Autotrunking is a function that enables one or more switch ports in a Cisco system of virtual local area networks (VLANs) to carry traffic for any or all of the VLANs accessible through a particular switch. Such a port is called a trunk port in contrast to an access port, which carries traffic only to and from the specific VLAN assigned to it. A trunk port marks each frame with a special identifying tag (either an ISL tag or an 802.1Q tag) as it passes between switches so that each frame can be routed to its intended VLAN.

In Cisco's Dynamic Trunking Protocol (DTP), a port can be set to autotrunking by default (DTP auto). However, it is also possible to disable this function (DTP off). Autotrunking can be a convenience because it simplifies the management of networks with many VLANs and multiple switches.

An administrator can take advantage of autotrunking to configure multiple VLANs in a large network. If autotrunking is not used, it is necessary to manually configure each VLAN in the system. On the other hand, if autotrunking is used for this purpose, it can render a network susceptible to a form of attack called VLAN hopping, in which rogue packets are sent to all VLANs accessible from the point of attack.

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This was last updated in June 2007

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