Dynamic Trunking Protocol

This protocol is used to negotiate trunk links between VLAN aware switches using dot1q or isl. The negotiation process will also determine which trunking protocol is used either dot1q or isl. By default no trunking protocol is configured on either end. ISL is configured if supported by each switch.

DTP advertieses VTP domain, the status of the interface and its DTP type. These packets are transmitted on the native VLAN every 60 seconds.

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.


The following switch port mode settings exist:

Access — Puts the LAN port into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The LAN port becomes a nontrunk port even if the neighboring LAN port does not agree to the change.

CLI command: switchport mode access

Trunk — Puts the LAN port into permanent trunking mode and negotiates to convert the link into a trunk link. The LAN port becomes a trunk port even if the neighboring port does not agree to the change.

CLI command: switchport mode trunk

Dynamic Auto — Makes the LAN port willing to convert the link to a trunk link. The LAN port becomes a trunk port if the neighboring LAN port is set to trunk or desirable mode.

CLI command: switchport mode dynamic auto

Dynamic Desirable — Makes the LAN port actively attempt to convert the link to a trunk link. The LAN port becomes a trunk port if the neighboring LAN port is set to trunk, desirable, or auto mode. This is the default mode for all LAN ports.

CLI command: switchport mode dynamic desirable

Nonegotiate — Puts the LAN port into permanent trunking mode but prevents the port from generating DTP frames. You must configure the neighboring port manually as a trunk port to establish a trunk link.

CLI command: switchport mode nonegotiate

VTP Basics

VLAN Trunking Protocol(VTP) allows switches to share VLAN information automatically. Thus an administrator only need to change VLAN on one switch which will be propagated to all switches. However, this does not reduce the need to assign switchports to different VLANs.

A VTP domain can only support 255 vlans. A switch is enabled in server mode by default. VTP advertisements are sent every 5 minutes by default or when there is a change in configuration revision number caused by addition or deletion of vlans.

VTP Modes:

Server Mode - In VTP server mode you can create, modify and delete vlans. VLAN information is synchronized with other VTP servers and clients on the VTP domain. You can have multiple VTP servers in the VTP domain and VLAN information is synchronized according to the server with the highest configuration revision number. VLAN information is stored in the the vlan.dat file in NVRAM/Flash memory.

Client Mode - Switches in VTP client mode receive and synchronize VLAN database information from other VTP servers and VTP clients in the VTP domain. A VTP client can update a VTP server if it has a higher configuration revision number. VLAN information is stored in the "Running-Config" or DRAM. If a switch in client mode is restarted then all VLAN information, including the VTP revision number on the switch is lost and must be relearned from the VTP server once the client has restarted.

Transparent Mode - Switches in transparent mode receive updates from other servers and clients but do not participate in the VTP Domain, rather they allow the VTP updates and advertisements, to pass through the switch on to other switches in the VTP domain. Transparent mode switches do not synchronize their VLAN information with other VTP servers and clients, but maintain their own separate VLAN configurations.


VTP Configuration Commands:


This command shows the vtp status like vtp version, configuration revision number, number of vlans, etc.

Switch# show vtp status

This configures the domain name of vtp. There can be more than one domain in a topology. VTP does not work across domains i.e. if the client is in a different domain than server, it does not learn about vlan from the server.

Switch(config)# vtp domain
Changing VTP domain name from NULL to


Switch(config)# vtp mode

To make two switches talk to one another, the ports connecting them should be in trunk mode. Either of the ports can be configured to trunk mode and the other will be automatically be in trunk mode as the ports are in dynamic-auto mode.

Switch(config)#int fa0/1
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport  trunk allowed vlan 1-99

Now go to the other switch and configure vtp client mode using the vtp domain and vtp mode command.