Virtual LANs

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A virtual LAN (VLAN) is a broadcast domain created by one or more switches. On a switch, a VLAN is a collection of ports among which received broadcast frames are forwarded. Such frames are never forwarded outside the member port group . VLANs are what make a switch so versatile: They give the switches the capability to operate in different broadcast domains. Remember, all ports on a bridge constitute a broadcast domain. Well, with VLANs, you can configure certain ports on a switch to be in one broadcast domain and then configure other ports to be in a different broadcast domain. Each port on a switch is still a collision domain, but, depending on how you configure the switch, one or multiple broadcast domains (or IP network subnets) are possible. Figure 5-2 illustrates a four-port switch configured for two VLANs. Ports 1 and 2 are configured for VLAN 1. Ports 3 and 4 on the switch are configured for VLAN 2.

Figure 5-2. Switch Diagram with Two VLANs

NOTE

The Spanning-Tree Protocol will not have to be configured in the lab. The purpose of the Spanning-Tree Protocol is to dynamically discover one active path through a redundant bridged or switched network. Because the lab scenario does not have multiple switches, you do not have to concern yourself with redundant bridging paths, so the Spanning-Tree Protocol will not be covered in this book. If you need more information on the Spanning-Tree Protocol, refer to Chapter 5 of Steve McQuerry's book Interconnecting Cisco Network Devices , from Cisco Press.

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