Layer 2 Switching

The term Layer 2 switching (L2 switching) refers to the typical switch-processing logic covered in Chapter 9. A switch receives a frame and looks at the destination MAC address. If the MAC table has an entry for that destination, it forwards the frame; if not, or if the frame is a broadcast, it forwards the frame out all ports, except the port in which the frame entered the switch.

When VLANs are used, an L2 switch uses the same logic, but per VLAN. So, there is a MAC address table for each VLAN. Because the MAC address tables are separate, unicasts sent inside one VLAN cannot be forwarded out ports in another VLAN. Likewise, broadcasts in one VLAN cannot be forwarded out ports in another VLAN.

In short, L2 switches cannot forward traffic between VLANs. The last few pages of this chapter cover a few alternatives for how you can forward traffic between VLANs.

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