Specific protocols, or rules, are used to implement trunking. Trunking provides an effective method to distribute VLAN ID information to other switches.

Using frame tagging as the standard trunking mechanism, as opposed to frame filtering, provides a more scalable solution to VLAN deployment. Frame tagging is the way to implement VLANs according to IEEE 802.1Q.

VLAN frame tagging is an approach that has been specifically developed for switched communications. Frame tagging places a unique identifier in the header of each frame as it is forwarded throughout the network backbone. The identifier is understood and examined by each switch before any broadcasts or transmissions are made to other switches, routers, or end-station devices. When the frame exits the network backbone, the switch removes the identifier before the frame is transmitted to the target end station. Frame tagging functions at Layer 2 and requires little processing or administrative overhead.

It is important to understand that a trunk link does not belong to a specific VLAN. The responsibility of a trunk link is to act as a conduit for VLANs between switches and routers.

ISL is a protocol that maintains VLAN information as traffic flows between the switches. With ISL, an Ethernet frame is encapsulated with a header that contains a VLAN ID.

Web Links Understanding and Configuring VLAN Trunking Protocol (VTP) http://www.cisco.com/en/US/tech/ tk389/tk689/technologies_tech_note 09186a0080094c52.shtml