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I'm trying to understand how a switch handles two vlan's on the same port, in the case of a voice vlan + access vlan.

If you connect an ip phone to a switch and then a laptop to the switch at the back of that ip phone, and on your cisco switch configure the interface to which the ip phone is connected as a voice vlan x + access vlan x, how does the switch know which vlan the data is coming from?

How does the switch know whether the data it just received from said interface, came from the voice vlan or the access vlan?

Thank you for all your answers

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You almost answered your own question: VOIP packets are tagged with the VOIP VLAN tag, internet packets are untagged, since that's the native VLAN on that port.

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    No, you're wrong. The switch only tags outgoing frames. The VOIP phone will tag the VOIP frames going to the switch with the VOIP VLAN tag.
    – Teun Vink ♦
    Sep 14, 2017 at 14:41

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