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The Forwarding Path

Here’s the next logical question: How is traffic forwarded between VLANs? For the answer, we’ll take a look at all three connectivity models discussed earlier.

For option 1, let’s assume that two devices need to communicate—one is configured into VLAN 10, and the other is configured into VLAN 20. We’ll also assume that the Fa0/0 interface communicates with VLAN 10 traffic and the Fa0/1 interface communicates with VLAN 20 traffic. In this scenario, traffic from the VLAN 10 device will flow to the VLAN 20 device using the path shown in Figure 4.

Figure 4

Figure 4 Traffic path (option 1).

Now let’s condense this layout a little and look at how the forwarding would work with a ROAS configuration. In Figure 5, notice that the path looks very similar, but without needing an extra interface.

Figure 5

Figure 5 Traffic path (option 2).

Finally, let’s look at how this design works with a Layer 3 switch. This layout is a bit different because it doesn’t need a separate routing device. In this case, the routing mechanism is built into the same switch, and SVIs route the traffic (see Figure 6).

Figure 6

Figure 6 Traffic path (option 3).

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