Content

| Overview

Promotion of VLANs in the 1990s by the trade journals and the workforce created confusion. Authors had different interpretations of the new network terminology that were not consistent or in agreement with each other. Vendors also took varied approaches to creating VLANs, which led to further confusion.

VLAN designs have changed over the last decade as a result of advances in technology. In the mid-1990s, deployment of campus-wide VLANs was acknowledged as the best design methodology. Local or geographic VLANs are considered the better design methodology, because VLANs are separated by geographic location and not by function.

The speed of Layer 2 switching influenced campus-wide VLAN design. Multilayer switches were able to perform Layer 3 switching at rates approximating that of Layer 2 switching. Hardware-based Layer 3 switching provided speeds of hundreds of millions of packets per second (pps). Although campus-wide VLANs play a role in special circumstances, designing a stable, scalable campus network has become more important.

With the creation of high-performance Layer 3 switches, the campus-wide model became obsolete and is no longer recommended in most network designs. The CPU-intensive Spanning-Tree Protocol (STP) causes convergence and redundancy problems in some large networks making them susceptible to network instabilities.

With a local VLAN model, Layer 3 switching allows the users' traffic to be carried based on the network layer address, such as the IP address. VLANs then become a simple mechanism of segmenting an IP subnet at Layer 2. For example, the 172.16.10.0/24 subnet is mapped to VLAN 10, the 172.16.20.0/24 subnet is mapped to VLAN 20, and so forth. A critical tenet of campus design declares that user VLANs terminate at a Layer 3 switch, which is typically located in a data center or basement of a building.

In this module, the student will examine the following tasks:

- Clarify the role of VLANs and VLAN protocols in modern campus networks.
- Present definitions and categorizations for VLANs.
- Explain how to configure VLANs.
- Describe situations when VLANs are appropriate.
- Clarify misinformation about VLANs.
- Introduce VLAN management tools such as the VLAN Trunk Protocol (VTP).