switches as well as the encapsulation type of either 802.1q or ISL (Generally, 802.1q is used because ISL has more overhead than 802.1q). Of course, It is a layer 2 (data link) protocol and is enabled by default. 

Here are the different options available while configuring a switch interface:- 

switchport mode access (DTP mode OFF) – 
This mode puts the switch interface into permanent non-trunking mode regardless of whether the neighbouring interface is a trunk port or trying to become a trunk port that is why it is known as DTP mode OFF. The port is a dedicated layer 2 access port. 

switchport mode trunk (DTP mode ON) – 
It puts the interface into trunking mode. The interface will become a trunk interface even if the neighbouring ports are trunk or not that is why it is called DTP mode ON. 

switchport mode dynamic auto – 
This is a default mode on the older CISCO switches. This mode makes the interface able to convert to a trunk link. The interface will become a trunk link if the neighbouring interface is set to trunk or desirable mode. If both switches interface mode is auto, then the trunk will not be formed. 

switchport mode dynamic desirable – 
By this mode, the interface will actively attempt to convert the link into a trunk link. The interface will become a trunk port if the neighbouring interface is set to trunk, desirable or auto. 

switchport nonegotiate – 
This mode prevents the interface from generating DTP frames. This command is used only when the switch port mode is access or trunk. You must manually configure the neighbouring interface as a trunk interface to establish a trunk link. 

Now, let’s see the scenarios in which the switch interface will either become a trunk or access interface. 

 Dynamic Auto 
Dynamic desirable 
Dynamic Autoaccesstrunktrunkaccess
Dynamic desirabletrunktrunktrunkaccess
trunktrunktrunktrunklimited connectivity
accessaccessaccesslimited connectivityaccess

By observing this, it is clear that whenever you receive a DTP packet that requests to form a trunk, your interface will be in trunk mode.

Here are some of its features:

  • Automatic negotiation: DTP allows switches to automatically negotiate the formation of a trunk link without requiring manual configuration.
  • Four modes: DTP has four modes: “dynamic auto”, “dynamic desirable”, “trunk”, and “access”. The mode selected on each switch determines the behavior of the negotiation process.
  • Proprietary protocol: DTP is a proprietary protocol developed by Cisco and is only supported on Cisco devices.
  • Can pose security risks: DTP can pose security risks if not configured properly, as it can allow unauthorized devices to form a trunk link with a switch.
  • Can improve network performance: DTP can improve network performance by allowing switches to form trunk links automatically and efficiently manage network traffic.
  • Can cause issues in mixed environments: DTP can cause issues in mixed network environments where non-Cisco devices do not support the protocol or do not behave as expected.

Advantages of Dynamic Trunking Protocol (DTP)

  • DTP simplifies the process of configuring and managing VLANs on a network. It enables network administrators to dynamically negotiate trunk links between switches without manual configuration.
  • DTP reduces the risk of misconfiguration errors and simplifies the task of managing VLANs in large networks with multiple switches.
  • DTP allows for automatic creation and deletion of VLANs on switches, which can save time and reduce the risk of configuration errors.
  • DTP provides a quick and easy way to connect switches together and establish VLAN communication.

Disadvantages of Dynamic Trunking Protocol (DTP)

  • DTP can create security vulnerabilities in the network. It enables automatic negotiation of trunk links, which can lead to unauthorized switches being connected to the network, potentially compromising network security.
  • DTP can result in increased network traffic due to the constant negotiation of trunk links between switches.
  • DTP can lead to misconfiguration of VLANs in situations where a switch negotiates a trunk link with a different VLAN configuration than intended.
  • DTP is a Cisco proprietary protocol, which means it may not be compatible with non-Cisco switches or other network devices. This can limit its usefulness in heterogeneous networks.


Last Updated : 05 May, 2023
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