Chapter 5. VLANs

This chapter covers the following key topics:

• What is a VLAN? Provides a practical and technical definition of virtual LANs.

• VLAN Types Describes how Layer 2, 3, and 4 switching operate under a VLAN.

• 802.1Q: VLAN Interoperability Describes the IEEE 802.1Q committee's effort to develop a vendor-independent method to create virtual bridged local area networks via shared VLANS (SVLs).

• Justifying the Need for VLANs Describes how network security, broadcast distribution, bandwidth utilization, network latency from routers, and complex access lists justify the need for configuring VLANs. This section also details the improper motivation for setting up a VLAN.

• Catalyst VLAN Configuration Describes how to plan for, create, and view VLAN configurations.

• Moving Users in VLANs Describes how VLANs simplify moving a user from one location to another.

• Protocol Filtering Describes how to control flooding of unneeded protocols.

When the industry started to articulate virtual LANs (VLANs) in the trade journals and the workforce, a lot of confusion arose. What exactly did they mean by VLAN? Authors had different interpretations of the new network terminology that were not always consistent with each other, much less in agreement. Vendors took varied approaches to creating VLANs which further muddled the understanding. This chapter presents definitions for VLANs as used in the Catalyst world and explains how to configure VLANs. It also discusses reasons to use and not use VLANs and attempts to clarify misinformation about them.