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Another switch feature is the ability for a port to form a trunk. A trunk is configured between two devices when they need to carry traffic from multiple VLANs. A VLAN is what switches create in order to make a group of workstations appear to be on its own segment or broadcast domain. Trunk ports make these VLANs extend across multiple switches so that a single VLAN can cover an entire campus. In order to extend the VLANs in this way, the trunk ports add tags to the packets that indicate the VLAN to which the packet belongs.

There are different types of trunking protocols. If a port can become a trunk, there is a possibility that the port can trunk automatically. And, in some cases, the port can even negotiate the type of trunking to use on the port. DTP provides this ability to negotiate the trunking method with the other device. The precursor to DTP is a protocol named Dynamic Inter-Switch Link Protocol (ISL) (DISL). If these protocols run, they can delay when a port on the switch becomes active.

Usually, a port that connects to a workstation belongs to only one VLAN. Therefore, the port does not need to trunk. If a port has the ability to negotiate the formation of a trunk, the port usually defaults to the auto mode. If you change the port trunking mode to off, you can further reduce the delay of when a switch port becomes active.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00800b1500.shtml