Key Point

Just like devices on different LANs, those on different VLANs require a Layer 3 mechanism (a router or a Layer 3 switch) to communicate with each other.

A Layer 3 device can be connected to a switched network in two ways: by using multiple physical interfaces or through a single interface configured as a trunk. These two connection methods are shown in Figure 2-9. The diagram on the left in this figure illustrates a router with three physical connections to the switch; each physical connection carries traffic from only one VLAN.

Figure 2-9. A Router, Using Either Multiple Physical Interfaces or a Trunk, Is Required for Communication Among

VLANs

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Figure 2-9. A Router, Using Either Multiple Physical Interfaces or a Trunk, Is Required for Communication Among

VLANs

[View full size image]

The diagram on the right in Figure 2-9 illustrates a router with one physical connection to the switch. The interfaces on the switch and the router have been configured as trunks; therefore, multiple logical connections exist between the two devices. When a router is connected to a switch through a trunk, it is sometimes called a "router on a stick," because it has only one physical interface (a stick) to the switch.

Each interface between the switch and the Layer 3 device (whether physical interfaces or logical interfaces within a trunk) is in a separate VLAN (and therefore in a separate subnet for IP networks).

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