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DTP (Dynamic Trunking Protocol)


Dynamic Trunking Protocol (DTP) is used for negotiating trunking on a link between two devices, as well as negotiating the encapsulation type of either 802.1Q or ISL. We use the no negotiate command to disable trunking on an interface, use the switchport mode access command, which sets the port back to a dedicated layer 2 switch port.

switchport mode dynamic auto

This mode makes the interface able to convert the linkto a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. This is now the default switchport mode for all Ethernet interfaces on all new Cisco switches.

switchport mode dynamic desirable

This one makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. I used to see this mode as the default on some older switches, but not any longer. The default is dynamic auto now.

switchport mode trunk

Puts the interface into permanent trunking mode and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface isn’t a trunk interface.

switchport no negotiate Prevents

The interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.

Command for DTP

Switch # Show interface fastethernet 0/1 switchport

Switch 0 configuration

Switch # Config terminal

Switch (Config) # interface fastEthernet 0/1

Switch (Config-if) # switchport mode dynamicdesirable/auto/no negotiate






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