A VLAN is a broadcast domain created by one or more switches. The network design in Figures and requires three separate broadcast domains.

Figure shows how three separate broadcast domains are created using three separate switches. Layer 3 routing allows the router to send packets to the three different broadcast domains.

In Figure , a VLAN is created using one router and one switch. However, there are three separate broadcast domains.  In this scenario there is one router and one switch, but there are still three separate broadcast domains.

In Figure , three separate broadcast domains are created. The router routes traffic between the VLANs using Layer 3 routing.

The switch in Figure  forwards frames to the router interfaces:

• If it is a broadcast frame.
• If it is in route to one of the MAC addresses on the router.

If Workstation 1 on the Engineering VLAN wants to send frames to Workstation 2 on the Sales VLAN, the frames are sent to the Fa0/0 MAC address of the router. Routing occurs through the IP address on the Fa0/0 router interface for the Engineering VLAN.

If Workstation 1 on the Engineering VLAN wants to send a frame to Workstation 2 on the same VLAN, the destination MAC address of the frame is the MAC address for Workstation 2.

Implementing VLANs on a switch causes the following to occur:

• The switch maintains a separate bridging table for each VLAN.
• If the frame comes in on a port in VLAN 1, the switch searches the bridging table for VLAN 1.
• When the frame is received, the switch adds the source address to the bridging table if it is currently unknown.
• The destination is checked so a forwarding decision can be made.
• For learning and forwarding the search is made against the address table for that VLAN only.

Web Links Overview of Routing between Virtual LANs http://www.cisco.com/univercd/cc/ td/doc/ product/ software/ ios113ed/113ed_cr/ switch_c/xcvlan.htm