Overview
Early VLANs were difficult to implement across networks. Most VLANs were defined on each switch, which meant that defining VLANs over an extended network was a complicated task. Every switch manufacturer had a different idea of the best ways to make their switches VLAN capable, which further complicated matters. VLAN trunking was developed to solve these problems.

VLAN trunking allows many VLANs to be defined throughout an organization by adding special tags to frames to identify the VLAN to which they belong. This tagging allows many VLANs to be carried across a common backbone, or trunk. VLAN trunking is standards-based, with the IEEE 802.1Q trunking protocol now widely implemented. Cisco’s Inter-Switch Link (ISL) is a proprietary trunking protocol that can be implemented in all Cisco networks.

VLAN trunking uses tagged frames to allow multiple VLANs to be carried throughout a large switched network over shared backbones. Manually configuring and maintaining VLAN Trunking Protocol (VTP) on numerous switches can be challenging. The benefit of VTP is that, once a network is configured with VTP, many of the VLAN configuration tasks are automatic.

This module explains VTP implementation in a VLAN switched LAN environment.

VLAN technology provides network administrators with many advantages. Among other things, VLANs help control Layer 3 broadcasts, they improve network security, and they can help logically group network users. However, VLANs have an important limitation. They operate at Layer 2, which means that devices on one VLAN cannot communicate with users on another VLAN without the use of routers and network layer addresses.

Students completing this module should be able to:

  • Explain the origins and functions of VLAN trunking
  • Describe how trunking enables the implementation of VLANs in a large network
  • Define IEEE 802.1Q
  • Define Cisco ISL
  • Configure and verify a VLAN trunk
  • Define VTP
  • Explain why VTP was developed
  • Describe the contents of VTP messages
  • List and define the three VTP modes
  • Configure and verify VTP on an IOS-based switch
  • Explain why routing is necessary for inter-VLAN communication
  • Explain the difference between physical and logical interfaces
  • Define subinterfaces
  • Configure inter-VLAN routing using subinterfaces on a router port