Dynamic Trunking Protocol

• Switch interfaces automatically negotiate/configure
  • the switchport states to be access or trunk; also
  • the encapsulation methods to be ISL or 802.1Q.
• DTP will not form a trunk with a router, PC, etc. The switchport will be in access mode.
• switchport mode access will disables DTP (Negotiation of Trunking) on the interface.
• To ensure DTP negotiation working, two switches' interfaces must send and receive DTP frames. It means either of interfaces with DTP (Negotiation of Trunking) off will result in misconfiguration.

DTP Negotiation (Switchport Mode)

Switchport Modes	access	auto	desirable	trunk
access	access	access	access	X
auto	access	access	trunk	trunk
desirable	access	trunk	trunk	trunk
trunk	X	trunk	trunk	trunk

DTP Negotiation (Encapsulation)

Encap Modes	Negotiate	802.1Q	ISL
Negotiate	ISL	802.1Q	ISL
802.1Q	802.1Q	802.1Q	X
ISL	ISL	X	ISL

Commands & Syntax

SW1(config-if)#switchport mode dynamic {auto | desirable}                    //Defualt is 'auto'
SW1(config-if)#switchport mode nonegotiate                                   //prevent DTP (negotiation) packets from being sent out the interface
SW1(config-if)#switchport trunk encapsulation {negotiate | isl | dot1q}      //Configue DTP encapsulation method. By defualt, 'negotiate' is enabled and ISL is favored.